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The Evolution of Life

This section provides information about the evolution of life on Earth. For reasons of simplicity some important events and life forms were selected for each geological time span. However this should not give the impression that the evolution of life on Earth is or was a sequential event. Many life forms that were not mentioned in this text were very successful and have evolved or in some cases maintained a very successful life strategy. Prokaryotes, for example are the most successful and abundant organisms on Earth, in both numbers and biomass and still make up to 90% of the total weight of living things. For further information on each geological period or special life forms mentioned, there are links to other websites about this time.

The evolution of life is organised into 20 sections referring to their biological and geological significance. You will find a list of useful reference websites at the very end of the sections

Birth of Earth - approximately 4600 million years ago - The Hadean Period (4600 m.y. - 3800 m.y.)

Scientists estimate that our planet, Earth, formed around 4600 million years ago. The oldest rocks dated so far are from the Acasta Gneiss Complex near Great Slave Lake, Canada with an age of about 4030 million years. The Isua Supracrustal rocks in West Greenland are dated at about 3700 to 3800 million years old. The best age estimate for the Earth's final formation is about 4540 million years ago, based on data from the Canyon Diablo meteorite. In addition, mineral grains called zircon from sedimentary rocks from west-central Australia have recently been reported with an age of about 4400 million years.

While we don't know exactly how the Earth was formed, we have some really good evidence to support the following hypothesis: The Earth began as part of the accretion of the solar system. The formation of the planets (including Earth), asteroids, meteors, comets and the central sun - formed through the tendency of matter to clump together, until finally there were substantial bodies, the planets and their moons, sweeping up all the 'left-over' bits in their orbits. Studies of the Moon's surface show that in its early history it was subjected to bombardment by giant meteorites. Earth's early history must have been even more violent because of the greater gravitational forces involved. None of those early bombardments can be seen today, as they have been transformed by Earth's erosional forces. Most of the meteorite craters which are evident on Earth are much younger.

This was the so-called "Hadean Period" from 4600 - 3800 million years ago. During this period the heavier molten iron sank to the middle of the newly forming Earth, to become the core. The lighter material rose to the surface, the lightest of all becoming the crust on the surface. There was also an outgassing of volatile molecules such as water, methane, ammonia, hydrogen, nitrogen, and carbon dioxide, which formed the early atmosphere of the Earth. The initial steam atmosphere was made of water from comets and hydrated minerals from volcanic eruptions. Rain fell into proto-ocean about 4300 to 4400 million years ago. All terrestrial planets are thought to have had a similar process in their early histories.

During the long interval of the Precambrian Era (which includes approximately 90% of geologic time and includes three Eons: the Hadean, the Archean and the Proterozoic) the only inhabitants of the Earth were simple microscopic organisms, many of them comparable in size and complexity to modern-day bacteria. The conditions under which these organisms lived differed greatly from those prevailing today, but the mechanisms of evolution were the same. Genetic variations made some individuals better fitted than others to survive and to reproduce in a given environment. The emergence of new forms of life through this principle of natural selection exerted great changes on the physical environment, thereby altering the conditions of evolution.

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The earliest cells and stromatolites - The Archaean Period (3800 m.y. - 2500m.y.)

The middle era of Precambrian time, spanning the period between 3800 and 2500 million years ago is called the Archaean, meaning ancient. Life arose on Earth during the early Archaean, as indicated by the appearance of fossil bacteria in rocks thought to be about 3500 million years old.

While evidence preserved in rock layers in present-day Greenland tell us that life existed on Earth during that time it doesn't explain how it came to exist. The classic experiment demonstrating the mechanisms by which inorganic elements could combine to form the precursors of organic chemicals was the 1950 experiment by Stanley Miller. He undertook experiments designed to find out how lightning - simulated by repeated electrical discharges - might have affected the primitive earth atmosphere. He discharged an electric spark into a mixture thought to resemble the primordial composition of the atmosphere. In a water receptacle, designed to model an ancient ocean, amino acids appeared. Amino acids are widely regarded as the building blocks of life.

Although the primitive atmosphere is no longer believed to be as rich in hydrogen as was once thought, the discovery that the Murchison meteorite contains the same amino acids obtained by Miller, and even in the same relative proportions, strongly suggests that his results are relevant.

It may seem surprising that bacteria can leave fossils at all. However, one particular group of bacteria, the cyanobacteria or "blue-green algae," have left a fossil record that extends far back into the Precambrian - the oldest cyanobacteria-like fossils known are nearly 3500 million years old and are among the oldest fossils currently known. Cyanobacteria are larger than most bacteria, and many secrete a thick cell wall. More importantly, cyanobacteria may form large layered structures, called stromatolites (more or less dome-shaped) or oncolites (round). These structures form as a mat of cyanobacteria growths in a marine environment, trapping sediment and sometimes secreting calcium carbonate. When sectioned very thinly, fossil stromatolites may be found to contain exquisitely preserved fossil cyanobacteria and algae.

These early cells belonged to the group of prokaryotic cells (in contrast to the more complex structures of eukaryotic cells). Prokaryotes are small cells which lack the complex internal structures, like mitochondria and chloroplasts, found in eukaryotic cells. Although prokaryotes possess DNA on a chromosome, it is not enclosed in a nucleus.

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The first algae - 1200 million years ago - The Proterozoic Era (2500 m.y. - 544 m.y.)

The final Era of the Precambrian, the Proterozoic Era, spans the time between 2500 million and 544 million years ago. Fossils of both primitive single celled and more advanced multicellular organisms begin to appear in abundance in rocks from this era. The name, proterozoic, means "early life."

The oldest multi-cellular algae fossil dates to 1200million years. At this time biological diversity increased greatly to become eukaryote cells. Different to the prokaryotic cells, the eukaryotic cells are larger and have a complex internal organisation and which includes a nucleus housing the DNA on the chromosomes and specialised structures known as organelles. The oldest fossil evidence of multi-cellular animals, or metazoans, are burrows which suggest they were made by smooth, wormlike creatures. Theses fossils have been found in rocks in various places including China, Canada, India. The imprints of these soft bodied animals reveal little else but their basic shape. Oxygen must have been freely available by the time the first eukaryotic cells arrived, released through the proliferation of cyanobacteria, earlier in the Precambrian.

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The Vendian animals - 570 million years ago - The Vendian Period (650 m.y. - 544 m.y)

The first animals in the fossil record appeared between 620 and 550 million years ago. This period is called the Vendian after the stratigraphic sequence in Russia where rocks of this age are especially well developed. The Vendian period is also known as the Ediacaran period (after a site in Australia) and is distinguished by a characteristic collection of fossils from complex soft-bodied animals. These fossils have been found at several localities around the world.

The Ediacarian/Vendian faunas have puzzled many palaeontologists, because although some of these animals may have belonged to groups that survive today, others don't seem at all related to animals we know. There are two confusing aspects of the Vendian or Ediacaran organisms: firstly there does not seem to be any evidence for any skeletal hard parts, in any of those fossils, i.e. the organisms were soft bodied. Secondly, is the issue of which group of animals these group of fossils belong to. Although many have been compared to modern day jelly fish or worms they have been also compared to be somewhat like analogous to a 'mattress' with tough outer walls and fluid filled internal cavities, something like a sponge. However, Simon Conway Morris, of Cambridge University, argued them to be higher developed animals than sponges, as there were no sponge fossils in the Vendian fauna. Sponges are regarded as quite primitive animals and should have appeared before the Vendian soft bodied organisms.

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The Cambrian Explosion - The Cambrian Period (544m.y.- 505 m.y.)

The earliest period of the Palaeozoic era is called the Cambrian Period. It is named after Cambria, the Roman name for Wales, where rocks of this age were first identified by the nineteenth century geologist Adam Sedgwick. Cambrian sediments, however, are by no means restricted to Wales but found in many other parts of the world.

As recently as the middle of last century the earliest known fossils had all come from the Cambrian. The Cambrian fossils include animals with body plans similar to those of a number of living animals and they represent the lineages of almost all animals living today. This stunning and unique evolutionary flowering is termed the "Cambrian explosion". But it was not as rapid as an explosion: the changes seem to have happened during about 30 million years, and some stages took 5 to 10 million years.

The emergence of many kinds of creatures during the transition from the Precambrian to the Cambrian radically changed the nature of the relations among animals, including the development of more complex predator-prey relationships. Animals that fed on living matter, rather than scavenging on dead organic matter or relying on symbiotic relationships with photosynthesising algae, became much more common and even predators, such as Anomalocaris, evolved to eat those who could not escape them.

The best record of the Cambrian diversification is the Burgess Shale in British Columbia. Laid down in the middle-Cambrian, when the "explosion" had already been underway for several million years, this formation contains the first appearance in the fossil record of brachiopods, with clamlike shells, as well as trilobites, molluscs, echinoderms, and many odd animals that probably belong to extinct phyla.

The cause of the Cambrian "explosion" is a matter of debate among scientists. Some point to the increase in oxygen that began around 2000 million years ago supporting a higher metabolic rate and allowing the evolution of larger organisms and more complex body structures. Others propose that an extinction of life at the end of the Vendian period opened up ecological roles that the new forms exploited. A change in ocean chemistry may have occurred, allowing for the first time the development of hard body parts such as teeth and supporting skeletons. Genetic factors were also crucial. Recent research suggests that the period prior to the Cambrian explosion saw the gradual evolution of a "genetic tool kit" of genes (the homeobox or "hox" genes) that govern developmental processes. Once assembled, this genetic tool kit enabled an unprecedented period of evolutionary experimentation -- and competition.

Many forms seen in the fossil record of the Cambrian disappeared without trace. Future evolutionary change was then limited to acting on the body plans that remained in existence.

Recently many scientists have begun to question whether the Cambrian explosion was a real event, or a reflection of the patchiness of this ancient fossil record. Genetic data suggest that multicellular animals evolved around 1000 million years ago; this is supported by fossil embryos from rocks in China that date back 600 million years. In addition, trilobites were a very diverse group even early in the Cambrian, and some scientists suggest that this indicates that the arthropod group must have had a much earlier evolutionary origin.

New Zealand Example evolutionary evidence from the Cambrian Period:

During the Cambrian, evolution was rapid and within a few million years the Earth was populated with many animal groups. Fossils found elsewhere indicate that the marine ancestors of New Zealand's ancient land dwelling caterpillar-like Peripatus were alive at that time. Cambrian deposits in the Cobb Valley, in north-west Nelson of the South Island are the oldest accurately dated geological formations in New Zealand. Some associated sequences occur in small areas in the southwest of Fiordland. The fauna typically consist of Trilobites, Brachiopods, Sponges and Ostracods. Be aware that the rocks in the Cobb Valley are protected and specimens cannot be removed.

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The Rise of the fish - The Ordovician Period (505 m.y.-440 m.y.)

The time between 505 and 440 million years ago is called the Ordovician. It is named after a Celtic tribe called the Ordovices. At this time, the area north of the tropics was almost entirely ocean, and most of the world's land was collected into the southern super-continent Gondwana. Throughout the Ordovician, Gondwana shifted towards the South Pole and much of it was submerged underwater.

During the Ordovician the first plants appeared. But it was not until the late Silurian before they resembled modern plants. It is widely assumed that the first eukaryotic cells were non-photosynthetic descendants of Archaebacteria. The theory of endosymbiosis proposes that mitochondria (and chloroplasts) were derived from symbiotic, aerobic Eubacteria, and were engulfed by ancestral eukaryotic cells.

The Ordovician is best known for the presence of its diverse marine invertebrates, including graptolites, trilobites, brachiopods, and the conodonts (early vertebrates). A typical marine community consisted of these animals, plus red and green algae, primitive fish, cephalopods, corals, crinoids, and gastropods. A burst of evolution went on to triple the diversity of marine animal life in the space of 50 million years.

Fish are members of the chordate phylum because they display certain defining characteristics: a backbone that replaces the notochord of the 'simpler' chordates, a dorsal nerve, gills and a tail. Agnathans, or jawless fish were the earliest fish and the first true vertebrates and they appeared around 480 million years ago. One of the Agnathan lineages were the Ostracoderms, the earliest jawless fish, dating back around 510 million years. They were bottom-feeders and were almost entirely covered in armour plates. As jaws evolve in the bony fish and early sharks around 450 million years, jawless fish had trouble competing. Hagfish and lampreys are the only jawless fish alive today. While sharks are not plentiful until the Devonian period (410 to 360 million years ago), their fossil scales date the existence of the earliest sharks to the late Ordovician.

With these new groups of Palaeozoic fauna the ocean ecology reorganised and the new species adapted to use resources more efficiently. After the reorganisation of changing lifestyles, species lasted longer and extinction occurred less frequently than among the Cambrian ancestors.

From the Early to Middle Ordovician, the Earth experienced a milder climate in which the weather was warm and the atmosphere contained a lot of moisture. However, when Gondwana finally settled on the South Pole during the Late Ordovician, massive glaciers formed causing shallow seas to drain and sea levels to drop.

It has been estimated that at least 70 percent of oceanic species became extinct at the end of the Ordovician period, in what may have been the second largest of all mass extinctions. Compare this with several mass extinctions which culled early animals during the 50 million years of the Cambrian but from which they bounced back to their previous levels.

New Zealand Example fossils from the Ordovician Period:

During the Ordovician New Zealand was situated about 20 north of the equator. All life on Earth was still in the sea. The sediments that were to become New Zealand formed a shallow sea shelf off the coast of Gondwana. Well defined Ordovician rocks, characterised by Graptolite faunas can be found in the Nelson region and the south west of Fiordland.

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Venturing on land - the Silurian Period (440 m.y. - 410 m.y.)

The Silurian period, named after a Celtic tribe called the Silures, was the time when some plants and animals left the water and colonized the land for the first time. Why they left the water is still being debated but it was probably the result of competition in the marine ecosystems, escaping predators and the availability of new land-based environments. Once animals and plants became established on land they contributed to changes in the nature of the physical and chemical processes on Earth. But living on land required new strategies for survival, such as obtaining nutrients and water, avoiding desiccation, carrying out gas exchange, and reproduction.

Early vascular land plants - so named for their internal system of tubing that circulates water and nutrients - evolved around 425 million years ago. Most grew only a few centimetres tall but were still tall enough to send shoots skyward to capture sunlight and release reproductive spores to the winds. With deeper root systems than earlier plants (rhizoids, not true roots) and a rigid vertical stem, they were now equipped to colonise more of the Earth's surface. An example for one of the simple vascular plants is Cooksonia.

Arthropods were the first animals to adapt to the land, appearing there around about 420 million years ago. Fossil arthropod footprints of arthropods from western Australia, that were made in the sandy flats surrounding temporary lakes, indicate that these animals may have accompanied the landward march of plants. In most ways they were pre-adapted to life on land. By the time they moved ashore, they had already evolved lighter bodies and spindly but strong legs to counteract the force of gravity. Their hard outer shells, called cuticles, provided protection and retained moisture. Spiders, centipedes and mites were among the earliest land variants. Some of them were the giants of their joint-legged kind. The longest kind of its species was Slimonia, a relative of the scorpions, which was the size of a man. This animal was still too big and too heavy and the walking legs too small to venture onto land and probably lived in marginal marine (deltaic) environments.

New Zealand Example fossils from the Silurian Period:

By Silurian times Gondwana had brought New Zealand into the southern Hemisphere. As marine creatures began to adapt to changing temperatures and salinity, they were preparing themselves for life on land. At Hailes Knob, to the west of Motueka, a few Silurian fossils such as shellfish and trilobites have been found. There is a boulder by the roadside before the entrance to the Takaka Gorge, in the Cobb Valley, Nelson, which shows folds of marble and sandstone. It is possible to see a few tiny shellfish and crinoid stems in this rock.

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Invasion of the land - the Devonian Period (410m.y. - 360m.y.)

The Devonian period was named after Devonshire, England, where rocks of this age were first studied. During the Devonian, early arthropods and vertebrates continued to colonise the land. The animals had to solve the same problems that plants faced when they moved to the land, such as reducing water loss and maximising oxygen uptake. The evolutionary advances that solved these problems not only allowed animals to invade land, but also to radiate over the continents.

During the Devonian, there were three major continental masses: North America and Europe sat together near the equator with much of their current land underneath the seas. To the north lay a portion of modern Siberia. A composite continent of South America, Africa, Antarctica, India, and Australia - Gondwanaland - dominated the southern hemisphere.

During the Devonian, two major animal groups dominated the land. The first tetrapods, or land-living  vertebrates, appeared during the Devonian, as did the first terrestrial arthropods, including wingless insects and the earliest arachnids which had already ventured onto land during the Silurian. In the oceans, brachiopods flourished.

One of the first amphibians was Ichthyostega, which lived during the late Devonian in what is now Greenland. Its skull was nearly identical to that of the fish Eusthenopteron and it also retained a deep tail with fins. Ichthyostega had four strong limbs and its head was clearly separated from its body by a neck. Its strong backbone and ribcage helped to support the animal's weight on land.

While arthropods  and vertebrates strived to occupy land, the sea was also teeming with life. One result of this marine struggle for survival was the adaptive radiation of the fish, which were adapting to live in the largest ecosystem on earth, water.

By the Late Devonian plants with true roots and leaves, many of them rather tall, had evolved. Archaeopteris, was a large tree with true wood. In fact it is the oldest such tree known, and produced some of the world's first forests. By the end of the Devonian, the first seed plants had appeared. This rapid appearance of so many plant groups and growth forms has been called the "Devonian Explosion".

New Zealand Example fossils from the Devonian Period: Today there are only two small areas of Devonian rocks to be found in New Zealand, the Baton formation, north-west of Nelson and the Reefton Mudstones and Limestone in the South Island. Devonian corals in the region indicate that the seas were warm. Approximately 6.5 kilometres southeast of Reefton and 1.4 kilometres from Lankey Creek, the main road to Lewis Pass cuts through a dark limestone bluff beside the Inangahua River, where corals can be seen in the limestone. Some Devonian fish have been found in rocks of the Waitahu Valley, near Reefton.

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Reptiles and Conifers - the Carboniferous Period (360m.y. - 286m.y.)

The Carboniferous Period extended from 360 to 286 million years ago. The term "Carboniferous" comes from England, in reference to the rich deposits of coal (carbon) that occur there.

In Carboniferous times, the continents were coming together to form a smaller group of more or less continuous landmasses with connections from Europe to North America, and from Africa to South America, Antarctica and Australia. The collisions of the continents produced the Appalachian mountain belt of eastern North America and the Hercynian Mountains in the United Kingdom. A further collision of Siberia and eastern Europe created the Ural Mountains.

The conditions were ideal for the beginnings of coal and several major biological, geological, and climatic events occurred during this time. One of the greatest evolutionary innovations of the Carboniferous was the amniotic egg which allowed early reptiles to move away from waterside habitats and colonize dry regions. The amniotic egg allowed the ancestors of birds, mammals, and reptiles to reproduce on land by preventing the embryo inside from drying out, so eggs could be laid away from the water. It also meant that in contrast to the amphibians the reptiles lay fewer eggs, they had no larval stage and fertilisation was internal.

Hylonomus and Paleothyris were very early cotylosaurs (primitive reptiles). They were quite little, lizard-sized animals with amphibian-like skulls, shoulder, pelvis, & limbs, and intermediate teeth and vertebrae. The rest of the skeleton was reptilian. Many of these new "reptilian" features are also seen in little amphibians which also sometimes have direct-developing eggs laid on land, so perhaps these features just came along with the small body size of the first reptiles. Here you will find picture of a Hylonomus skeleton. One New Zealand frog has direct-developing eggs. The remaining late Devonian trees were now the most striking aspect of the landscape, with giant club mosses up to 30 or 40 metres tall, horsetails growing up to 15 metres, and equally tall tree ferns. Trees like the conifers still used the massive sporing strategies of marine plants by releasing millions of pollen grains to fertilise the female cone. These wind blown strategy required masses of pollen to achieve results and worked better where trees were massed together without much competition.

New Zealand Example fossils from the Carboniferous Period: The Carboniferous period of Gondwana produced immense coalfields in Australia. New Zealand was still submarine and continued to travel southwards. The seas around New Zealand were very cold, forcing the sea life to adapt or die. The New Zealand seascape, lying off the south eastern coast of Gondwana included a chain of volcanic islands that were very active late in the period. This volcanic activity probably obscured any deposits of sediments of that time, explaining the absence of Carboniferous rocks in New Zealand.

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Pangea - the Permian Period (286m.y.- 248m.y.)

By Permian times, the continents were moving even closer together than during the Carboniferous as the northern and southern supercontinents of Laurasia and Gondwana began to assemble into a single great landmass, called Pangaea. The Permian period was the final period of the Paleozoic era and is named after the province of Perm, Russia, where rocks of this age were first studied.

The global geography of the Permian included massive areas of land and water. Models indicate that the interior regions of this vast continent were probably dry, with great seasonal fluctuations, because of the lack of the moderating effect of nearby bodies of water, and that only some parts of the supercontinent received rainfall throughout the year. The ocean itself still has little known about it. There was extensive glaciation in southern parts of the landmass, shown by glacial striations on Permian rocks from what are now Africa, South America, and Antarctica, and extensive deposits of windblown soil indicate a very dry climate. However, there are indications that the climate of the Earth shifted at this time, and that glaciation decreased, as the interiors of continents became drier.

One of the most striking transitions in the evolution of life occurred when mammals evolved from one lineage of reptiles. This transition began during the Permian, when the reptile group that included Dimetrodon gave rise to the "beast-faced" therapsids. These mammal-like reptiles in turn gave rise to the cynodonts, e.g. Thrinaxodon , of the Triassic period. The development of a key mammalian trait, the presence of only a single bone in the lower jaw (c.f. several in reptiles) can be traced in the fossil history of this lineage. It includes the excellent transitional fossils, Diarthrognathus and Morganucodon , whose lower jaws have both reptilian and mammalian articulations with the upper. However, at the end of the Permian it was the dinosaurs, not the mammal-like reptiles, which took advantage of the newly available niches to diversify into the dominant land vertebrates. Among plants, Lepidodendron and Sigillaria became rare, but ferns and conifers persisted. The widely distributed "seed fern," Glossopteris, which was apparently successful in resisting glacial conditions, was the most conspicuous development in the Permian flora. A single leaf of Glossopteris has been found in New Zealand sediments that formed when our country was still under the sea off the east coast of Australia.

The distinction between the Paleozoic and the Mesozoic is made at the end of the Permian in recognition of the largest mass extinction recorded in the history of life on Earth. It affected many groups of organisms in many different environments, but its greatest affects were felt by marine communities as it caused the extinction of 90-95% of marine species of the time. On land the extinction cleared the way for other forms to dominate, and led to what has been called the "Age of Dinosaurs". Although the cause of the Permian mass extinction remains a debate, numerous possible explanations have been formulated to explain the events of the extinction. Glaciation, the formation of Pangaea and volcanism are some of the theories as well as extraterrestrial causes like an impact with an asteroid.

New Zealand Example fossils from the Permian Period: Permian rocks are widely distributed in New Zealand's South Island and also occur in the Northland region of the North Island, where they are the oldest known rocks. In some places the deep sedimentary marine series that was laid down is up to 20 kilometres thick and one of the most complete Permian sequences preserved anywhere in the world. The biggest build up of volcanic rocks made the Takitimu Mountains near Redcliff, Waiau Valley, where the pile of tuff layers is 14 kilometres thick. Unfortunately Permian fossil outcrops are hard to find, but at Arthurton, near Gore, complete shells of Atomodesma, a bivalve, can be found. And of course a whole leaf of Glossopteris plant has been found at Productus Creek.

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Mammals and dinosaurs - the Triassic Period (248m.y.-213m.y.)

The Triassic period was the earliest period of the three Mesozoic eras (Triassic - Jurassic - Cretaceous).The name Triassic refers to the threefold division of rocks of this age in Germany. Mesozoic means "middle animals", and is the time during which the world fauna changed drastically from that which had been seen in the Paleozoic. Dinosaurs, which are perhaps the most popular organisms of the Mesozoic, evolved in the Triassic, but were not very diverse until the Jurassic. Except for their ancestors the birds, dinosaurs became extinct at the end of the Cretaceous.

In many ways, the Triassic was a time of transition. The world's landmasses were still combined in the supercontinent of Pangaea, altering global climate and ocean circulation. Large areas of the landmass were arid. The Triassic also followed the largest extinction event in the history of life (the extinction at end of the Permian), and so is a time when the survivors of that event spread and recolonized both the land and radiated into the niches left vacant by that event.

The organisms of the Triassic can be considered to belong to one of three groups: those that survived the mass extinction in the late Permian, new groups which flourished briefly, and new groups which went on to dominate the Mesozoic world. The survivors included plants like the lycophytes and glossopterids , and mammal-like reptiles like dicynodonts. While those that went on to dominate the Mesozoic world include modern conifers , cycadeoids , and the dinosaurs . During the Triassic period major changes were taking place in the posture of several groups of reptiles. They were shifting from the standard "sprawling mode" to an "errect" posture. The dinosaurs, or "terrible lizards", fall into two initial groups on the basis of their hip structure: the saurischians and the ornithischian. Saurischians are further subdivided into theropods (such as Coelophysis and Tyrannosaurus rex) and sauropods (e.g. Apatosaurus). Most scientists agree that birds evolved from theropod dinosaurs. The ornithischian dinosaurs include Triceratops, Iguanodon, Hadrosaurus, and Stegosaurus.

Mammals are advanced synapsids. Synapsida is one of two great branches on the amniote family tree. This is the branch that includes us. Synapsids are characterized by having extra openings in the skull behind the eyes; this opening gave the synapsids stronger jaw muscles and jaws (the jaw muscles were anchored to the skull opening) than previous animals. "Pelycosaurs" (non-therapsid synapsids) were once considered reptiles, but we now know that their lineage had separated very early. Pelycosaurs (like Dimetrodon and Edaphosaurus) were early synapsids, they were mammal-like reptiles. Later synapsids include the therapsids and the cynodonts . The cynodonts led to the true mammals. Over time, the synapsid way of walking became more upright and tail length decreased.

New Zealand Example fossils from the Triassic Period: New Zealand was still mainly a marine environment, but parts began to rise out of the sea, while volcanic activity continued. Some of the ancient forms of kauri, rimu, totara and kahikatea trees colonised the land. In the rocky shore platform at Kiritehere Beach on the Waikato west coast, beds of a scallop-like bivalve, Monotis and prehistoric mussels can be found. No terrestrial vertebrates were preserved in the sediments, but Ichthyosaurus, a marine reptile, has been found in the Mt Potts region in the South Island. And at Nothosaur Stream, Mt Harper, Canterbury, a Nothosaurus, a primitive amphibious sea lizard, has been found.

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Dinosaurs and birds - the Jurassic Period (213 m.y. - 145 m.y.)

The Jurassic period was the middle period of the Mesozoic era, spanning the time between 213 and 145 million years ago. It is named after the Jura Mountains between France and Switzerland, where rocks of this age were first studied. The Jurassic has become a household word with movies like Jurassic Park. But outside of Hollywood, the Jurassic is still important to us today, both because of its wealth of fossils and because of its economic importance -- the oilfields of the North Sea, for example, are Jurassic in age.

About 200 million years ago, continental drift, driven by upwelling heat from deep within the Earth, caused the supercontinent Pangaea to break up. The first division was into northern Laurasia and southern Gondwanaland, and rift valleys appeared where the continents separated. Over the following 160 million years, fragments repositioned themselves as today's continents. The opposing coastlines of South America and Africa are a good example to show where these two continents were once joined when they were part of Pangaea.

Studies of oxygen isotopes, the extent of land flora, and marine fossils indicate that climates during Jurassic times were mild, perhaps 8C warmer than those of today. No glaciers existed during this period. The plant life of the Jurassic was dominated by the cycads, but conifers, ginkgoes, horsetails, and ferns were also abundant. Creeping about in this foliage were a number of early mammals, no bigger than rats. Of the marine invertebrates, the most important were the ammonites. The dominant animals on land, in the sea, and in the air were the reptiles. Dinosaurs, more numerous and more extraordinary than those of the Triassic period, were the chief land animals; crocodiles, ichthyosaurs, and plesiosaurs ruled the sea, while the air was inhabited by the pterosaurs, the flying relatives of the dinosaurs.

A particularly important discovery is Archaeopteryx lithographica, found in the Jurassic Solnhofen Limestone of southern Germany, which is marked by rare but exceptionally well-preserved fossils. It has long been accepted that Archaeopteryx was a transitional form between birds and reptiles, and that it was the earliest known bird. Lately, scientists have realised that it bears even more resemblance to its ancestors, the Maniraptora (a group of dinosaurs), than to modern birds; providing a strong phylogenetic link between the two groups. A recent discoveries of a variety of early birds and feathered dinosaurs in northeast China provide support for the theory that theropods evolved feathers for warmth before birds used them in flight. Archaeopteryx is one of the most important fossils ever discovered.

New Zealand Example fossils from the Jurassic Period: In the Jurassic sediments that had accumulated offshore between the east of Antarctica and the west of Australia were squeezed together by several tectonic plates. They were consolidated and pressured into huge folds and uplifted and formed a New Zealand microcontinent called Tasmantis. It is likely that plants and animals travelled freely across the land and the subsequent isolation of New Zealand suggests that the archaic frogs, large land snails, tuatara and peripatus are all living fossils that stem back from those times. An ancestor to the New Zealand weta has been found in late Jurassic rocks at Port Waikato. The first late Jurassic dinosaur was found in the Huriwai Plant Beds. The bone, that is similar in shape and size to that of a Compsognathus, is very rare but indicates that dinosaurs were living around New Zealand.

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The final season of the dinosaurs - The Cretaceaous Period (145m.y.-65m.y.)

The last episode of the Mesozoic era was the Cretaceous period. The name is derived from the Latin word for chalk ("creta") and was first given to the extensive chalk deposits of this age that form white cliffs along the English Channel between Great Britain and France. This period lasted longer than the whole of the succeeding and as yet unfinished era, the Cenozoic. It was a time when many of the typically Mesozoic lifeforms - ammonites, belemnites, gymnosperms, ichtyosaurs, plesiosaurs, pterosaurs and dinosaurs - were in decline. But all of these groups radiated and diversified during some or all of their time and towards the end of the late Cretaceous they showed a variety of patterns of extinction.

Few changes have affected the landscape and ecology of the Earth more than the arrival of angiosperms, or flowering plants, approximately 130 million years ago. Flowering plants, which include hardwood trees and grasses, are distinguished from other plants by the flowers they produce. While some are wind pollinated, most use colour, scent or both to attract insects (and pollen collectors). Nectar may have evolved as a reward for performing this function. Animals deliver pollen more efficiently than wind, so plants that attract them improve their chances of reproducing. Angiosperms are not the only plants to evolve animal pollination, however as some cycads are pollinated by insects such as beetles. The origin of flowering plants during the early Cretaceous seems to have triggered a second great radiation of insects; new groups, such as butterflies, moths, ants and bees arose and flourished. These insects drank the nectar from the flowers and developed in the case of the ants and bees highly complex colonial structures.

The mass extinction at the end of the Cretaceous period, 65 million years ago wiped out the dinosaurs along with every other land animal that weighed much more than 25 kg. However it was less catastrophic than the previous mass extinction at the end of the Permian period yet it has attracted more research than any of the other extinction events. No one can say for sure just why the dinosaurs died out. The recent research interests were triggered by a remarkable article published in the journal Science in 1980 by Luis Alvarez and collaborators that said that the Earth had been struck by an asteroid, 10 km in diameter 65 million years ago. Their evidence came from an iridium spike - a rare element in the Earth's crust that is only present from meteor showers - and the same phenomenon has been demonstrated at many more K-T sections worldwide. This theory has been supported by a crater that was found in 1991on the Yukatan Peninsula in Central America. An alternative suggested cause for the K-T event is that the iridium spikes were caused by massive outpouring of lava which were occurring in the Deccan Traps, India.

New Zealand Example fossils from the Cretaceous Period: About 120 million years ago waterways developed between the edge of Gondwana and the new uplifted Tasmantis including the piece that became New Zealand. Because of the increasing distance that separated ancestral New Zealand from Antarctica the biota it took was purely Cretaceous which subsequently experienced an evolution-in-isolation. Some of the Cretaceous concretions contain bones of marine reptiles. In inland Hawkes Bay, enough bones have been collected to identify a new species of mosasaur. The dinosaur fauna, of New Zealand was discovered through the efforts of Joan and Pont Wiffen, of Hawkes Bay. Also a single vertebra was found to be that of an upright carnivorous land dinosaur. At Oaro, south of Kaikoura, the sea is eroding late Cretaceous rocks just north of Amuri (or Haumuri) Bluff. They contain layers of belemnites, bones and shark teeth. Examples of the Cretaceous-Tertiary boundary can be seen in the rocks of the Te Uri Stream in Hawkes Bay and in Waipara, North Canterbury as well as south of Chancet Rocks at Woodside Creek. However in all those three locations the passage of time is shown by microscopic fossils, mostly Foraminifera which are intercepted by a layer of clay that contains Iridium.

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Archaic mammals and early primates - the Palaeocene Epoch (65m.y.- 55.5m.y.)

The Palaeocene was the earliest epoch of the Tertiary period, spanning the time between 65 and 55.5 million years ago. It is named after the Greek words "palaois" (old) and "ceno" (new), indicating the presence of new fauna and flora associated with the old ones from the Cretaceous. The world at that time was a much more equable place than it is today, with a tropical or subtropical type of climate reaching to the Polar Regions. Patterns of rainfall may have changed dramatically after the extinction of the dinosaurs, with much higher levels spread more evenly through the seasons of the year.

The Palaeocene is a crucial time in the history of mammals, it was a world without dinosaurs. Unfortunately, mammal fossils from this epoch are either scarce or entirely unknown in many parts of the world. Thus we can only speculate how the fauna of whole continents looked after the extinction of dinosaurs. Even where fossils occur, most species are only known from their characteristic teeth, and entire skeletons are only known for a few forms.

Mammals appeared first in the late Triassic, at about the same time as dinosaurs. Throughout the Mesozoic, most mammals were small, fed on insects and lead a nocturnal life, whereas dinosaurs were the dominant forms of life on land. After the abrupt changes about 65 million years ago, when dinosaurs disappeared with the exception of their descendants, the birds, the world was practically without larger sized terrestrial animals. This unique situation was the starting point for the great evolutionary success of the mammals. Only ten million years later, at the end of the Palaeocene, they had occupied a large part of the vacant ecological niches, often competing with giant carnivorous birds, especially in South America. By this time, the landscape was teeming with small insectivorous and rodent-like mammals, medium sized mammals were searching the forests for any kind of food they could cope with, the first large (but not yet gigantic) mammals were browsing on the abundant vegetation, and carnivorous mammals were stalking their prey.

This first flush of mammal radiations in the Palaeocene contained mainly groups that are termed "archaic" because they were not the direct ancestors of any surviving animal group. These mammals were still on a primitive level of anatomy in comparison to mammals of today. Often they showed only the first beginning of specializations that characterize their descendants from later epochs, such as optimization of the teeth for a special kind of food or adaptations of the limbs to fast running. Archaic conditions require archaic designs and the replacement of those early designs by mammals that we consider now "modern" reflects the changes produced by a later, more seasonal world.

Where and when the first primates - the group to which we belong - appeared remains uncertain, but the oldest confirmed primate fossils date to about 60 million years ago. It is widely agreed that primates emerged from archaic terrestrial and nocturnal insectivores (shrew-like animals) with early primates resembling lemurs or tarsiers and probably lived in trees in tropical or subtropical areas. Many of their characteristic features are well suited for this habitat: hands specialised for grasping, with five digits and, in most primates, opposable thumbs, rotating shoulder joints and stereoscopic (three dimensional) vision. Other traits include a large brain cavity and nails instead of claws. Modern primates range from prosimians such as the pygmy mouse lemur, through the monkeys, to anthropoid apes such as the gorilla- and humans.

New Zealand Example fossils from the Paleocene Epoch: During the early Palaeocene the seafloor spreading in the Tasman Sea had stopped. Much of the New Zealand landscape eroded away and sank. This happened probably because New Zealand's now separated continental crust was thinner (about 26 km) than that of the Gondwana landmass (about 37 km). New Zealand fossils from the Palaeocene include the marker fossil Conchorthyra, an ancestor of the ostrich foot shell which survived throughout the K-T event. Another survivor of the K-T boundary were turtles which lineage goes back 230 million years. Fossil turtles bones have been found in New Zealand from Cretaceous up until Miocene rocks.

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Whales and horses - The Eocene Epoch (55.5 m.y. - 33.7m.y.)

The epoch after the Palaeocene is called the Eocene. Its name derives from the Greek words "eos" (dawn) and "ceno" (new), i.e., the dawning of new fossil forms. Towards the end of the Palaeocene and until about 50 million years ago in the early Eocene, the global climate grew notably warmer. The range of the tropical type of vegetation expanded, pushing the tropical rainforest inside the Arctic Circle to create jungles at the poles. Many of our present day fauna made their first appearance in the early Eocene, among them true primates and even- and odd- toed hoofed mammals (ungulates). By the latest Eocene, ice began to accumulate in Antarctica, and this began the latest of the Earth's ice ages. The Earth has been in an ice age ever since - glacial and interglacials represent waxing and waning of the amount of ice, but not the complete removal of the ice.

The oldest known fossils of most of the modern orders of mammals appear in a brief period during the Early Eocene and all were small, under 10 kg. Both groups of modern ungulates (hoofed animals), the Artiodactyla and the Perissodactyla became prevalent mammals at this time, due to a major radiation between Europe and North America. Horses began as small, four-toed woodland animals (Hyracotherium aka Eohippus) and underwent considerable radiation to end as big one-toed grassland gallopers. Horses evolved in North America & colonised Europe in successive waves, only to become extinct in America prior to human arrival, and later reintroduced with the Spanish conquistadors. Most perissodactyl lineages went extinct in the late Eocene or Oligocene. Those that remained include the horses and zebras (Equidae; eight living species), rhinos (Rhinocerotidae; five living species), and tapirs (Tapiridae; four living species). Most of the species that remain-notably, all five living species of rhinoceros-are in danger of extinction; others, like the Quagga, have already been driven to extinction.

Whales are one of evolution's great enigmas: after life went to all the trouble of adapting to dry land, some mammals decided life was better in the water after all. Most of the fossil evidence suggests that the distant ancestors of whales were mesonychids, which underwent a radical change of habitat. Mesonychids were hoofed, hyena-like, land-dwelling mammals, the size of wolves, but had skull the size of bear skulls. They had four short legs, big feet, and 5-8 cm notched, triangular teeth similar to those of early predatory whales. Another early ancestor of the whale, Ambulocetus, may have evolved from mesonychids. Ambulocetus natans, meaning "walking whale that swims" was discovered in 1993 and showed back feet bigger than those in the front. Although Ambulocetus was still clearly a tetrapod, its ear capsule was isolated from the rest of its skull - just like that of a modern whale. With powerful jaws and shark-like teeth, a small brain, and a pelvis fused to its backbone. But going back to the oceans required many adaptations for living in the water, including a backwards and upwards shift of the nostrils, coverings for the nostrils, a streamlined shape, loss of the rear limbs, change of the forelimbs into flippers, addition of flukes for swimming, modification of senses for use in the water, loss of most hair, and addition of a layer of insulating blubber. The Archaeoceti were the first primitive whales to appear. They had tiny heads and pointed snouts with teeth. Basilosaurus was one of the most common primitive whales from the Archeoceti group.

New Zealand Example fossils from the Eocene Epoch: At the beginning of the Eocene, New Zealand and New Caledonia were remotely connected by a series of islands, sharing plant and animal life from their joint Gondwana heritage. New Zealand continued to erode and sink while drifting northwards into a warmer climate. This is the time when the coalfield of Huntley and the natural gas and oil fields of southern Taranaki were formed. Leaves of pohutukawa and rata, manuka, kanuka and eucalyptus have been found in Eocene coal deposits as well as fossil pollen of Seaforthia the ancestor of the nikau palm tree. New Zealand's ancient whales such as Basilosaurus were long slender giants whose vertebrae and teeth are occasionally found in Eocene limestones of South Canterbury.

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Grasses and primates - the Oligocene Epoch (33.7m.y. - 23.8m.y.)

The Oligocene epoch of the early Tertiary period, spanned the time between 33.7 and 23.8 million years ago and is named after the Greek words "oligos" (little, few) and "ceno" (new) indicating that there were only a few new fossil types. The Oligocene is thus a relatively short span of time, though a number of changes occurred during this time. These include the appearance of the first elephants with trunks and the appearance of many grasses -- plants that would produce vast tracts of grasslands in the following epoch, the Miocene.

The transition from Eocene to Oligocene is characterised by major changes. The global climate changed from generally wet and tropical to more seasonal, drier and subtropical. These were the first hints of the Tertiary cooling trend. In North America and Europe, the Oligocene was mainly an erosional episode, after the major mountain-formation events of the Eocene. In Asia, the Indian plate collided with the Eurasian plate during the Middle Oligocene and the first mountain-formation events of Himalayan cycle began. These events had probably a serious effect to Middle and East Asian Oligocene environments. New Zealand in the Oligocene was heavily eroded and submerged, to the extent that 2/3 of modern NZ would have been covered by sea. This is the time when Antarctica began to accumulate large volumes of ice that caused the climatic cooling.

At the start of the Oligocene the world was growing rapidly cooler and more seasonal. Waves of extinction overtook the mammals that had been better suited to the more tropical world of the earlier Eocene. By the early Oligocene the polar broad leaved deciduous forests were gone and Antarctica was ice capped. In the oceans, some of the marine biotic provinces became more fragmented as marine life capable of withstanding cooler temperatures congregated to places further from the warmer equator, where other species could better survive. The cooling trend was also responsible for the reduced diversity in marine plankton, the foundation of the food chain. By the mid-Oligocene there was a worldwide marine regression (due to rapid ice accumulation in Antarctica), marked by a decline in the total number of marine species, including many planktonic and invertebrate species.Marine mammals, like the primitive (archaeocete) whales go extinct and are replaced by their modern relatives. The planktic and invertebrate life forms were affected as well.

The now much cooler and drier climate favoured the evolution of grasses, which became one of the most important groups of plant organisms on the planet. As they spread extensively over several million years they fed herds of grazing animals gave shelter to smaller animals and birds, stabilised the soil and with that reduced erosion. They are high fibre, low protein plants and must be eaten in large quantities to provide adequate nutrition. But because they contain tiny silica fragments they wear animal teeth down. This in turn drove the evolution of grazing animals with teeth adapted to cope with such a diet e.g. the more recent horses such as Merohippus. Over several million years, as the grasses spread, they fed herds of grazing animals, gave shelter to smaller animals and birds, and stabilised the soil, reducing erosion. Other than flowering plants grasses do not rely on animal pollination but make use of the wind.

In Western Europe, a sudden change in the fauna, known as the Grand Coupure, occurred. The Grand Coupure refers to the time near the end of the Eocene when many faunal groups, including primates, became extinct in the Northern Hemisphere and involved the immigration from areas to the east of many new taxa, Artiodactyla and Perissodactyla in particular and the extinction of many Eocene genera and species. During this time at least 17 generic extinctions, 20 first appearances, and 25 unaffected genera of mammals are represented across the Eocene-Oligocene boundary in Western Europe. The late Oligocene period, marked by the expansion of grasslands saw domination on land by mammals such as horses, deer, camel, elephants, cats, dogs, and primates. The continuation of land mammal faunal migration between Asia and North America was responsible for the dispersion of several lineages onto new continents, except in Australia.

Some DNA evidence suggests that the ancestors of modern apes - and humans - evolved between 22 and 33 million years ago, but abundant fossils do not appear until the Miocene. Chimps, gorillas and orang-utans (the great apes) and gibbons and siamangs (the lesser apes) are classified with humans in the taxon Hominoidea.

New Zealand Example fossils from the Oligocene Epoch: Two thirds of modern day New Zealand were submerged during the Oligocene, the movement of the tectonic plates in the north of New Zealand caused big areas of oceanic crust to be submerged. The little land that was left during the Oligocene was home to a decreasing number of species. Many died out but some snails, peripatus, frogs, tuatara and ratites survived. A similar scenario happened to the plants on land, and here the warmth loving beech trees became dominant. Large sea urchins, giant oysters, crayfish, molluscs and giant sharks (up to 13 metres) thrived in the shallow seas. The fossilised shells of giant oysters can be found in limestones around the Te Kuiti district.

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Kelp forests and horses - the Miocene Epoch (23.8 m.y. - 5.3m.y.)

The Miocene was named from the Greek words "meion" (less) and "ceno" (new). During this period there were less new fossil forms than during the following epoch, the Pliocene. It was a time of generally warmer global climates than those in the preceding Oligocene, or the following Pliocene. By the Miocene the Drake's Passage opened up between Antarctica and South America, as had the passage between Tasmania and Antarctica, making way for a circumpolar current of cold water. This significantly reduced the mixing of warmer tropical water and cold polar water, and permitted the further build up of the Antarctic polar cap.

The shrinkage of shallow inland seas over the continents, e.g. the Tethys Sea, which was closed off by a land bridge between Africa and Eurasia, damming the Mediterranean Sea, was a further influence on the world's changing climate. With more landmass exposed there was less sea to buffer the global climate from extreme heat or cold.

Communities of large brown algae, called kelp supported evolving marine life, such as sea otters, as well as established groups of fishes and invertebrates. Tough kelp is a plant, it is not closely related to its land counterparts. Its cells use different pigments for photosynthesis. Kelp grows in cool shallow waters where it attaches to rocks and coral or sometimes floats freely. Because marine plants do not preserve well over time, scientists can date kelp only to the Miocene, when animals it supports are known to appear but it may have existed in earlier periods.

Plant studies of the Miocene have focussed primarily on spores and pollen. Such studies show that by the end of the Miocene 95% of modern seed plant families existed, and that no such families have gone extinct since the middle of the Miocene. A mid-Miocene warming, followed by a cooling is considered responsible for the retreat of tropical ecosystems, the expansion of northern coniferous forests, and increased seasonality. With this change came the diversification of the modern graminoids, especially grasses and sedges. The overall pattern of biological change for the Miocene is one of expanding open vegetation systems (such as deserts, tundra, and grasslands) at the expense of diminishing closed vegetation (such as forests). Mammals and birds in particular developed new forms, whether as fast-running herbivores, large predatory mammals and birds, or small quick birds and rodents. Horses first appeared in the early Eocene as cat sized herbivores, feeding on leafy vegetation. As coarse grasses replace the woodlands during the Oligocene some species evolve larger jaws and deep rooted teeth with protective enamel. They also evolve larger guts, to be able to digest the large quantities of grass. The grazing Oligocene horses are now larger with longer legs and hooves that enable them to run faster than those with padded feet. They quickly spread from North America to Europe and Asia and from there to Africa where some species become today's horses.

New Zealand Example fossils from the Miocene Epoch: Around the mid-latitudes of the Southern Hemisphere prevailing westerly winds established during the Miocene. These winds, and the associated ocean currents, aided the transfer of Australian plants and animals across the Tasman Sea, and have been very important in bringing South American taxa to Australasia e.g. kowhai (Sophora) has salt-resistant seeds and colonized New Zealand from Chile. The long-tailed bat reached New Zealand from Australia in the Miocene, as did the geckos and the ancestor of the takahe, which later evolved both gigantism and flightlessness as adaptations to island life. In the seas, the giant crab Tumidocarcinus giganteus , five times the size than the modern purple rock crab, flourished. Fossilised crabs have been found near Taumaranui in the North Island and Motunau and Glenafric in the South Island.

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The first hominids - the Pliocene Epoch (5.3 m.y. - 1.8 m.y.)

The final epoch of the Tertiary period is called the Pliocene epoch and is named after the Greek words "pleion" (more) and "ceno" (new) meaning that there were more new fossil forms than previous epochs. The gradual cooling that began in the Eocene continued through the Miocene and into the Pliocene; it represents the final stages of a global cooling trend that led up to the Quaternary glaciations. While the Pliocene world was still rather warmer than at present, by about 2 million years extensive ice sheets covered both poles, and during the Pleistocene glaciers repeatedly advanced and retreated over large areas of the globe. The Panamanian land-bridge between North and South America appeared during the Pliocene , allowing migrations of plants and animals into new habitats. This had a substantial effect on the biota of both continents, as placental mammals spread south across the land bridge and marsupials migrated north.

The primates continued to diversify. Humans and chimpanzees shared their last common ancestor around 7 million years ago, and have since followed separate evolutionary paths. We share about 98.8% of our DNA with chimpanzees, which are thus our closest relatives amongst the primates. The first known hominids or humanlike primates evolve in eastern Africa about 5.2 million years ago. Hominids feature prominent jaws and most species have large brains relative to those of apes. Most hominids probably lived in groups either in or near forests and some later species made and used tools. The oldest fossils, a jawbone teeth and a toe bone found in Ethiopia, date to 5.3 million years. A younger near complete hominid skeleton named Lucy by its discoverers and a set of remarkably preserved footprints in Hadar, Tanzania revealed more about their appearance and one of their most distinctive traits: even the earliest hominids could walk upright on two legs. This adaptation afforded certain advantages such as the ability to see over the top of high vegetation and to easily carry food or tools and weapons while traveling.

New Zealand Example fossils from the Pliocene Epoch: New Zealand's geography changed rapidly during the Pliocene. There was widespread volcanic activity, particularly in the North Island (although Banks Peninsula and Port Chambers, both volcanic calderas, formed at this time), and both the Southern Alps and the Kaikoura Ranges began to form. This was important in the evolution of New Zealand's plants and animals as it split the South Island longitudinally into a wetter western side and a drier, flatter eastern side. The late Pliocene was also a period of rather high sea levels, so that NZ was divided into a series of islands. The consequent isolation of species populations that could then diverge into separate species or subspecies accounts for some of the biodiversity and also many of the distribution patterns of our modern biota. The Hebe family made its first appearance during the Pliocene while many warmth loving plants were extinct. Fossils of giant spider crabs have been found in the Wanganui River, Cape Kidnappers and in rocks of inland central Hawke's Bay. These animals preferred deep, cold water and are an indication that New Zealand's climate was moving towards a glaciation.

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Glaciation, the Moa and Homo sapiens - the Pleistocene Epoch (1.8 m.y. - 10,000 y.)

The Pleistocene was the penultimate epoch of the Quaternary period, spanning the time between 1.8 million years ago up to the beginning of the Holocene at 10,000 years ago. It is named after the Greek words "pleistos" meaning "most" and "ceno" meaning "new" fossil forms.

By the start of the Pleistocene, the world had entered a cooler period of alternating glacial and interglacial phases. The Northern Hemisphere showed arctic vegetation: the tundra inside the Arctic Circle and taiga - coniferous evergreen forest - in a band below it. Tundra is a world of permanently frozen soil, permafrost, with a very short growing season for plants that are mainly mosses, lichens and sedges. In lower latitudes the drier climate brought desert type vegetation. Vast sheets of glacial ice covered and then partly uncovered the Earth's higher latitudes, particularly in the Northern Hemisphere. With a large landmass close to the Arctic and the possibility to channel glaciers southwards the Northern Hemisphere became a big icing machinery. The Antarctic, while equally cold, was separated from the southern continents and therefore southern glaciations in New Zealand, Chile and Tasmania mainly took the form of valley glaciers and small ice caps.

The exact timing of the onset of mid-latitude Northern Hemisphere glaciation is uncertain, but some oxygen isotope records suggest a date towards the end of the Pliocene (about 3 to 2 million years ago). Variations in planktic microfossil abundances indicate that large changes in sea surface temperature were occurring prior to 2.4 million years whilst ice-rafted debris from the Norwegian Sea has been dated at 2.8 to 2.6 million years ago. Interestingly, recent discoveries of ice-rafted debris in marine sediments from around Greenland have been dated to about 7 million years ago and this may indicate more extensive Northern Hemisphere ice accumulation during the Miocene.

Many palaeontologists study Pleistocene fossils in order to understand the climates of the past. The Pleistocene was not only a time during which climates and temperatures shifted dramatically; Pleistocene fossils are often abundant, well-preserved, and can be dated very precisely. The Pleistocene fauna included giant marsupials, such as the rhino-sized wombat-related Diprotodon , and the giant monitor lizard Megalania. New Zealand's giant flightless bird Dinornis maximus or Moa was herbivorous and reached a height of 3 metres. The Pleistocene also saw the evolution and expansion of our own species, Homo sapiens, and by the close of the Pleistocene, humans had spread through most of the world. A fossil jaw found in Mauer, Germany, of Homo heidelbergensis dates these early humans to approximately 500,000 years ago. They display physical characteristics of modern humans, with an increased brain capacity, smaller teeth and a face that slopes less than that of other hominid ancestors. About 130,000 years ago modern humans (Homo sapiens) disperse throughout Africa, the Middle East and Europe. They were characterised by a more gracile skeleton, and higher, domed skull than their European contemporaries, the Neanderthals. Cave paintings suggest that by 40,000 years ago they had developed a sophisticated culture; some authors equate this to the appearance of complex spoken language.

New Zealand Example fossils from the Pleistocene Epoch: Throughout the Pleistocene there were about twenty cycles of cold glacial and warm interglacial periods at intervals of about 100,000 years. During the glacial times glaciers dominated the landscape, snow and ice extended into the lowlands, transporting huge quantities of rock with them. During these periods the South Island was extensively glaciated, and there were small glaciers on the Tararua Ranges and Central Plateau. Because a lot of water was locked up in ice, the sea levels dropped during the glacials (by up to 135m lower than at present),. Extensive land bridges joined the main and many offshore islands, allowing the migration of plants and animals. During the warmer periods large areas became submerged again under water. These repeated episodes of environmental fragmentation drove rapid adaptive radiation in many NZ species, especially (but not exclusively) the alpine plants. Coastal areas such as Hawke's Bay, Bay of Plenty, Wairarapa, Wanganui, Marlborough and North Canterbury have only been pushed up to become land in the last 50,000 years. Rich deposits of beautifully preserved Pleistocene sea shell fossils can be found from Te Piki (East Cape), Te Mata Peak, Cape Kidnappers and Castlecliff (Hawke's Bay), Castlepoint (Wairarapa Coast), Hawera and Wanganui (Castlecliff), Motunau Beach (Marlborough) and at Titarangi, (Chatham Islands). They are evidence for shallow, sandy bottom seas. The cliffs of the Rangitikei River represent layers of Plio- and Pleistocene rocks, uplifted sea floors that have been eroded by the river exposing many shell and mollusc fossils.

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The Age of Humans - the Holocene Epoch (8,000 years - present)

The final epoch of the Quaternary period, spanning the time from the end of the Pleistocene (10,000 years ago) to the present is the Holocene epoch, named after the Greek words "holos" (entire) and "ceno" (new), indicating it contains entirely new fossil assemblages.

Ice core evidence suggests the Holocene epoch has been a relatively warm and has endured only small scale climate shifts. One of these, known as the Little Ice Age begins about 650 years ago (1350 AD) and lasted only about 550 years. Some scientists think that the current warm global climatic conditions may be only temporary - that we are simply in an interglacial period of a continuing Ice Age.

The Holocene is sometimes called the "Age of Man." This is somewhat misleading: modern humans evolved and spread over the planet well before the Holocene began. However, since the rise of the first civilizations - perhaps 12,000 years ago - humans have influenced the global environment in a manner quite unlike that of any other organism.

While all organisms influence their environments to some degree, few have ever changed the globe as much, or as fast, as our species is doing. Some estimates indicate that as many as 20 percent of all plant and animal species present today will be extinct by the year 2025. More information is needed to determine whether the current and expected levels of extinction are in line within the natural background levels of species replacement, or whether these have been accelerated by human practices such as hunting, pollution, flood control and deforestation into what has been described as the sixth major mass extinction event. In addition, the vast majority of scientists agree that human activity is responsible for "global warming," an observed increase in mean global temperatures that is still going on and which may have totally unpredictable effects.

Yet the Holocene has also seen the great development of human knowledge and technology, which can be used -- and are being used -- to understand the changes that we see, to predict their effects, and to stop or ameliorate the damage they may do to the Earth and to us. Palaeontologists are part of this effort to understand global change. Since many fossils provide data on climates and environments of the past, palaeontologists are contributing to our understanding of how future environmental change will affect the Earth's life.

New Zealand Example fossils from the Holocene Epoch: The Holocene period represents the last 10,000 years, a very short time for fossils to form. Holocene fossils are often modern species that have fallen down holes into underground caves and been preserved. Extinct species like the Moa, the giant eagle, a native goose, reptiles, frogs and bats have all been found in Holocene limestone caves. Fossil bones of the king shag, Leucocarbo carunculatus, are reported from the late Holocene dune deposits of Tokerau Beach, Doubtless Bay, Northland.


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