The earliest fossils we have are of prokaryotes - a major taxonomic grouping that includes both bacteria and members of the Archaea (things like blue-green algae, aka cyanobacteria). And like modern prokaryotes, those early life-forms were tiny. Most of us are far more familiar with some of the eukaryotes, and perhaps a major reason for this is that we can see them: they are orders of magnitude bigger than microbes. And an interesting question is: what sort of trajectory took some forms of life from the tiny to the ginormous? Was there a smooth upward trend in the maximum size of living things? Or did things progress like a learner driver - by bunny-hops?
Well, what you see when you look at the fossil record is not a smooth upwards progression. Instead there are two points where the maximum size of organisms increases rapidly, both followed by periods of relative stasis. This is something I've talked about in my first-year lectures for quite a while, but it's been documented very thoroughly in a recent paper (Payne et al. 2009). Here's the relevant figure from their article.
You can see from this that there was a big jump in maximum size about 2 billion years ago, & then another leap about a billion years later. What was going on?
Well, part of the answer's given in the figure's legend: those marked increases in size happened around the time when atmospheric oxygen levels increased as a result of photosynthesis, an environmental change that supported more active metabolisms and bigger living things.
The first jump marks the appearance of eukaryote fossils. Remember that one of the characteristics of eukaryotes is the presence of mitochondria, organelles that carry out aerobic respiration (a process that releases more chemical energy than the anaerobic respiration typical of most prokaryotes). All the evidence points to mitochondria having originated by the process of endosymbiosis: they were once free-living aerobic bacteria that entered into a symbiotic relationship within other prokaryote cells, and over time lost the ability to survive outside their hosts, This was an ancient event indeed - but a significant one, because when the levels of available oxygen had reached a certain level, these new hybrid organisms, the early eukaryotes, were at an advantage. They could generate more ATP than their prokaryote competitors, & some of the extra energy made available by aerobic respiration supported an increase in cell size and complexity.
Maximum body size leapt up again a billion years later, after another surge in atmospheric oxygen levels. This timeframe sees the appearance of Ediacaran and Cambrian organisms - multicellular organisms. Some of those evolutionary experiments - the Ediacarans - died out, but many of the others are still with us today. And the evolution of multicellularity opened a new door for upward changes in body size: a single cell can be only so big before it encounters punishing difficulties with obtaining food & oxygen and disposing of waste. This is because single-celled organisms must carry out these processes across their body surfaces. But relative to volume, surface area increases more slowly as organisms get bigger, & this places an upper limit on the size of single-celled organisms. (Ostrich eggs aside, one of the largest single-celled organisms I can think of is the alga Caulerpa, which can be several metres long with hundreds of large flat fronds. It can do this because, while long, the cell is also broad & extremely thin, giving a high surface area: volume ratio.) Whereas with multicellular organisms we see the evolution of guts, gas exchange surfaces, circulatory and excretory systems.
And there hasn't been a lot of movement since then. We might like to think of ourselves as large animals, who share the Earth with even larger animals (blue whales, anyone?) and plants (such as the sequoia), & are much larger (& more sophisticated ;-) ) than those Cambrian critters. Yet that second growth spurt levels off about 450 million years ago, when the largest things around were big cephalopods (with shells up to 5m long!). As Payne et al. comment, [the] maximum size of animals has increased by only 1.5 orders of magnitude since the Ordovician; the giant sauropods of the Mesozoic and even the extant blue whale add comparatively little to the size range of animals. The largest living individual organism, the giant sequoia, is only 3 orders of magnitude bigger than the largest Ordovician cephalopod and one and a half orders of magnitude bigger than the blue whale.
J.L. Payne, A.G. Boyer, J.H. Brown, S. Finnegan, M. Kowalewski, R.A. Krause Jnr, S.K. Lyons, C.R. McClain, D.W. McShea, P.M. Novack-Gottshall, F.A. Smith, J.A. Stempien & S.C. Wang (2009) Two-phase increase in the maximum size of life over 3.5 billion years reflects biological innovation and environmental opportunity. Proceedings of the National Academy of Sciences 106(1): 24-27 doi/10.1073/pnas.0806314106
And if you're interested in the evolution & significance of mitochondria, you can't go past Nick Lane's excellent book:
N. Lane (2005) Power, sex, suicide: mitochondria and the meaning of life. Oxford.