You learn something new every day.
One of the big talking points in palaeontology is the 'Cambrian explosion' - the seemingly rapid appearance (over 'just' a few million years!) of complex animal life, which occurred around 490-540 million years ago. Discussion ranges over the causes of this diversification and whether the apparent 'explosion' really happened at all.
We do know that, over those few million years (the blink of an eye on the geological time scale), a large number of animal groups become visible in the fossil record. And this visibility is largely due to the presence of hard body parts - teeth, shells, armoured carapaces. This is understandable - soft bits don't normally fossilise particularly well. But even those are present in the Cambrian fossil assemblages, from a few special locations such as the Burgess Shale of the Canadian Rockies. (I was first introduced to the story of the Burgess Shale & its exquisitely preserved organisms in Stephen Jay Gould's book Wonderful Life. Published in 1989, this book is still well-worth reading, even though interpretations of some of those fossils have changed as new information's become available.) There are still older fossils of multicellular organisms: the enigmatic Ediacaran animals, first discovered in Australian rocks that date back around 600 million years. But these are few, and much less varied than the Cambrian remains.
Part of the debate hinges around whether this 'explosion' is real, or simply an artefact: after all, soft-bodied organisms may have been very common, but just didn't fossilise well. And there is evidence that at least some animals must have been around for much longer - the trilobites, even in the early Cambrian, are a very diverse group, and this suggests a much earlier origin. As do what appear to be multicellular animal embryos from pre-Cambrian rocks in China.
But what's behind the relatively sudden appearance of those teeth, spines, shells, & armoured bodies? There've been several different explanations put forward. One relates to changes in the oxygen concentration of the Cambrian oceans & atmosphere. Oxygen levels had been trending upwards for at least a billion years, since blue-green algae evolved the capacity for aerobic photosynthesis, producing oxygen as a by-product. Perhaps the amount of oxygen available finally reached the point where larger, more active animals could evolve. Or there may have been a change in oceanic chemistry, allowing animals to use calcium carbonate in forming their hard parts.
In The eye: a natural history (yes, I'm still working through it in instalments), Simon Ings describes another possibility - that the evolution of vision set things up for an evolutionary arms race. How cool! & I hadn't even thought of that! There's no evidence that the Ediacaran organisms had vision, nor that any of them were anything but passive gatherers of nutrients. (This is not to say that they were unable to detect & respond to light: even some bacteria are capable of this.) But the Cambrian animals were different - they had eyes. And as Ings says:
Like Adam & Eve caught naked and defenceless in the Garden, Life became aware, for the very first time, that it was being watched.
And he goes on:
There's no point in having legs if you can't see where you're going. (The other senses will give you some idea of your environment, but only vision will give you the instantaneous information that makes agile movements possible.) Teeth and armour, too, are the product of an arms race that could not have got started without vision. When the chances of getting snared by a predator are little more than random, there's no reason to evolve heavy, expensive defenses against misfortune. But once the predator can see you, and pursue you, the chances of falling victim to its attentions are astronomically increased and a suit of armour becomes a worthwhile investment.
After all, if predator & prey alike were blind, the only defences necessary may well have been keeping quiet & trying not to release too much in the way of odours. But once that early light-sensitivity provided the basis for primitive eyes, all bets were off.
S.J. Gould (1989) Wonderful Life. Norton
S. Ings (2008) The eye: a natural history. Bloomsbury