In the Beginning . . .

​​For nearly a century, geologists have believed that complex animal life first emerged in the Cambrian Explosion, some 500 million years ago. More recent research shows that it can be traced back much further than that, and is hinting at some of the factors that drove evolution.

When the delightfully named Charles Doolittle Walcott discovered fossils of strange creatures in the Burgess Shale in Canada, in 1909, geologists believed that they had found evidence for the first flowering of complex life on Earth. Dated to about 500 million years ago, the event became known as the Cambrian Explosion, and that is exactly what they seemed to imply – that complex life had suddenly exploded on Earth, in many different forms, out of almost nothing.

Interpreting the fossils posed some serious challenges. All were new to science, and few seemed to have obvious descendants. Amongst the most unusual was one appropriately named Hallucigenia, which seemed to have 14 unarticulated legs and a row of curling breathing tubes on its back. Only later, when it was eventually turned up the other way, did it become clear that its tubes were legs and its legs were spines, and that it wasn’t quite so stiff-limbed after all!

As more fossils of a similar age were found elsewhere in the world, over following decades, the case for this first blossoming of life seemed to be well-proved. But over the last thirty years or so, older fossils have started to emerge. The first of these discoveries, in 1946, was from the Ediacarian rocks of Australia, which date to the late Pre-Cambrian (ca. 540 million years ago). Since then, further finds have been made in Siberia, Namibia, Brazil, Canada and China. Some, such as the Chinese sites (at a possible 635 million years) are not yet firmly dated. But a well-dated site in Newfoundland shows that complex life existed at least as far back as 571 million years ago. 

The fossils from these Ediacarian sites are not so glamorous as the Burgess Shale finds. The oldest all seem to be soft-bodied creatures, rather like algae, and were probably sedentary. Some, however, bear scratch marks that suggest they might have been grazed by predators, and the first evidence of burrowing animals also appear. 

But life did not stay like that. By 550 million years ago, the first animals with skeletons appear in limestone deposits in Siberia, Namibia and Brazil. The development of skeletons was a crucial innovation, for they offered several evolutionary advantages: in particular, protection from predators. It also shows that reef formation was possible for the first time. 

What drove this early evolution is still not wholly clear. But new geochemical techniques suggest that for most of the Pre-Cambrian the oceans were subject to major swings in the levels of oxygen. At the end of ‘Snowball Earth’ about 630 million years ago (when the planet is believed to have been largely ice-bound), oxygen levels were low. Over the next 90 million years, however, there seem to have been at least four brief periods when oxygen levels rose high enough to allow animals to allow calcium carbonates to form, and skeletons to be made. In each of these, life flourished; each time oxygen levels fell again, mass extinction occurred. 

As all this shows, the early evolution of life on Earth goes back much further, and is far more complex, than we had previously imagined. It also suggests that geochemical changes in the oceans may have been a major driver for evolution in these stages. One other thing also seems certain. There is still much more to learn.

For more on this story, see:

Rachel Wood 2019 The rise of animals. Scientific AmericanVol 320, No. 6, June 2019, pp. 20-25.