LITMIR.BIZ

      Kimberella seems the clearest case of a self-propelled Ediacaran, but there were very likely others. Near a Dickinsonia fossil, one often finds a sequence of fainter traces bearing the same shape. The animal seemed to sit and feed for a while at one spot, then move on. Some reconstructions of Ediacaran scenes show a few animals swimming, including Spriggina, named after Reg, their discoverer, but Gehling thinks this scenario is unlikely, because Spriggina fossils are always found the same way up. If a Spriggina swam, then whenever some tiny disaster killed it, it would have had some chance of landing the other way up. So Gehling thinks that Spriggina, like Kimberella, crawled.

      Some biologists have argued that the Ediacarans are members of an animal-like evolutionary experiment, but not properly animals themselves. Rather than sitting on the animal branch of the tree of life, they exhibit a different way that cells can come together to yield an organism. Those strange three-sided forms and quilted fronds might support such a view. A more standard interpretation is that some Ediacarans, like Kimberella, were members of familiar animal groups, while other fossils represent abandoned evolutionary detours, together with ancient algae and other kinds of life. One theme that has emerged fairly consistently, though, is that the Ediacaran world was a rather peaceful one, a world largely without conflict and predation.

      The word “peace” might not be apt, as it suggests a kind of considered friendship or truce. Rather, the Ediacarans appear to have had very little to do with each other. They munched on the mat, filtered food from the water, and in some cases roamed around, but if the fossil evidence is any guide, they hardly interacted at all.

      Perhaps the fossil record is not a good guide; back in the first part of this chapter I discussed how the world of single-celled organisms now seems full of hidden interactions, mediated by chemical signals. The same may have been true in Ediacaran times, and this mode of interaction would leave no fossil trace. And certainly the Ediacarans competed with each other in an evolutionary sense – that is inevitable in a world of reproducing organisms. But some of the most conspicuous forms of interaction between one organism and another do seem to be absent. In particular, there is no evidence of predation – no half-eaten animal remains. (A few fossils show possible signs of predation-related damage in one animal, Cloudina, but even this case is unclear.) This was in no sense a dog-eat-dog world. Instead, in a phrase coined by the American paleontologist Mark McMenamin, it seems to have been “the Garden of Ediacara.”

      We can also learn something about life in the garden from Ediacaran bodies. These creatures do not seem to have large and complex sense organs. There are no large eyes, no antennae. Almost certainly they had some responsiveness to light and chemical traces, but they made little investment, as far as we can tell, in this sort of machinery. There are also no claws, spikes or shells – no weapons, and no shields with which to fend weapons off. Their lives seem not to have been lives of conflict and complicated interaction; they certainly didn’t evolve the familiar tools used in such interactions. It was a garden of relatively self-contained and self-possessed beings. Macarons that pass in the night.

      This is vastly unlike animal life now. Our animal cousins are highly alert to their environment; they track friends, foes, and countless other features of the landscape. They do that because what’s going on around them matters; often it’s a matter of life and death. Ediacaran lives show no evident sign of this moment-to-moment engagement with the environment. If so, this makes it likely that our Ediacaran ancestors put their nervous systems – when they had them – to different uses from those seen in more recent animals. Specifically, this might have been a time when the role played by those nervous systems fits the second of the theories of nervous system evolution I introduced above, the view based on internal coordination rather than sensory-motor control. Nervous systems were for shaping movements, maintaining rhythms, crawling and (perhaps) swimming. This would have included some sensing of the environment, but perhaps not very much.

      Those inferences might be mistaken; perhaps a great deal of sensing and interaction was going on, using organs made of soft materials that leave no trace. Something else that has always puzzled me in discussions of the peaceful Ediacaran is the role of jellyfish. Sprigg’s own fossils were not jellyfish, as he’d thought, but jellyfish are believed to have been around at this time, usually leaving no traces. Cnidarians in general, but especially jellyfish, have stinging cells, and a garden of stinging jellyfish, as any Australian will insist, is far from Edenic.

      When the Royal Society of London held a conference on early animals and the first nervous systems in 2015, the age of the first jellyfish stings was a topic of puzzled discussion. It does seem that cnidarian stings evolved early – this we infer from the fact that the evolutionary split between two major branches of this group appears to date to the Ediacaran or even before, and animals on both sides of the split have the same sort of stingers. Cnidarian stings are weapons. Were they offensive or defensive? Neither the prey nor the foes of modern cnidarians existed back then. So who were the stings aimed at? We do not know.

      Even if Ediacaran life was not as peaceful as has sometimes been supposed, a very different world was around the corner.

      The “Cambrian explosion” began around 542 million years ago. In a relatively sudden series of events, most of the basic animal forms seen today arose. These “basic animal forms” did not include mammals, but did include vertebrates, in the form of fish. They also included arthropods – animals with an external skeleton and limbs with joints, such as trilobites – along with worms, and various others.

      Why did it happen then, and why did it happen so fast? The timing may have had to do with changes to the Earth’s chemistry and climate. But the process itself may have been largely driven by a kind of evolutionary feedback, due to interactions between organisms themselves. In the Cambrian, animals became part of each other’s lives in a new way, especially through predation. This means that when one kind of organism evolves a little, it changes the environment faced by other organisms, which evolve in response. From the early Cambrian onward there was definitely predation, together with everything that predation encourages: tracking, chasing, defending. When prey starts to hide or defend itself, predators improve their ability to track and subdue, leading in turn to better defenses on the prey side. An “arms race” has begun. From the early part of the Cambrian, the fossil record of animal bodies contains exactly what was not seen in the Ediacaran – eyes, antennae, and claws. The evolution of nervous systems was heading down a new path.

      The revolution in behavior seen in the Cambrian also occurred, in large part, through the unfolding of possibilities inherent in a particular kind of body. A jellyfish has a top and bottom but no left and right. It is said to have radial symmetry. But humans, fish, octopuses, ants, and earthworms are all bilaterians, or bilaterally symmetrical animals. We have a front and back, and hence a left and right, as well as a top and bottom. The first bilaterians, or at least some early ones, might have looked bit like this: