Feature THE BIZARRE SPECIES THAT ARE REWRITING ANIMAL EVOLUTION Early fossils with guts, segmented bodies and other sophisticated features reveal a revolution in animal life — before the Cambrian explosion. By Traci Watson he revolutionary animal lived and earthworms is one possibility — Yilingia is died in the muck. In its final hours, it helping to fill in key details about the evolu- inched across the sea floor, leaving tion of animals. Most importantly, Yilingia a track like a tyre print, and finally shows that some quintessential animal traits went still. Then geology set to work. had appeared half a billion years ago, earlier Over the next half a billion years, sed- than previous definitive evidence, Xiao says. iment turned to stone, preserving Yilingia is not the only creature from that the deathbed scene. The fossilized region to provide some of the earliest fossil creature looks like a piece of frayed rope meas- evidence for an important animal feature. In T 2 uring just a few centimetres wide. But it was a 2018, Xiao and his team reported on tracks trailblazer among living things. found in the Yangtze Gorges consisting of two This was the earliest-known animal to show parallel rows of dimples. The researchers pro- unequivocal evidence of two momentous pose that the trails were made by an animal innovations packaged together: the ability to from 550 million years ago that might have roam the ocean floor, and a body built from been able to burrow and had multiple pairs segments. It was also among the oldest known of appendages — which would make it one of to have clear front and back ends, and a left the earliest-known animals with legs. side that mirrored its right. Those same fea- These Chinese fossils hail from a time right Organisms named Fractofusus cover the sea floor some 560 million years ago, in a reconstruction of fossils from Newfoundland, Canada. tures are found today in animals from flies to before the Cambrian explosion, the evolution- flying foxes, from lobsters to lions. ary transformation when most of the animal The insights into the Ediacarans’ powers Palaeontologist Shuhai Xiao marvels at the groups that populate the planet today first lend support to a provocative idea: that the tracks left by this creature, Yilingia spiciformis, made their appearance in the fossil record. Sci- Cambrian explosion, that iconic evolution- and how they captured evidence of its move- entists long regarded the boundary between ary burst, was actually less revolutionary than ment. In his cluttered office at Virginia Tech the Cambrian period and the Precambrian as a many had thought. in Blacksburg, he shows off a slab of beige dividing point in evolution — a transition from The Cambrian explosion “is just another resin — a reproduction of the fossil, which was a world in which simple, strange organisms phase of evolution”, says palaeobiologist found in China’s Yangtze Gorges region and is flourished, to a time when the seas teemed Rachel Wood at the University of Edinburgh, now kept in a Chinese research institute. The with complex creatures that are the forebears UK. “It’s not a single flash event. It could not replica captures a snapshot of a moment from of nearly everything that followed. have happened without previous waves of 550 million years ago. Xiao, whose team for- But a growing number of findings reveal innovation.” mally described Yilingia last year1, traces the that the time slice just before the Cambrian, bumpy tracks it made immediately before its known as the Ediacaran (635 million to 541 mil- Tumultuous times death. “It was just moving around, and it died lion years ago), was a pivot point of animal The Ediacarans’ innovations came against a suddenly,” he says. evolution — a period that includes the earli- backdrop of planetary cataclysms. During this But that’s not the end of this creature’s story. est fossil records of anatomical innovations, time, Earth was still recovering from a long, Although nobody knows which category of such as guts and legs, and the first appearance shivery chapter when ice covered much of the life it belonged to — the group that includes of complex behaviours such as burrowing. seas. A gigantic meteor slammed into what is 662 | Nature | Vol 586 | 29 October 2020 ©2020 Spri nger Nature Li mited. All rights reserved. ©2020 Spri nger Nature Li mited. All rights reserved. CHARLOTTE G. KENCHINGTON CHARLOTTE Organisms named Fractofusus cover the sea floor some 560 million years ago, in a reconstruction of fossils from Newfoundland, Canada. now Australia and probably kicked up enough ‘Darwin’s dilemma’ would remain unsolved about the identity of Ediacaran organisms. As dust to trigger catastrophic changes around for a century. In the 1930s and 1940s, research- more were discovered, scientists tried valiantly the globe. The planet’s very surface was split- ers found intriguing imprints in rocks in Aus- to place them on the tree of life. Some of the ting: during the Ediacaran, one supercontinent tralia and elsewhere, but those rocks were fossils were towering structures that stood broke apart and another took shape as land not definitively Precambrian. Then, several one metre tall; others resembled deflated air masses smashed together. On the continents, English schoolchildren finally gave the Edi- mattresses. They have been called lichens and no plants grew. In the ocean, oxygen levels acarans their big break in 1957. Scrambling algae, fungi and bacterial colonies. “Basically swung wildly. through a local quarry, the students noticed any interpretation you can name has been sug- Scientists once thought that complex a leaf-shaped imprint in the ancient stone. gested,” says geobiologist Lidya Tarhan at Yale life did not start until after all this tumult. Geologist Trevor Ford at the University of University in New Haven, Connecticut. In Charles Darwin’s day, no fossils had been Leicester, UK, went to see it — and recognized Finally, an audacious theory broke through found below the rock layers documenting the that it had been made by a living thing. Ford’s the welter of competing claims. In the 1980s Cambrian explosion. That blank rock record paper3 about the imprint provided definitive and 1990s, palaeontologist Adolf Seilacher troubled Darwin, who reasoned that if his the- evidence that large, complex species lived in at the University of Tübingen in Germany ory of evolution were correct, there must have the Precambrian. He ventured that the type of proposed4,5 that many Ediacaran life forms been life before the Cambrian’s riches. “The organism was probably “an algal frond”. were not animals, but instead belonged to a case at present must remain inexplicable,” he It almost certainly wasn’t. Ford’s proposal was single, bizarre group that he called the Vendo- wrote in On the Origin of Species in 1859. among the first in a long list of mistaken ideas bionta. These organisms were “an evolutionary Nature | Vol 586 | 29 October 2020 | 663 ©2020 Spri nger Nature Li mited. All rights reserved. ©2020 Spri nger Nature Li mited. All rights reserved. Feature experiment that failed” when formidable intractable imprints. Take the baffling organ- same time as the Dickinsonia specimens that predators arrived on the scene, Seilacher isms in the genus Dickinsonia. Rounded and Bobrovskiy’s team studied, or perhaps earlier7. wrote. His ideas have fallen out of favour, flat, they resembled segmented bath mats only This discovery resolves a long-standing but they challenged researchers to question a few millimetres thick, although they could Ediacaran whodunnit: what made the narrow, their assumptions. “At the time it was brilliant reach nearly 1.5 metres in length. Their strange twisting burrows that cut through Ediacaran thinking,” says geobiologist Simon Darroch at construction spawned theories that they sediments? They are among the most common Vanderbilt University in Nashville, Tennessee. were protists — a diverse group of mostly sin- Ediacaran calling cards, but are so small — only “Before that, everyone assumed they were all gle-celled organisms that includes protozoa 1.5–2 millimetres wide — that they must have jellyfish, which was even more wrong.” and some algae — or lichens, although many been created by an elusively tiny organism. Now, most scientists are reaching agree- researchers suspected that they were animals. “We never thought we’d see it,” says palae- ment that the Ediacarans were a grab bag of To try to settle the long-standing dispute, ontologist Mary Droser at the University of disparate life forms, rather than the self-con- California, Riverside. Then she got her hands tained group proposed by Seilacher. “It’s on a 3D laser scanner. inappropriate to consider them a failed Droser and her colleagues used the scanner experiment,” says palaeontologist Frances to image hundreds of tiny blobs found near the Dunn at the University of Oxford, UK. “They twisting burrows. The team’s high-resolution represent the ancestors, probably, of lots of 3D reconstructions show that the blobs were, different things.” Many scientists — although BASICALLY ANY in fact, organisms7. They were smaller than not all — are also signing up to the idea that grains of rice, but they had left–right symme- some fraction of the Ediacaran organisms were INTERPRETATION YOU try and both a front and back end, and features probably animals, including some that don’t of the burrows suggest that the creatures could look like any animal alive today. CAN NAME HAS BEEN control where they moved. Previous analysis That idea dovetails with genetic evidence SUGGESTED.” showed that some burrows wend into and that animals, or metazoans, first appeared out of the buried bodies of larger organisms, more than 600 million years ago, well before implying that Ikaria was a scavenger — the the Ediacaran.