LIFE appeared on our planet more than 3.5 billion years ago and consisted exclusively of microbes for the next 3 billion years. Then, about 539 million years ago, everything changed.
In the geological blink of an eye, the seas were filled with large and complex animals, including worms with legs and fearsome spikes, creatures with a trunk-like nose and five eyes, and giant shrimp-like predators with mouths like pineapple rings.
This evolutionary starburst is known as the Cambrian explosion. It is one of the most significant moments in life’s history on Earth because it is the point at which species that are clearly related to today’s animals first appeared. It is seen as evolution’s big bang.
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But over the past few years, geologists have begun to have second thoughts. Newly discovered fossils and careful analysis of ones found decades ago suggest that animals were thriving in the period before the Cambrian. As a result, some people are now arguing that the explosion of animal life started about 12 million years earlier. Others are questioning whether it is possible to define a distinct explosion at all.
You could be forgiven for thinking that shifting the dawn of the animal revolution from 539 to 551 million years ago isn’t that big a deal. But evolution can do a lot in that length of time: the entire span of human evolution probably fits within 12 million years, the length of time since our lineage separated from that of chimpanzees. What’s more, shifting life’s big bang back could have important implications for the quest to figure out what sparked evolution’s most spectacular spell of invention.
Scholars first worked out how to read the geological record in the 19th century, and they quickly noticed something puzzling. The oldest rocks they could find seemed devoid of fossils. Biologically complex marine animals, including woodlouse-like trilobites, suddenly appeared in abundance in the rocks assigned to the Cambrian period.
The pattern troubled Charles Darwin because it clashed with his idea of evolution by natural selection as a slow and gradual process. To make sense of it all, he suggested that simpler life forms must have evolved before the Cambrian but left little or no fossil evidence of their existence.
“Ediacarans were as strange to our eyes as life on another planet would be”
We now know Darwin’s hunch was correct. Geologists have spotted signs of microbial life in rocks more than 3 billion years old. They have also identified an important transition roughly 2 billion years ago, when those microbes became slightly larger and more biologically complex. This was a necessary step on the way to animals, broadly defined as organisms that are multicellular, capable of locomotion and responsive to their environment.
But the Cambrian explosion still seemed to mark the sudden blossoming of animal life. This remained the case even though, in the mid-20th century, geologists began finding fossils of large organisms, some a metre or more across, in rocks that predate the Cambrian explosion by 30 million years. These organisms were dubbed the Ediacaran biota because they date to the Ediacaran geological period, which precedes the Cambrian. But we couldn’t quite figure out what to make of them.
This wasn’t only because none of these organisms seemed to possess obvious animal features like a gut or a mouth. Some, including those in a group called the rangeomorphs, also had a very unanimal-like fractal anatomy, in which tiny parts of the organism looked like miniature versions of larger parts (see “Pushing back the clock”).
The influential palaeontologist Adolf Seilacher argued that the Ediacaran organisms were so clearly unrelated to the animals of the Cambrian that they were effectively as strange to our modern eyes as life on another planet would be. Seilacher was one of many researchers who felt that Ediacaran species and ecology looked so alien that it was impossible to escape the conclusion that the Cambrian was indeed a dramatic explosion of familiar animal life.
In the past 10 years, however, geologists have shifted their thinking. Sophisticated analytical techniques have started to suggest that some of the weird species of the Ediacaran were animals after all, and that they behaved uncannily like modern creatures. “We used to be obsessed with the Ediacaran-Cambrian boundary,” says Simon Darroch at Vanderbilt University in Tennessee. “Now it’s looking smoother than previously thought.”
Perhaps the most compelling clues come from rocks in Newfoundland, Canada, that contain traces of the earliest Ediacaran communities. Here, you don’t just stumble on the occasional nicely preserved specimen – you walk over bedrock exposures 120 square metres in area that each contain thousands of fossils.
“There probably wasn’t a Cambrian explosion worthy of the name after all”
Each giant slab is a Pompeii-like snapshot of the deep-sea floor community as it was 570 million years ago. “It’s absolutely astonishing in terms of preservation,” says Emily Mitchell at the University of Cambridge. It is what the rocks reveal about Ediacaran organisms that really surprised her, though.
Mitchell and her colleagues mapped the size and distribution of fossils of an oval-shaped rangeomorph called Fractofusus. This Ediacaran grew up to 40 centimetres in length and was covered in peculiar, fractally repeating pleats. The data, , showed that the largest individuals were scattered randomly across the ancient sea floor, surrounded by halos of smaller and smaller individuals. This suggests that Fractofusus reproduced in a sophisticated way. It generated waterborne offspring that would drift and land on an empty bit of sea floor. Then, as the offspring developed into adults, each would form a series of tentacle-like fingers, the ends of which would then grow into a clone of the adult.
Surprisingly modern
That is a little like how certain modern deep-sea animals such as sponges and corals reproduce, says Mitchell. Fractofusus may have had a fractal-like anatomy unlike that of any modern animal, but it apparently reproduced like some of today’s animals do. That might hint that it was related to those animals, although Mitchell says we can’t rule out the possibility that Fractofusus was instead related to fungi, which sometimes also reproduce this way.
In any case, it isn’t just Fractofusus that behaved surprisingly like a modern animal. Another Ediacaran organism called that suggest it trundled around, grazing on microbial mats on the sea floor, which is a strikingly animal-like way to behave. Darroch and his colleagues have used computer models to show that another Ediacaran, , just as many modern shellfish do. In a sense, it doesn’t even matter whether these Ediacaran organisms were animals or not: they were behaving and reproducing like modern marine animals do, which suggests that Ediacaran ecology was more like today’s than we previously thought.
There might also be good reason to believe that at least some strange Ediacaran organisms really were animals. The strongest evidence for this came last year. Ilya Bobrovskiy at the Australian National University in Canberra and his colleagues analysed the chemistry of rocks containing an Ediacaran organism called Dickinsonia. The rock around the fossils had a chemical signature associated with algae, which would make sense because the shallow sea floor on which Dickinsonia lived was probably coated in mats of algae. But the molecules within the fossils themselves included a particular type of steroid that is produced only by animals, implying that Dickinsonia was an animal.
These conclusions don’t necessarily lead to a defusing of the Cambrian explosion. Some people suspect that the . If so, it is possible that there was still an evolutionary explosion of sorts 539 million years ago, just one that involved a sudden blossoming of different sorts of animals, namely recognisably modern ones.
Palaeontologists have, however, begun to find evidence that the Ediacaran seas did contain animals that probably were related to modern ones. “We’re sucking down the [species] that were previously known in the Cambrian into the Ediacaran,” says Rachel Wood at the University of Edinburgh, UK.
For instance, in 2017, Wood and her colleagues announced they had found tiny fossils of what were previously assumed to be dating to the final 10 million years of the Ediacaran. Tiny burrows that could have been produced by nematode-like worms have also been seen in Ediacaran rocks from Brazil dating back at least 550 million years.
That is an important discovery because nematodes, primitive though they may seem, are relatively advanced animals. One study even suggests that they are : animals, like spiders and lobsters, with legs and an exoskeleton. If nematodes were around in the Ediacaran, it is plausible arthropods were too.
Indeed, just last year, were reported stretching several centimetres in rocks from south China thought to be up to 551 million years old. And earlier this year, a team caught a rare glimpse of another possible early arthropod in the same rocks: a 25-centimetre-long segmented creature called Yilingia that seems to have had primitive legs.
In light of all this, there probably wasn’t a Cambrian explosion 539 million years ago after all. Animals, both familiar and weird, really were thriving millions of years earlier. This revelation is so fresh that opinion is still divided on how to recast the rise of the animals. Earlier this year, a team including Wood and Mitchell argued that animals actually became dominant by diversifying through a series of relatively small evolutionary changes over tens of millions of years. As such, they concluded that it is debatable whether there really was any explosion worthy of the name.
“I can absolutely see their argument,” says Darroch. Even so, he still thinks there was a distinct evolutionary explosion, albeit one that began much earlier than we had thought. , he and his colleagues argued that this explosion didn’t take place 539 million years ago but 12 million years earlier, when the Ediacaran period was still in full swing. It is then that we see the first clear signs that tiny yet unmistakably modern animals were scuttling around in the shadow of the larger Ediacaran organisms.
If there was a distinct explosion, our chances of working out why it happened would be immeasurably improved if we could figure out when and where to look for clues. For comparison, by 54 million years ago, mammals were thriving across the world and the first primates had just appeared. But our explanations for this explosion of mammal life are lacking if they don’t acknowledge the dinosaur-ending asteroid impact that had occurred 12 million years earlier.
As far as we know, there was no asteroid impact to trigger the evolutionary explosion 551 million years ago. But we do know that huge changes were afoot at the time. The problem is that they are frustratingly mysterious. Geochemists studying the chemical isotopes locked away in 551-million-year-old rocks have found signs of what they describe as . It is known as the Shuram event.
Carbon shifts often indicate ecosystems in flux. At other times in our planet’s past, they have been linked to – and with mass extinction events. But the size of the carbon shift during the Shuram event is so large that it has , even after 25 years of study. And deciphering the event has now taken on new significance, given the realisation that it might have been the trigger for the blossoming of animal life as we know it.
Some geologists argue that the Shuram event reflects what they describe as “turmoil” from dramatic changes to the paths that water took as it slowly circulated around the ancient oceans. Others suspect that it represents a huge global warming event that released carbon-containing methane into the oceans and atmosphere. Either of these environmental disturbances might somehow have triggered the dawn of modern ecosystems, but we still don’t know quite how.
Alternatively, the Shuram event might reflect a sudden rise in atmospheric oxygen. Conventionally, a surge in oxygen levels has been viewed as a potential trigger for the sudden flourishing of animal life – although these days, many biologists suspect that the story is more complicated.
It is also exasperatingly unclear how animal life responded to the Shuram event. Darroch says geologists have struggled to find rocky outcrops that both record the Shuram geochemical signal and contain enough Ediacaran fossils to show how ecosystems reacted. “The rock record is not being as helpful as we’d like it to be,” he says.
Darroch thinks we will eventually find those elusive rocks. One reason for optimism is that a number of new outcrops of Late Ediacaran rocks have come to light in the past couple of years: details of , and Darroch and his colleagues are in the process of studying a fresh locality in South Africa.
Evidence from these sites might finally help explain when and why the most dramatic event in the history of life on Earth occurred – or it could indicate that the story of early animal life is so complex that there wasn’t a neatly definable Cambrian explosion after all. “It might just be that we’ve been trying to impose artificial patterns and boundaries on the rock record,” says Darroch.
