THE “tink, tink” of chisel on rock echoes across the valley. High up in the Rocky Mountains of Canada, half a dozen palaeontologists are patiently splitting chunks of shale. Sunburnt, covered in rock dust, with hands blistered from their labours, they have been living rough here near Marble Canyon for four weeks now. The season is short, and they spend almost every daylight hour at the quarry, a rock face about 4 metres deep. But their efforts are paying off.
Before long, a block falls open to reveal an almost perfectly preserved arthropod from half a billion years ago – a distant cousin of today’s insects and crustaceans. Hour after hour, the finds continue – more than 3000 so far. And the richest strata are yet to come. “Starting now, we are expecting an amazing array of new animals,” says Jean-Bernard Caron of the Royal Ontario Museum in Toronto, Canada, who leads the project.
Marble Canyon was discovered just two years ago. It is part of the famous Burgess Shale, perhaps the richest source of information about early animal life on Earth. The tale the shale tells is one of a sudden, dramatic burst of animal diversification known as the Cambrian explosion, evolution’s equivalent of the Italian Renaissance. These creatures are so bizarre-looking that 25 years ago, palaeontologist Stephen Jay Gould argued in his book Wonderful Life that they represent a failed experiment that left few descendants in today’s world. Since then, most of the Burgess fauna have been recognised as members of modern animal groups – albeit odd, experimental ones. That means that in just a few tens of millions of years – a geological instant – almost every major animal group we know made its first appearance in the fossil record, and the ecology of the planet was transformed forever.
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Excavations at Marble Canyon, Canada, are revealing fossils of animals half a billion years old (Image: ROM/Zak Rogers)
The original Burgess Shale in Canada’s Yoho National Park was discovered over a century ago, and has yielded some 200 animal species to date. Marble Canyon looks set to surpass even this, and last year Caron’s team reported that some of the creatures they found there match those in China’s Chengjiang fossil beds, suggesting that some Cambrian species had a worldwide distribution ().
Scientists have struggled to explain what sparked this sudden burst of innovation. Until recently, most efforts tried to find a single trigger, but over the past year or two, . The Cambrian explosion appears to have been life’s equivalent of the perfect storm. Instead of one trigger, there was a whole array of them amplifying one another to generate a hotbed of animal evolution the likes of which the world has never seen before or since.
Even without these particularly striking fossils, it is clear that something dramatic happened at the beginning of the Cambrian period, 542 million years ago (see diagram). For most of the 3 billion years before then, life was unicellular. The first sign of multicellular animals is in rocks about 750 million years old, which contain fossilised biomolecules found today only in sponges. Then another 150 million apparently uneventful years passed before the appearance of the Ediacaran fauna. This enigmatic group of multicellular organisms of uncertain affinities to other lifeforms flourished in the oceans up to the beginning of the Cambrian. Then all hell broke loose.FIG-mg30100601.jpg
“Something happened, right at the base of the Cambrian,” says Caron. The Ediacarans vanished abruptly, in what may have been the first mass extinction. What took their place is a mystery because we have few whole-animal fossils from the earliest part of this period. But trace fossils – the preserved remnants of burrows, tracks and the like – clearly show that big changes were happening ().
Throughout the Ediacaran period burrows were simple, barely more than shallow trenches ploughed in the seafloor. Palaeontologists can pick out seven distinct types, which they assume correspond to a few different body forms – grazing worms, actively foraging worms, suspension-feeding worms and surface-scratching mollusc-like animals. At the beginning of the Cambrian, that number abruptly jumped to 22.
“In terms of basic forms, you have tripled, very quickly. That is a huge thing,” says Gabriela Mangano at the University of Saskatchewan in Saskatoon, Canada. The trace fossils from this time include burrows of all different kinds: deep, vertical, meandering, U-shaped – even burrows that wind back and forth, systematically exploring a patch of ground. At the same time, fragments of fossilised shells, spines and other hard body parts become much more abundant. These indicate that a diverse set of animals evolved early in the Cambrian.
By the time the first whole-body fossils turn up, in shales from about 520 million years ago, almost all the major groups of animals are already present. In other words, virtually all modern groups seem to have originated between about 540 and 520 million years ago. “You go from seemingly nothing to everything in a few million years,” says Kevin Peterson, a molecular palaeobiologist at Dartmouth College in New Hampshire.
Why did this sudden burst of evolution happen when it did? One possibility is it simply took that long for animals to evolve the genetic toolkit necessary to build themselves into complex, mobile forms. “In my mind, 200 million years to figure out all the body plans and get things going seems about right,” says Nick Butterfield, a palaeontologist at the University of Cambridge. He describes the gene networks that control development from an egg into a multicellular animal as the most complex algorithms ever developed. “At some stage, everything’s going to line up and take off. I call that the Cambrian explosion.”
But that explanation may be too simple. Studies of “molecular clocks” – which use the gradual accumulation of genetic changes to estimate when particular evolutionary branches diverged – suggest that animal complexity emerged before the Cambrian (). For example, humans and starfish have a common ancestor back in the Ediacaran period, implying that the shared details of their biology – including body cavities, guts, nervous systems and more – must have been present then, too. “Developmentally, everything’s there long before the Cambrian,” says Peterson.
So why didn’t the Cambrian explosion happen earlier? If the genetic toolkit was in place, why the lag? “It’s necessary, but not sufficient. It’s not driving,” says at the Smithsonian National Museum of Natural History in Washington DC. In other words, other triggers were required. He points to two huge ecological innovations that make their debut in the Cambrian fossil record.
The first is the ability to burrow into the sea floor. These burrows would have broken up the stagnant microbial mats that dominated the Ediacaran sea floor and brought oxygenated water down into the sediment, stimulating productivity. “By ventilating the sediment, you’re basically producing food,” says Erwin. The many kinds of burrows also make for a more complex environment, so that animals have a wider range of ways to make a living.
The second innovation was predation. The beginning of the Cambrian features the first examples in the fossil record of true predators – animals that catch and eat other animals. “I don’t think that’s a coincidence,” says Peterson. With predation comes a much richer and more intense set of evolutionary pressures, as predators and their prey get caught up in “arms races”. This spurs the development of faster-moving animals, hard-bodied prey, stronger predators, better burrowers, sharper sense organs and the like.
Taken together, these probably led to an evolutionary snowball effect, as one original design begat another and diversity begat greater diversity – leading, within a few million years, to the profusion of new body plans and lifestyles evident in Caron’s Marble Canyon fossils. In effect, the rapidly evolving animals of the Cambrian were inventing whole ecologies on the fly, says Erwin.
Such a burst of creativity would be unlikely in today’s world, which is already teeming with animals of all sorts and sizes. But the ecologically sparse Cambrian world would have presented less competition, allowing animals to work out the “growing pains” that are inevitable with any radical redesign. Erwin points out that you see similar evolutionary experimentation after mass extinctions. For example, 252 million years ago, in which over 90 per cent of all species became extinct, was followed by a spate of innovation in vertebrate form that led to the dinosaurs, turtles and others.
This case for an ecological trigger has been around for several years, but it still doesn’t explain the exact timing of the animal explosion. What sparked the key innovations that were needed for a biological boom? What else were these early creatures waiting for?
Fantastic fertiliser
One intriguing possibility is that they were waiting for fertiliser. Geological evidence suggests that rising sea levels during the Cambrian could have increased erosion, boosting levels of nutrients such as calcium, phosphate and potassium in the oceans. This fertilisation would have sparked an increase in productivity, providing more resources, generating longer, more intricate food chains and allowing the ecological positive feedbacks to take off. But there’s a problem with this idea. “Nutrients alone don’t tell you how you get increases in ecological complexity and diversity,” says Erwin. “If you put nutrients into an ecosystem today, what you get is a huge increase in abundance of a small number of species.” Today’s polar oceans, for example, are among the most productive and least diverse marine ecosystems.
Nevertheless, calcium was key for one Cambrian innovation: high levels dissolved in the oceans, combined with carbonate ions, would have made it easier for animals to form hard shells and skeletons, helping spark the ecological arms race. In fact, these high calcium levels might even be the reason the Burgess Shale fossils are so well preserved. But most species that appear in the Cambrian explosion are entirely soft-bodied. “What about the rest of the animals?” says Caron. Some further ingredient is needed.
One likely candidate is oxygen. For most of Earth’s history, oxygen gas was exceedingly rare. A study published in October concluded that until 800 million years ago, oxygen levels were less than 0.1 per cent of present levels, which would have precluded any sort of active animal life (). Atmospheric oxygen levels crept up gradually after that. And although geochemists do not yet agree on the specifics, at some stage oxygen levels in the oceans would have risen to the point where active, predatory lifestyles became possible.
To learn more about where this point was, Erik Sperling at Harvard University and his colleagues looked to modern sea floor habitats. Sure enough, sites that were almost devoid of oxygen had very few predators, while those with more oxygen supported more diverse communities of predators, they reported in 2013 (). The crucial threshold seemed to be between 1 and 5 per cent of present oxygen levels. Geochemists’ best guess at when the ancient oceans reached this point is about 550 million years ago – just in time to kick off predation and its resulting ecological feedback. “That’s about what you’d expect if oxygenation was a trigger for the Cambrian explosion,” says Sperling.
Most biologists think that photosynthetic plankton were responsible for the gradual increase in oxygen. Last year, however, a few researchers argued that animals may have had a hand in fashioning the trigger, too (). The idea, proposed by Butterfield, goes like this: Precambrian oceans were full of single-celled algae and bacteria. When these small cells died, they would have started to sink, decomposing quickly as they went – and because decomposition consumes oxygen, this would have kept ocean waters anoxic. Filter-feeding sponges, which evolved sometime before the Ediacaran,then started clearing these cells out of the water column before they died and decomposed. The sponges themselves, being larger, were more likely to be buried in the sediment after death, allowing oxygen to remain in the water. Over time, this would have led ever more of the ocean to become oxygenated. “The sponges basically march from shallower to deeper water, oxygenating as they go,” says Erwin.
Like any good mystery, the Cambrian explosion has several prominent suspects. It is becoming increasingly clear that they worked together, but we still don’t know exactly how. “It’s very difficult to separate what are the triggers and what are the consequences,” says Caron. Nevertheless, as palaeontologists find more fossils, and develop more techniques for inferring past environments from geological clues, the answers may emerge quickly. “There are so many really good people working on this,” says Erwin. “.”
“Like any good mystery, the Cambrian explosion has several key suspects”
Caron and his team at Marble Canyon are in the vanguard of this progress. In just two short excavation seasons they have found thousands of fossils from some 80 species, 10 to 15 of them never seen before. Their finds will shed more light on the perfect storm of factors behind the Cambrian explosion, but Caron still can’t resist wondering what sparked the storm. “I think that’s the million-dollar question,” he says. And occasionally he worries there may be no way of knowing. “It’s possible that what we’re looking at here is a random event.”
This article appeared in print under the headline “Sparks of life”
