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WHAT SPARKED the CAMBRIAN EXPLOSION? an Evolutionary Burst 540 Million Years Ago Filled the Seas with an Astonishing Diversity of Animals

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WHAT SPARKED the CAMBRIAN EXPLOSION? an Evolutionary Burst 540 Million Years Ago Filled the Seas with an Astonishing Diversity of Animals JOHN SIBBICK/NATURAL HISTORY MUSEUM HISTORY JOHN SIBBICK/NATURAL WHAT SPARKED THE CAMBRIAN EXPLOSION? An evolutionary burst 540 million years ago filled the seas with an astonishing diversity of animals. The trigger for that revolution is finally coming into focus. BY DOUGLAS FOX series of dark, craggy pinnacles rises oceans held so little oxygen that modern fish modern anatomical features. The Cambrian 80 metres above the grassy plains would have quickly foundered and died there. explosion, as it is called, produced arthropods of Namibia. The peaks call to mind A gooey mat of microbes covered the sea floor, with legs and compound eyes, worms with Asomething ancient — the burial mounds of and on that blanket lived a variety of enigmatic feathery gills and swift predators that could past civilizations or the tips of vast pyramids animals whose bodies resembled thin, quilted crush prey in tooth-rimmed jaws. Biologists buried by the ages. pillows. Most were stationary, but a few mean- have argued for decades over what ignited this The stone formations are indeed monu- dered blindly over the slime, grazing on the evolutionary burst. Some think that a steep rise ments of a faded empire, but not from anything microbes. Animal life was simple, and there in oxygen levels sparked the change, whereas hewn by human hands. They are pinnacle reefs, were no predators. But an evolutionary storm others say that it sprang from the development built by cyanobacteria on the shallow sea floor would soon upend this quiet world. of some key evolutionary innovation, such as 543 million years ago, during what is known Within several million years, this simple eco- vision. The precise cause has remained elusive, as the Ediacaran period. The ancient world system would disappear, and give way to a world in part because so little is known about the occupied by these reefs was truly alien. The ruled by highly mobile animals that sported physical and chemical environment at that time. 268 | NATURE | VOL 530 | 18 FEBRUARY 2016 © 2016 Macmillan Publishers Limited. All rights reserved FEATURE NEWS The Cambrian seas But over the past steps, approaching today’s sea-surface concen- them, he found that tiny worms survive in areas teemed with new types several years, discov- trations at the start of the Cambrian, around 541 of the sea floor where oxygen levels are very low of animal, such as the eries have begun to million years ago — just before more-modern — less than 0.5% of average global sea-surface predator Anomalocaris yield some tantalizing animals suddenly appeared and diversified. This concentrations. Food webs in these oxygen- (centre). clues about the end of supported the idea of oxygen as a key trigger for poor environments are simple, and animals feed the Ediacaran. Evi- the evolutionary explosion. directly on microbes. In places where sea-floor dence gathered from the Namibian reefs and But last year, a major study1 of ancient oxygen levels are a bit higher — about 0.5–3% other sites suggests that earlier theories were sea-floor sediments challenged that view. of concentrations at the sea surface — animals too simplistic — that the Cambrian explosion Erik Sperling, a palaeontologist at Stanford are more abundant but their food webs remain actually emerged out of a complex interplay University in California, compiled a database limited: the animals still feed on microbes rather between small environmental changes that trig- of 4,700 iron measurements taken from rocks than on each other. But around somewhere gered major evolutionary developments. around the world, spanning the Ediacaran and between 3% and 10%, predators emerge and Some scientists now think that a small, Cambrian periods. He and his colleagues did start to consume other animals4. perhaps temporary, increase in oxygen sud- not find a statistically significant increase in the The implications of this finding for evolution denly crossed an ecological threshold, enabling proportion of oxic to anoxic water at the bound- are profound, Sperling says.The modest oxygen the emergence of predators. The rise of carn­ ary between the Ediacaran and the Cambrian. rise that he thinks may have occurred just before ivory would have set off an evolutionary arms “Any oxygenation event must have been far, the Cambrian would have been enough to trig- race that led to the burst of complex body types far smaller than what people normally consid- ger a big change. “If oxygen levels were 3% and and behaviours that fill the oceans today. “This ered,” concludes Sperling. Most people assume they rose past that 10% threshold, that would is the most significant event in Earth evolution,” “that the oxygenation event essentially raised have had a huge influence on early animal evo- says Guy Narbonne, a palaeobiologist at Queen’s oxygen to essentially modern-day levels. And lution,” he says. “There’s just so much in animal University in Kingston, Canada. “The advent of that probably wasn’t the case”, he says. ecology, lifestyle and body size that seems to pervasive carnivory, made possible by oxygena- The latest results come at a time when sci- change so dramatically through those levels.” tion, is likely to have been a major trigger.” entists are already reconsidering what was The gradual emergence of predators, driven happening to ocean oxygen levels during this by a small rise in oxygen, would have meant ENERGY TO BURN crucial period. Donald Canfield, a geobiol- trouble for Ediacaran animals that lacked obvi- In the modern world, it’s easy to forget that ous defences. “You’re looking at soft-bodied, complex animals are relative newcomers to mostly immobile forms that probably lived Earth. After life first emerged more than 3 bil- “This is the most their lives by absorbing nutrients through their lion years ago, single-celled organisms domi- significant event in skin,” says Narbonne. nated the planet for most of its history. Thriving Studies of the ancient Namibian reefs suggest in environments that lacked oxygen, they relied Earth evolution.” that animals were indeed starting to fall prey to on compounds such as carbon dioxide, sulfur- predators by the end of the Ediacaran. When containing molecules or iron minerals that act ogist at the University of Southern Denmark palaeobiologist Rachel Wood from the Uni- as oxidizing agents to break down food. Much in Odense, doubts that oxygen was a limiting versity of Edinburgh, UK, examined the rock of Earth’s microbial biosphere still survives on factor for early animals. In a study published last formations, she found spots where a primitive these anaerobic pathways. month2, he and his colleagues suggest that oxy- animal called Cloudina had taken over parts of Animals, however, depend on oxygen — a gen levels were already high enough to support the microbial reef. Rather than spreading out much richer way to make a living. The process simple animals, such as sponges, hundreds of over the ocean floor, these cone-shaped crea- of metabolizing food in the presence of oxygen millions of years before they actually appeared. tures lived in crowded colonies, which hid their releases much more energy than most anaero- Cambrian animals would have needed more vulnerable body parts from predators — an eco- bic pathways. Animals rely on this potent, con- oxygen than early sponges, concedes Canfield. logical dynamic that occurs in modern reefs5. trolled combustion to drive such energy-hungry “But you don’t need an increase in oxygen Cloudina were among the earliest animals innovations as muscles, nervous systems and across the Ediacaran–Cambrian boundary,” he known to have grown hard, mineralized exo- the tools of defence and carnivory — mineral- says; oxygen could already have been abundant skeletons. But they were not alone. Two other ized shells, exoskeletons and teeth. enough “for a long, long time before”. types of animal in those reefs also had min- Given the importance of oxygen for animals, “The role of oxygen in the origins of animals eralized parts, which suggests that multiple, researchers suspected that a sudden increase in has been heavily debated,” says Timothy Lyons, unrelated groups evolved skeletal shells around the amount of the gas in the oceans, to near- a geobiologist at the University of California, the same time. “Skeletons are quite costly to modern levels, could have spurred the Cam- Riverside. “In fact, it’s never been more debated produce,” says Wood. “It’s very difficult to come brian explosion. To test that idea, they have than it is now.” Lyons sees a role for oxygen in up with a reason other than defence for why an studied ancient ocean sediments laid down evolutionary changes, but his own work3 with animal should bother to create a skeleton for during the Ediacaran and Cambrian periods, molybdenum and other trace metals sug- itself.” Wood thinks that the skeletons provided which together ran from about 635 million to gests that the increases in oxygen just before protection against newly evolved predators. 485 million years ago. the Cambrian were mostly temporary peaks Some Cloudina fossils from that period even In Namibia, China and other spots around that lasted a few million years and gradually have holes in their sides, which scientists inter- the world, researchers have collected rocks stepped up (see ‘When life sped up’). pret as the marks of attackers that bored into the that were once ancient sea beds, and analysed creatures’ shells6. the amounts of iron, molybdenum and other MODERN MIRRORS Palaeontologists have found other hints that metals in them. The metals’ solubility depends Sperling has looked for insight into Ediacaran animals had begun to eat each other by the late strongly on the amount of oxygen present, so oceans by studying oxygen-depleted regions Ediacaran. In Namibia, Australia and New- the quantity and type of those metals in ancient in modern seas worldwide.
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