This video shows a reconstruction of the extinct animal Opabinia.
The Cambrian explosion remains a puzzle
For Charles Darwin, trying to demonstrate his theory of natural selection, this sudden burst of evolution was a major problem. “The case must at present remain inexplicable; and may be truly urged as a valid argument against the views here entertained,” he wrote in On the Origin of Species in 1859.
To this day the Cambrian explosion remains a puzzle. But maybe a planet-encasing icy catastrophe could help explain it.
The evidence for a Snowball Earth first emerged in the early 1990s. Unexpectedly, geologists discovered evidence of glaciers – such as stones that had clearly been carried on ice rafts and then dropped – in the tropics. Since then, a growing body of evidence has shown that the global deep freeze began around 715 million years ago, and lasted nearly 120 million years.
Exactly how far the ice extended is still debated. Some argue that the entire Earth was encased in ice, with just a few small pockets of open water where hot springs bubbled up. Others believe that a belt of open water remained around Earth’s equator.
Regardless of how far the ice stretched, most scientists agree that the Snowball formed suddenly. It was probably caused by rapid weathering of Earth’s continents, which sucked carbon dioxide – a planet-warming greenhouse gas – out of the atmosphere and caused temperatures to plummet. There were two distinct pulses of extreme glaciation, interspersed with a 20-million-year warm period. Finally, around 660 million years ago, Earth’s volcanoes topped up the atmospheric carbon dioxide enough to haul the climate out of its frozen state.
So why on Earth would this period of extreme cold cause life to switch gear so rapidly? Maybe, say many geologists, because it pumped lots of life-giving oxygen into the air.
The idea is that the ice gave a boost to microscopic plants, which released oxygen as a waste product. During the Snowball, the glaciers would have worn huge amounts of phosphorus-rich dust away from the underlying rocks. Then, when the ice retreated at the end of the Snowball, rivers washed this dust into the oceans, where it fed the microbes.
“High phosphorus levels would have increased biological productivity and organic carbon burial in the ocean, leading to a build-up of atmospheric oxygen,” says Noah Planavsky of Yale University in New Haven, Connecticut. In 2010 he identified a massive spike in phosphorus levels in sediments from around the world, just as Snowball Earth was ending.
That was suggestive, but in 2014 Planavsky found more direct evidence. His team estimated oxygen levels prior to Snowball Earth, by studying chromium – which exists in different states depending on the amount of oxygen in the air – in ancient rocks. Until 800 million years ago, atmospheric oxygen levels were just one-hundredth of today’s levels.
Planavsky thinks that is far too low to support complex animal life. “In modern low-oxygen environments there is less ecosystem complexity and a more limited range of animal behaviours,” says Planavsky. “So it is reasonable to expect that an oxygen rise would pave the way for animal and ecosystem diversification.”
But there’s a problem with that idea. Experiments published in 2014 showed that some animals can survive with much less oxygen than previously thought. Sponges, one of the oldest kinds of animal, need just 0.5% of modern oxygen levels. That suggests oxygen wasn’t enough of a trigger.
In recent years another idea has come to prominence. Maybe it was the ice itself that drove the evolutionary leap, says Richard Boyle of the University of Southern Denmark in Odense. “There are no animals more complex than a sponge prior to the last of the Snowball glaciation events, and in my opinion this is not coincidence,” says Boyle.
For Boyle the real puzzle isn’t the appearance of multicellular animals. Instead, it’s the rise of cellular differentiation – cells with specific roles like liver, muscle and blood. These specialised cells allowed animals to become much more intricate. “What sets animals apart from plants and fungi is this irreversible cellular differentiation, which, for instance, is what allows animals to have more cell types,” says Boyle.
It’s hard to see how this could have evolved, because specialised cells lose the ability to reproduce on their own. Instead they have to be distinctly self-sacrificing, cooperating with other cells in the body for the greater good of the animal. Only the specialised reproductive cells, the sperm and eggs, get to create a new generation.
By contrast, plants don’t just rely on specialist sex cells to reproduce. They can also reproduce themselves from cuttings taken from their stems or roots. “You can’t take a cutting from an animal,” says Boyle. He thinks the severity of Snowball Earth may have pushed animal cells to abandon this flexibility, and specialise.
“During the Snowball period, life will have been confined to small geothermally heated areas, and will have experienced frequent extinctions and population crashes,” says Boyle. The populations that did survive were often reduced to just a handful of organisms. Boyle suggests that these little groups of survivors were often closely related, encouraging them to cooperate more than usual.
Biologists have long known that animals are more likely to help close relatives, because by doing so they can benefit their own genes, which the relative will also carry. For example, wild animals are likely to adopt orphans that are related to them, but not orphans that are unrelated. Boyle thinks that Snowball Earth may have forced cells to behave altruistically. “Until that point, the cost of being an animal cell had been too high,” he says.
Boyle’s notion is controversial and other scientists are sceptical. “Boyle’s melt-hole idea for the origin of animals is fun,” says palaeontologist Nick Butterfield of the University of Cambridge, UK. “But most geologists don’t buy the idea of a hard Snowball Earth anymore, so the isolated hot-spring refugia ponds wouldn’t have actually existed.”
Butterfield argues that life probably retreated to the open waters of the tropics during Snowball times, but otherwise carried on as normal.
It would really help to find some definitive fossils to resolve this. Unfortunately, the fossil record is very patchy in such ancient rocks. So far, the oldest definitive fossils of complex animals date to around 560 million years ago. That could fit with either hypothesis.
Genetics doesn’t help much either. By working backwards through the animal family tree and estimating rates of genetic change, scientists have estimated that the first animals are likely to have emerged around 750 million years ago. But these “molecular clock” estimates are notoriously unreliable.
Nonetheless, recent discoveries hint that animal life may have started to gain a foothold during Snowball Earth. In 2014, Malcolm Wallace of the University of Melbourne in Australia discovered strange clumps of fossils in remote regions of Australia and Namibia. In the remains of ancient reefs, Wallace found bubble-shaped fossils up to 3cm across. Many of the bubbles appeared to interconnect into a network of finger-like strands.
“These fossils are big and complex, but they don’t really fit exactly into any of the animal phyla,” says Wallace. They date from around 700 million years ago, soon after Earth first became a Snowball.
So Wallace and his colleagues think they may have found the precursors to animals – very early sponge-like creatures, which lived in low-oxygen waters and represented a halfway stage between single-celled microbes and multicellular animals. And they think it is no coincidence that these animal precursors appear right after the first major Snowball glaciation.
“Intuitively, you might think that Snowball Earth would hinder evolution, and yet animals appear soon after the big glaciations,” says Wallace. “It seems clear that these big glaciations have disrupted the Earth’s ocean-atmosphere system in some way that was favourable for complex life to develop.”
Boyle agrees that this kind of primitive animal life may have evolved before the end of Snowball Earth, but he argues that this wasn’t the crucial step. Instead, the key threshold is when individual cells forgo their ability to reproduce, and instead take on specific roles within an animal.
So far, animals more complex than sponges, with specialised organs that do different jobs, have only been found in rocks laid down after the Snowball. Boyle predicts that they will never be found in older rocks, certainly not in rocks laid down before the Snowball. “If such fossils are found then my hypothesis will be proven incorrect,” he says.
Butterfield agrees that such ancient animal fossils may never turn up, but that could simply be because they haven’t been preserved. He now suspects that Boyle, Planavsky and Wallace have got the whole story backwards. Instead of the ice creating complex animals, he suggests that the first animals appeared 750 million years ago and transformed the planet, cooling the climate. “I think there is a good case to be made for the evolution of animals actually triggering the glaciations,” says Butterfield.
“Animals have an enormous capacity to modify physical environments,” says Butterfield. So he thinks the first animals upset the delicate balance of ocean chemistry, with knock-on effects for the rest of the planet.
Animals can certainly have big effects on the planet. For instance, burrowing animals like worms can break up rocks faster. The resulting rock dust reacts with carbon dioxide in the air, and the minerals produced get washed into the oceans – removing the carbon dioxide from the air. Meanwhile, marine animals boost oxygen levels by eating the remains of dead organisms, which would otherwise consume oxygen. Butterfield also thinks animals may have driven the evolution of new microscopic plants that sank faster, taking carbon dioxide with them.
There is some evidence that the first animals could have thrown Earth into a deep freeze. In 2011, Eli Tziperman of Harvard University in Cambridge, Massachusetts and his colleagues modelled the chemical cycles in the ocean. They found that the evolution of new marine organisms could have helped transport more carbon to the ocean floor and forced a major change in climate. “It’s certainly not unreasonable to suggest that the evolution of animals initiated glaciation,” says Butterfield.
Right now there’s not enough information to decide whether animals created Snowball Earth, or Snowball Earth triggered animal evolution. But either way, the two events are linked.
They are also a sobering reminder of how quickly conditions on Earth can change. Our planet has been just right for us for thousands of years, but there is no reason to believe it will stay that way.
Right now our appetite for fossil fuels is hotting things up dangerously fast. But a large asteroid impact, like the one that did for the dinosaurs, would throw up enough dust to block sunlight and cause a dangerous chill. And because today’s oceans are cooler than they were in the dinosaurs’ time it’s conceivable that the oceans would freeze and Earth would revert to a Snowball state.
Whether our planet goes hot or cold, it will be a seriously bumpy ride. Maybe we should learn from those early animal cells, and learn to work together.