About 700 million years ago, Earth turned into a snowball. The polar ice sheets expanded until they engulfed the globe. The oceans turned to slush. The vast expanses of ice and snow reflected the sun's light back into space, exacerbating the endless winter. Temporary relief came in the form of massive volcanic eruptions, which spewed carbon dioxide into the atmosphere and triggered a period of global warming. But that, too, spiraled out of control. Earth became a greenhouse — its oceans hot enough to cook their inhabitants, its blighted landscape further ravaged by floods. Then, suddenly, something about the shifting continents or ash-darkened skies prompted the planet to cool again. The snowball returned.

It's hard to imagine a less auspicious time to be a tiny creature trying to eke out an existence. But this period, called the Cryogenian, is when complex animal life got going. From the wreckage of this ice-and-fire-scourged planet emerged the evolutionary group that would give rise to jellyfish and corals, mollusks, snails, fish, dinosaurs, beetles, birds and, eventually, all of us.

This is no coincidence, scientists say. In paper published this week in the journal Nature, researchers report that the appearance of complex, multicellular animals is inextricably linked to a boom in algae enabled by the same destructive forces that made the Cryogenian seem so hellish.

“The work helps us answer the question why we exist,” lead author Jochen Brocks wrote in an email. And it may help scientists determine what conditions would be needed for complex organisms to evolve on alien worlds.

Life has existed on Earth for almost as long as Earth has been around. Every few months, it seems, researchers uncover ever-more-ancient rocks they claim contain the world's oldest fossils. But the creatures that inhabited our planet for its first 3 billion years were tiny, simple — mostly single-celled bacteria.

But about 650 million years ago — during the brief warm spell between Earth's snowball periods — more sophisticated creatures started to evolve. They had rudimentary tissues, including neurons, and they swiftly diversified into the various animal groups we recognize today.

Scientists couldn't figure out why it took so long for complex life to appear, according to Brocks, a biogeochemist at the Australian National University. Was it just a matter of time? The evolution of an animal is incredibly difficult, and because evolution is random and undirected, it might understandably take a while for the right genetic flukes to occur. Or was there some environmental constraint inhibiting the growth of large, moving organisms?

Some scientists have proposed that, because animals breathe oxygen, they couldn't evolve until atmospheric oxygen levels reached a certain threshold. But there's no direct evidence of oxygen levels in the geologic record, Brocks said, so it's difficult to test this theory.

He was most interested in a third explanation. Animals can't make their own fuel, so they require a buffet of efficient energy sources — in other words, food. The boring bacterial ecosystem that existed for most of Earth's history couldn't supply it. Those creatures would be as small and unsatisfying to a complex multicellular organism as a flea would be to a tiger. Algae, on the other hand, have a cell volume 1,000 times bigger than a bacterium. Now that's a scrumptious meal.

Algae first emerged somewhere between 1 billion and 2 billion years ago, when a large cell known as a eukaryote swallowed a tiny photosynthetic critter called a cyanobacteria and put it to work. But Brocks wanted to know when algae became abundant, to pinpoint the moment when there were enough of these organisms floating around that they could start to fuel other creatures.

This was no easy task — tiny, soft algae decay quickly, so they don't leave an obvious mark on the fossil record. Instead, Brocks and his colleagues looked for the molecular traces that decaying organisms left behind. The team ground up rocks that formed from sediments at the bottoms of ancient oceans and mixed the resulting powder with a solvent to produce a bizarre chemical brew. They could then analyze their concoctions for signatures of bacteria and algae.

Their results indicated that algae remained relatively marginal for millions of years after they first evolved. But then his colleague Amber Jarrett found an immaculately preserved rock from the “inter-Snowball period” 650 million years ago, when Earth was momentarily warm. The chemical brew from this rock showed that algae abundances increased by a factor of 100 to 1,000 during this period. The Age of Algae had begun.

“We could not have made our discovery in any more exciting period,” Brocks wrote. “The close temporal connection between the melting of the Snowball, rising nutrient levels in the oceans, the rise of algae and the evolution of animals immediately suggest that these events must be linked.”

But how? As Brocks puzzled over the problem, he read a paper in the journal Nature linking the snowball periods to a dramatic increase in the global availability of the nutrient phosphate. That's when it clicked. During the interminable winter of the snowball periods, glaciers surged across the continents, gouging Earth's surface and grinding mountains down to dust. When the ice finally melted, all that crushed rock — and the nutrients within it — was washed into the oceans. The geologic upheaval likely released huge amounts of oxygen into the ocean and air. The sudden availability of nutrients probably caused massive algae blooms, and, eventually, complex animals emerged to take advantage of the feast.

At least, that's how the theory goes. Brocks anticipates that there will be debate about the exact causes of the rise of algae. Other researchers pin the snowball-era blooms on other nutrients, like nitrogen. Some might even turn the model on its head, he said — arguing that animals appeared at random, then precipitated the snowball period and the resulting algae age. The question of why we're here from is far from settled.

But in a commentary for Nature, Harvard geobiologist Andrew Knoll wrote that Brocks's paper “will change the conversation” about how animals evolved — offering a framework for studying the long-extinct ecosystems that preceded the world we now know.

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