Life on Earth goes back at least two billion years, but it was only in the last half-billion that it would have been visible to the naked eye. One of the enduring questions among biologists is how life made the jump from microbes to the multicellular plants and animals who rule the planet today. Now, scientists have analyzed chemical traces of life in rocks that are up to a billion years old, and they discovered how a dramatic ice age may have led to the multicellular tipping point.

Writing in Nature, the researchers carefully reconstruct a timeline of life before and after one of the planet's most all-encompassing ice ages. About 700 million years ago, the Sturtian glaciation created what's called a "snowball Earth," completely covering the planet in ice from the poles to the equator. About 659 million years ago, the Sturtian ended with an intense greenhouse period when the planet heated rapidly. Then, just as things were burning up, the Marinoan glaciation started and covered the planet in ice again. In the roughly 15 million years between the two snowballs, a new world began to emerge.

Jochen J. Brocks, a geologist from the Australian National University, Canberra, joined with his colleagues to track the emergence of multicellular life by identifying traces left by cell membranes in ancient rocks. Made from lipids and their byproducts, cell membrane "biomarkers" are like fossils for early microorganisms. By measuring chemical changes in these membranes, Brocks and his team discovered a "rapid rise" of new, larger forms of sea-going plankton algae in the warming waters after the Sturtian snowball. Some of these lifeforms were eukaryotes, meaning they had developed a nucleus—that's another necessary step on the road to multicellular life.

But multicellular life couldn't evolve without a major shift in the planet's geochemistry after the Sturtian. From the upper atmosphere to the deepest oceans, the planet's molecular composition had to change.

The great oxygen rush

The researchers suggest this transformation started when melting glaciers at the end of the snowball caused rapid erosion of landmasses, sending huge amounts of nutrients into the oceans. Slurries of icy minerals cascaded into the sea, sinking to the bottom and sequestering carbon.

That's when things got real. "Such massive burial of reduced carbon must have been balanced by a net release of oxygen into the atmosphere, initiating the protracted oxygenation of Neoproterozoic deep oceans," write the scientists. A world with very little oxygen in it was suddenly inundated with the stuff, both in and out of the water.

The rise of oxygen set off a cascade of linked events. It very likely led to the rise of phosphorous in the water, which is a key building block in DNA, and the energy-rich molecule ATP that provides fuel for our bodies. This meant more complex lifeforms like algae, which release oxygen during their digestive process. As algae diversified, lifeforms evolved to feed on the algae. Over time, new predators evolved to feed on those creatures, and so on. The more creatures who died and sank to the ocean floor, the more carbon was sequestered. As the researchers put it, the planet developed "a more efficient biological pump."

This oxygen- and phosphorus-driven change was unstoppable. Even after the Minoan glaciation's snowball, when the surface of the ocean heated up to as much as 60 degrees Celsius in the tropics, algae found its way to the poles and continued to diversify. Life as we know it appears to have emerged in the warm waters of a planet vacillating wildly between snowball and greenhouse. The climate became more stable about 550 million years ago, and we see the emergence of animals with heads, tails, and internal organs.

Harvard geobiologist Andrew Knoll, who was not involved in the study, wrote that this discovery "will change the conversation" about the emergence of complex life on Earth. Fundamentally, Brocks and his colleagues' work shows that environmental changes are key to the evolution of life. Without an oxygenated ocean, there would be no animals on this world.

That's why scientists are deeply concerned about the de-oxygenation of the seas today as a result of climate change and nutrient runoff from land. De-oxygenated areas called "dead zones" will slow or even halt the planet's biological pump. Earth is a glorious geochemical machine, running processes that take millions of years. Perturbations in those processes can completely transform the world. Sometimes that means the planet blooms with life, as it did during the rise of oxygen and phosphorous in the ocean. But sometimes it brings death.

Nature, 2017. DOI: 10.1038/nature23457