The origin of complex life on Earth just got a little less mysterious
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.