Fossil and molecular evidence suggests that complex multicellular organisms originated and proliferated during the Neoproterozoic Era (1,000-541 million years ago). Extreme glaciations during the Cryogenian period (720-635 million years ago), a phenomenon commonly referred to as Snowball Earth, led to a radical transformation of the Earth’s climate and oceans. New research suggests that Snowball Earth was an environmental trigger for the proliferation of complex multicellularity across several groups of eukaryotes.
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An artist’s impression of a ‘Snowball Earth.’ Image credit: NASA.
Why did multicellularity arise? Solving that mystery may help pinpoint life on other planets and explain the vast diversity and complexity seen on Earth today, from sea sponges to redwoods to human society.
Common wisdom holds that oxygen levels had to hit a certain threshold for single cells to form multicellular colonies.
But the oxygen story doesn’t fully explain why multicellular ancestors of animals, plants, and fungi appeared simultaneously, and why the transition to multicellularity took more than 1 billion years.
The new study shows how specific physical conditions of Snowball Earth — especially ocean viscosity and resource deprivation — could have driven eukaryotes to turn multicellular.
“It seems almost counterintuitive that these really harsh conditions, this frozen planet, could actually select for larger, more complex organisms, rather than causing species to go extinct or reduce in size,” said William Crockett, a Ph.D. student at MIT.
Using scaling theories, Crockett and his colleagues found that a hypothetical early animal ancestor — reminiscent of swimming algae that eat prey instead of photosynthesizing — would swell in size and complexity under Snowball Earth pressures.
By contrast, a single-celled organism that moves and feeds via diffusion, like a bacterium, would grow smaller.
“The world is different after Snowball Earth because there’s a new form of life on the planet,” said Santa Fe Institute’s Professor Christopher Kempes.
“One of the central questions of evolution is how do you go from nothing on a planet to things like us, and to societies? Is all of that an accident?”
“We think it’s not luck: there are ways to predict these major transitions.”
The study shows how the iced-over oceans during Snowball Earth would have blocked sunlight, reducing photosynthesis and thus draining the sea of nutrients.
Bigger organisms that processed more water had a better chance of eating enough to survive.
Once the glaciers melted, these larger organisms could expand further.
“Our study offers hypotheses of ancestor organism features to hunt for in the fossil record,” Crockett said.
The study was published in the Proceedings of the Royal Society B.
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William W. Crockett et al. 2024. Physical constraints during Snowball Earth drive the evolution of multicellularity. Proc. R. Soc. B 291 (2025): 20232767; doi: 10.1098/rspb.2023.2767
This article is a version of a press-release provided by Santa Fe Institute.
