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'Snowball Earth' Evolution Hypothesis Gains New Momentum

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3 months 2 weeks ago
The University of Colorado Boulder's magazine recently wrote: What happened during the "Snowball Earth" period is perplexing: Just as the planet endured about 100 million years of deep freeze, with a thick layer of ice covering most of Earth and with low levels of atmospheric oxygen, forms of multicellular life emerged. Why? The prevailing scientific view is that such frigid temperatures would slow rather than speed evolution. But fossil records from 720 to 635 million years ago show an evolutionary spurt preceding the development of animals... Carl Simpson, a macroevolutionary paleobiologist at CU Boulder, has found evidence that cold seawater could have jump-started — rather than suppressed — evolution from single-celled to multicellular life forms. That evidence is described in Quanta magazine: Simpson proposes an answer linked to a fundamental physical fact: As seawater gets colder, it gets more viscous, and therefore more difficult for very small organisms to navigate. Imagine swimming through honey rather than water... To test the idea, Simpson, a paleobiologist at the University of Colorado, Boulder, and his team conducted an experiment designed to see what a modern single-celled organism does when confronted with higher viscosity... In an enormous, custom-made petri dish, [grad student Andrea] Halling and Simpson created a bull's-eye target of agar gel — their own experimental gauntlet of viscosity. At the center, it was the standard viscosity used for growing these algae in the lab. [Green algae, which swims with a tail-like flagellum.] Moving outward, each concentric ring had higher and higher viscosity, finally reaching a medium with four times the standard level. The scientists placed the algae in the middle, turned on a camera, and left them alone for 30 days — enough time for about 70 generations of algae to live, swim around for nutrients and die... After 30 days, the algae in the middle were still unicellular. As the scientists put algae from thicker and thicker rings under the microscope, however, they found larger clumps of cells. The very largest were wads of hundreds. But what interested Simpson the most were mobile clusters of four to 16 cells, arranged so that their flagella were all on the outside. These clusters moved around by coordinating the movement of their flagella, the ones at the back of the cluster holding still, the ones at the front wriggling. "One thing that you learn about small organisms from a physics point of view is that they don't experience the world the same way that we do, as larger-bodied organisms," Simpson says in the university's article. It says that instead unicellular organisms are specifically "affected by the viscosity, or thickness, of sea water," and Simpson adds that "basically, that would trigger the origin of animals, potentially." Last year Simpson posted a preprint on biorxiv.org. (And he also co-authored an article on "physical constraints during Snowball Earth drive the evolution of multicellularity.") There's a video showing algae in Simpson's lab clumping together in viscous water. "This observed behavior adds evidence to Simpson's hypothesis that single-celled organisms clumped together to their mutual advantage during the 'Snowball Earth' period," says the video's description, "thus adding momentum to the rise of multicellular organisms." But Simpson says in the university's article, "To actually see it empirically means there's something to this idea." Simpson and colleagues have now received a $1 million grant to study grains of sand made from calcium carbonate and called ooids, since their diameter "could be a proxy measurement of Earth's temperature for the last 2.5 billion years," according to the university's article. Geologist Lizzy Trower says the research "can tell us something about the chemistry and water temperature in which they formed." And more importantly, "Does the fossil record agree with the predictions we would make based on this theory from this new record of temperature?" Trower and Simpson's work also has potential implications for the human quest to find life elsewhere in the universe, Trower said. If extremely harsh and cold environments can spur evolutionary change, "then that is a really different type of thing to look for in exoplanets (potentially life-sustaining planets in other solar systems), or think about when and where (life) would exist."

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