Evolution: multicellularity from snowball Earth (Evolution)

by David Turell , Wednesday, July 24, 2024, 18:45 (85 days ago) @ David Turell

A new reasonable theory:

https://www.quantamagazine.org/the-physics-of-cold-water-may-have-jump-started-complex-...

"To very small single-celled creatures, thick seawater would have posed some big problems. Bacteria feed by diffusion — the movement of nutrients through water from areas of high concentration to low concentration — and tend to wait for food to come to them. However, at low temperatures, diffusion slows down. Nutrients don’t travel as quickly or as far. For cells, living in a cold and more viscous fluid means getting less to eat. Even very small organisms that can propel themselves, such as cells with flagella, move more slowly in cold water. As a result, they encounter food less frequently.

"A bigger organism, on the other hand, can navigate thicker waters without much trouble. A cluster of cells has the benefit of inertia: Their combined mass is large enough to allow them to build up steam and barrel through thicker fluid. “At some point, you are too big for this to matter,” Simpson said.

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"There’s no way for biologists to travel back in time to test the real conditions of Snowball Earth, but they can try to re-create aspects of them in the lab. In an enormous, custom-made petri dish, 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. 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.

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"Simpson was particularly curious as to whether algae that made it into the highest viscosity ring would find ways to increase their swimming speed. The algae are photosynthetic, so they get energy from the sun. But they need to pick up nutrients such as phosphorus from the environment, so movement is still important to their survival. Maintaining the same level of nutrients in high-viscosity surroundings would require them to find a way to keep up their speed.

"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. (my bold)

Comparing the speed of these clusters to the single cells in the middle revealed something interesting. “They all swim at the same speed,” Simpson said. By working together as a collective, the algae could preserve their mobility.

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Intriguingly, when the scientists took these little clusters from the high-viscosity gel and put them back at low viscosity, the cells stuck together. They remained this way, in fact, for as long as the scientists continued to watch them, about 100 more generations. Clearly, whatever changes they underwent to survive at high viscosity were hard to reverse, Simpson said — perhaps a move toward evolution rather than a short-term shift.

Modern-day algae are not early animals. But the fact that these physical pressures forced a unicellular creature into an alternate way of life that was hard to reverse feels quite powerful, Simpson said. He suspects that if scientists explore the idea that when organisms are very small, viscosity dominates their existence, we could learn something about conditions that might have led to the explosion of large forms of life.

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From the time Simpson first realized that such limits on movement could be a monumental obstacle to microscopic life, he hasn’t been able to stop thinking about it. Viscosity may have mattered quite a lot in the origins of complex life, whenever that was. (my bold)

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it’s difficult to say how much importance to assign to this era. Butterfield also remarked on this uncertainty: “There’s no evidence of anything getting large until quite a bit after [Snowball Earth].”

That said, Brocks found Simpson’s experiment quite clever and beautiful. The fact that organisms might respond to high viscosity by developing collective behavior deserves to be better understood, he said — whether Snowball Earth led to the evolution of complex animal life or not.

Comment: A revolutionary idea to explain how multicellular life arrived. God, as designer, could have directed this method.