Aug 11, 2024 08:24 AM
(This post was last modified: Aug 11, 2024 05:41 PM by C C.)
https://www.wired.com/story/the-physics-...plex-life/
EXCERPTS: It is difficult to precisely date when animals arose, but an estimate from molecular clocks—which use mutation rates to estimate the passage of time—suggests that the last common ancestor of multicellular animals emerged during the era known as the Sturtian Snowball Earth, sometime between 717 million and 660 million years ago. Large, unmistakably multicellular animals appear in the fossil record tens of millions of years after the Earth melted following another, shorter Snowball Earth period around 635 million years ago.
[...] In 2021, he [Simpson] published his hypothesis that Snowball Earth viscosities would have put a significant strain on organisms’ ability to feed themselves and could have spurred some to evolve multicellularity. Then, with collaborators at the Santa Fe Institute, he designed mathematical models of small creatures—single cells that fed by diffusion and self-propelling cells that fed by moving around—living in thicker and thicker fluids. In the models, posted to biorxiv.org at the end of 2023 and recently published in the peer-reviewed Proceedings of the Royal Society B, the diffusion feeders responded to thicker fluids by shrinking in size. The self-propelling cells, equipped by the equations with the ability to cling together if needed, formed larger and larger multicellular groups. This suggested that if there were already multicellular organisms when Snowball Earth occurred—or at least organisms with the ability to take on multicellular forms—the thicker fluid could have given them a reason to get bigger.
The results were intriguing, but they were only computer models. Simpson thought: Well, what if they did this with real organisms?
The geologist Boswell Wing, a colleague at the University of Colorado, Boulder, had a colony of Chlamydomonas reinhardtii in his lab. These algae have twirling flagella that allow them to move under their own power. They are usually unicellular. But they can switch into a multicellular form under certain stressful conditions. Would higher viscosity, like that of the oceans during Snowball Earth, prove to be one of them?
Life in Thick Water
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.
[...] 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.
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. “I was really pleased,” he said. “With the coarse mathematical framework, there were a few predictions I could make. To actually see it empirically means there’s something to this idea.”
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... (MORE - missing details)
EXCERPTS: It is difficult to precisely date when animals arose, but an estimate from molecular clocks—which use mutation rates to estimate the passage of time—suggests that the last common ancestor of multicellular animals emerged during the era known as the Sturtian Snowball Earth, sometime between 717 million and 660 million years ago. Large, unmistakably multicellular animals appear in the fossil record tens of millions of years after the Earth melted following another, shorter Snowball Earth period around 635 million years ago.
[...] In 2021, he [Simpson] published his hypothesis that Snowball Earth viscosities would have put a significant strain on organisms’ ability to feed themselves and could have spurred some to evolve multicellularity. Then, with collaborators at the Santa Fe Institute, he designed mathematical models of small creatures—single cells that fed by diffusion and self-propelling cells that fed by moving around—living in thicker and thicker fluids. In the models, posted to biorxiv.org at the end of 2023 and recently published in the peer-reviewed Proceedings of the Royal Society B, the diffusion feeders responded to thicker fluids by shrinking in size. The self-propelling cells, equipped by the equations with the ability to cling together if needed, formed larger and larger multicellular groups. This suggested that if there were already multicellular organisms when Snowball Earth occurred—or at least organisms with the ability to take on multicellular forms—the thicker fluid could have given them a reason to get bigger.
The results were intriguing, but they were only computer models. Simpson thought: Well, what if they did this with real organisms?
The geologist Boswell Wing, a colleague at the University of Colorado, Boulder, had a colony of Chlamydomonas reinhardtii in his lab. These algae have twirling flagella that allow them to move under their own power. They are usually unicellular. But they can switch into a multicellular form under certain stressful conditions. Would higher viscosity, like that of the oceans during Snowball Earth, prove to be one of them?
Life in Thick Water
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.
[...] 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.
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. “I was really pleased,” he said. “With the coarse mathematical framework, there were a few predictions I could make. To actually see it empirically means there’s something to this idea.”
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... (MORE - missing details)