6.12: An evolutionary scenario for the origin of eukaryotic cells

When we think about how life arose, and what the first organisms might have looked like, we are moving into an area where data is fragmentary or unobtainable and speculation is rampant. These events took place billions of years ago. But there is relevant data present in each organisms’ genetic data, its genome, the structure of its cells, and their ecological interactions. It is this type of data that can inform and constrain our speculations.

Animal cells do not have a rigid cell wall. Its absence allows them to be active predators, moving rapidly and engulfing their prey whole or in macroscopic bits through phagocytosis. They use complex “cytoskeletal” and “cytomuscular” systems to drive these thermodynamically unfavorable behaviors (→). Organisms with a rigid cell wall can't perform such functions. Given that bacteria and archaea have cell walls, it is possible that cell walls were present in their common ancestor. This leads us to think more analytically about the nature of the earliest organisms and the path back to the common ancestor. A cell wall is a complex structure that would have had to be developed and assembled through evolutionary processes before it would be useful. If we assume that the original organisms arose in an osmotically friendly, non-challenging environment, then a cell wall could have been generated in steps, and once adequate it could enable the organisms that possessed it to invade new, more osmotically challenging environments. Another plausible scenario might be that the ancestors of the bacteria and archaea originally developed cell walls as a form of protection against predators. So who were these predators? Were they the progenitors of the phagocytic eukaryotes? If so, it might be that organisms in the eukaryotic lineage never had a cell wall, and that neither did the ancestor of bacteria and archaea. In this scenario, the development of eukaryotic cell walls by fungi and plants represents an example of convergent evolution. These structures would then be analogous rather than homologous to the cell walls of prokaryotes (bacteria and archaea).

But now a complexity arises, there are plenty of eukaryotic organisms, including microbes like the amoeba, that live in osmotically challenging environments. How do they deal with the movement of water into their cells? How could they follow their prey into the non-salty world? One approach is to actively pump the water that flows in back out using membrane pumps or perhaps an organelle similar to a "contractile vacuole". Water accumulates within a membrane-bounded intracellular membrane vesicle. As water accumulates the vesicle inflates. To expel this water, the vesicle connects with the plasma membrane and is squeezed by the contraction of a cytomuscular system; as a result the vesicle's contents are squirting out of the cell. The process of vacuole contraction is active; it involves work and requires energy.^[257] One might speculate that such as cytomuscular (contractile) system might originally been involved in enabling a cell to move its membranes so to surround and engulf other organisms (phagocytosis). The resulting vacuole became specialized in killing and digesting the engulfed prey. When digestion was complete, this micro-stomach could fuse with the plasma membrane to discharge the waste, a process involving an active contractile system. It turns out that the molecular systems involved in driving active membrane movement are related to the systems involved in dividing the eukaryotic cell into two during cell division; a system distinctly different from that used by prokaryotes.^[258] So which came first, distinct cell division mechanisms that led to differences in membrane behavior, with one leading to a predatory active membrane and the other to a passive membrane, perhaps favoring the formation of a cell wall? At this point it is hard (impossible?) to know for sure, we might best be agnostic while still able to make scientifically sensible speculation.

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