6.13: An evolutionary scenario for the origin of eukaryotic cells

When we think about how life arose, and what the first organisms looked like, we are moving into an area where data is fragmentary and speculation is rampant. These are also, dare we remind you, events that took place billions of years ago. But such obstacles do not mean we cannot draw interesting, albeit speculative conclusions – there is relevant data present in each organisms’ genetic data (its genotype), the structure of its cells, and their ecological interactions. This is 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 (see above). They use complex “cytoskeletal” and “cytomuscular” systems to drive these thermodynamically unfavorable behaviors (again, largely beyond our scope here). 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 the common ancestral organism. But 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 built through evolutionary processes before it would be useful. If we assume that the original organisms arose in an osmotically friendly, that is, 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 (dilute) environments - like most environments today.

For example, one plausible scenario is that the ancestors of the bacteria and the archaea developed cell walls originally as a form of protection against predation. So who were the predators? Where they the progenitors of the eukaryotes? If so, we might conclude that organisms in the eukaryotic lineage never had a cell wall, rather than that they had one once and subsequently lost it. In this scenario, the development of eukaryotic cell walls by fungi and plants represents an example of convergent evolution and that these structures are analogous (rather than homologous) to the cell walls of prokaryotes (bacteria and archaea).

But now a complexities 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? One approach is to actively pump the water that flows into them back out using an organelle known as a contractile vacuole. Water accumulates within the contractile vacuole, a membrane-bounded structure within the cell; as the water accumulates the contractile vacuole inflates. To expel the water, the vacuole connects with the plasma membrane and is squeezed out by the contraction of a cytomuscular system. This squirts the water out of the cell. The process of vacuole contraction is an active one, it involves work and requires energy. One might speculate that such as cytomuscular system was originally involved in predation, that is, enabling the cell to move its membranes, to surround and engulf other organisms (phagocytosis). The resulting vacuole became specialized to aid in killing and digesting the engulfed prey. When digestion is complete, it can fuse with the plasma membrane to discharge the waste, using either a passive or 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; distinctly different systems than is used prokaryotes¹⁸⁷. So which came first, different cell division mechanisms, which led to differences in membrane behavior, with one leading to a predatory active membrane and the other that led to a passive membrane, perhaps favoring the formation of a cell wall?

Contributors and Attributions

• Michael W. Klymkowsky (University of Colorado Boulder) and Melanie M. Cooper (Michigan State University) with significant contributions by Emina Begovic & some editorial assistance of Rebecca Klymkowsky.