#1. The Origin of the Eukaryotes [evolution]

EV

Red, theory; black, fact

A fossil (oncoid or stromatolite) of a colony of prokaryotes. Photographed in situ.

The eukaryotic cell may have arisen from a clonal array of prokaryotes that selectively lost some of its internal partition walls while following "the colony path to complexity."* The remaining partitions gave rise to the internal membrane systems of present-day eukaryotes. Those prokaryote colonists specializing in chemiosmotic processes such as oxidative phosphorylation and photosynthesis could not lose any of their delimiting walls because of the need to maintain concentration gradients, so they remain bacterium-like in morphology to this day. This is an alternative to the phagocytotic theory of the origin of mitochondria and chloroplasts. Modern blue-green algae genetically resemble the DNA in chloroplasts, and modern aerobic bacteria have genetic resemblances to the DNA in mitochondria, but this is not necessarily differential support for the phagocytosis theory. The resemblances can be accounted for by convergent evolution or by the existence of an ancestor common to the modern organisms and the ancient colony formers I suppose here.

The transition of the colony of prokaryotes to the eukaryotic cell would have required the breakup of a continuous system of negative spaces located between the colonists into a discontinuous system of positive spaces that forms the modern internal membrane structures. The negative spaces would originally have served as passages for trafficking substances to and from the colony’s outer surface. The first step in the breakup would have been the evolution of fenestrations in the double partition walls that allowed direct communication between the cytoplasms of two adjacent colonists. A pre-adaptation for fenestration may have been a capacity for bacterial conjugation. Progressive multiplication and widening of the fenestrations would then reduce the negative spaces to a three-dimensional network of tubular channels requiring mechanical reinforcement by a proto-cytoskeleton. In the final step, the tube network breaks up into vesicles being transported along the cytoskeleton by motor proteins: the modern situation.

 

Going even further back in time, a capacity for conjugation may be the evolutionary vestige of a capacity for complete merger of two hypothetical pre-prokaryotes, each possessing only part of the genetic instructions needed for a complete metabolism. The viruses may have emerged at this stage.

Prior to that, metabolic completion would have happened by diffusion of small-molecule metabolic intermediates in the extracellular fluid, basically at the ecosystem level.

The prokaryote colonies that became the eukaryotes would have originally reproduced by sporulation, not mitosis, which would have come later. The "spores" would be actively-metabolizing prokaryotes and before growing into further colonies, would be subject to natural selection. In the spore phase, the rapid evolvability of typical prokaryotes would have been recovered, allowing the formation of large, slow-growing colonies without sacrifice of the high evolvability of the original solitary prokaryotes. Modern-day eukaryotes often secrete tiny bodies called exosomes containing all the macromolecules of life. Exosomes may be the evolutionary vestige of the sporulation phase of the original eukaryotes.

* phrase by E. O. Wilson, from Sociobiology: the New Synthesis, 1975.