A New Aspect to the Origin and Evolution of Eukaryotes

Abstract

One of the most important omissions in recent evolutionary theory concerns how eukaryotes could emerge and evolve. According to the currently accepted views, the first eukaryotic cell possessed a nucleus, an endomembrane system, and a cytoskeleton but had an inefficient prokaryotic-like metabolism. In contrast, one of the most ancient eukaryotes, the metamonada Giardia lamblia, was found to have formerly possessed mitochondria. In sharp contrast with the traditional views, this paper suggests, based on the energetic aspect of genome organization, that the emergence of eukaryotes was promoted by the establishment of an efficient energy-converting organelle, such as the mitochondrion. Mitochondria were acquired by the endosymbiosis of ancient alpha-purple photosynthetic Gram-negative eubacteria that reorganized the prokaryotic metabolism of the archaebacterial-like ancestral host cells. The presence of an ATP pool in the cytoplasm provided by this cell organelle allowed a major increase in genome size. This evolutionary change, the remarkable increase both in genome size and complexity, explains the origin of the eukaryotic cell itself. The loss of cell wall and the appearance of multicellularity can also be explained by the acquisition of mitochondria. All bacteria use chemiosmotic mechanisms to harness energy; therefore the periplasm bounded by the cell wall is an essential part of prokaryotic cells. Following the establishment of mitochondria, the original plasma membrane-bound metabolism of prokaryotes, as well as the funcion of the periplasm providing a compartment for the formation of different ion gradients, has been transferred into the inner mitochondrial membrane and intermembrane space. After the loss of the essential function of periplasm, the bacterial cell wall could also be lost, which enabled the naked cells to establish direct connections among themselves. The relatively late emergence of mitochondria may be the reason why multicellularity evolved so slowly.

The Dispersed Archaeal Eukaryome and the Complex Archaeal Ancestor of Eukaryotes

“…The origin of these genes from methanogens also is compatible with the results of some phylogenetic analyses (Moreira and Lopez-Garcia 1998;Horiike et al 2004). Others have argued that the archaeal ancestor of eukaryotes lies outside the known diversity of Archaea, in accord with the standard model, on the basis of biological considerations (Vellai et al 1998) or phylogenetic analysis results (Hedges et al 2001;Tekaia and Yeramian 2005;Ciccarelli et al 2006;Fukami-Kobayashi et al 2007). …”

Origin and Evolution of the Mitochondrial Proteome

“…In particular, it was recognized that a free-living bacterium such as Paracoccus would probably not be able to actively transport ATP to a prospective host because bacteria do not in general have ATP exporters (10,20,124,185). Two additional observations, to which we return below, are relevant.…”

Multiple secondary origins of the anaerobic lifestyle in eukaryotes

“…The presence of this eubacterial endosymbiont allowed the host to colonize aerobic environments. A variety of mechanisms have been suggested as to why this should be so; the most popular, and one that features in a variety of different formulations for mitochondrial origins (Woese 1977;Martin & Mü ller 1998;Vellai et al 1998;Kurland & Andersson 2000), being the removal of cytosolic oxygen by the endosymbionts' respiratory activity. The feasibility of this idea, at least as a mechanism, is supported by a modern analogue among contemporary ciliate protozoa.…”