Evolution of cells

See also: Eukaryotes, Archaea, Bacteria

"The evolution of modern cells is arguably the most challenging and important problem the field of Biology has ever faced. In Darwin's day the problem could hardly be imagined. For much of the 20th century it was intractable. In any case, the problem lay buried in the catch-all rubric "origin of life"---where, because it is a biological not a (bio)chemical problem, it was effectively ignored. Scientific interest in cellular evolution started to pick up once the universal phylogenetic tree, the framework within which the problem had to be addressed, was determined (refs. 3 and 4; Fig. 1). But it was not until microbial genomics arrived on the scene that biologists could actually do much about the problem of cellular evolution." (Carl Woese, 2002)

The common ancestor of eukaryotes, bacteria, and archaea may well have been a community of organisms that readily exchanged components and genes. It would have contained:

• Autotrophs that produced organic compounds from CO2 either photosynthetically or by inorganic chemical reactions;
• Heterotrophs that obtained organics by leakage from other organisms;
• Saprotrophs that absorbed nutrients from decaying organisms; and phagotrophs that were sufficiently complex to envelop and digest prey.

How the current lineages of microbes evolved from this postulated community is currently unsolved but subject to intense research by biologists, stimulated by the great flow of new discoveries emerging from genome science.

Hypothesis for the origin of eukaryotic (nucleated) cells, from fusion of two distinct non-nucleated prokaryotes that were partners of a postulated ancient symbiosis.

Instead of relying a single gene such as the SSU rRNA gene to reconstruct early evolution, or a few genes, scientific effort has shifted to exploiting the comprehensive information from the many complete genome sequences of organisms that are now available.

It is now clear that trees based only on SSU rRNA alone do not capture the events of early eukaryote evolution accurately, and the origins of the first nucleated cells are still uncertain. For instance, careful analysis of the complete genome of the eukaryote yeast shows that many of its genes are more closely related to bacterial genes than they are to archaea, and it is now clear that archaea were not the simple progenitors of the eukaryotes, in stark contradiction to the findings based on SSU rRNA and limited samples of other genes.

One imaginative but disputed hypothesis getting some support from recent computer-assisted studies of complete genome DNA sequences is that the first nucleated cell arose from two distinctly different ancient prokaryotic (non-nucleated) species that had formed a symbiotic relationship with one-another to carry out different aspects of metablism. One partner of this postulated symbiosis is proposed to be a true bacterial cell, and the other an archaean cell. It is postulated that this symbiotic partnership progressed via the cellular fusion of the partners to generate a chimeric or hybrid cell with a membrane bound internal structure that was the forerunner of the nucleus. The next stage in this scheme was transfer of both partner genomes into the nucleus and their fusion with one-another. Several variations of this hypothesis for the origin of nucleated cells have been suggested,

But some biologists dispute this conception and argue for the need for a shift in conceptual framework if this problem is to be solved - and point out that early living communities would compise many different entities to extant cells, and would have shore their genetic material more extensively than current microbes.