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where did eukaryotic cells come from: The Theory

Endosymbiotic Theory of Evolution of Eukaryotic Cells

THE THEORY: The mitochondria and chloroplasts of Eukaryotic cells arose from the symbiotic incorporation of prokaryotic cells by a proto-eukaryotic cell.


More than 4 billion years ago, it is argued, primitive cells had one of at least three modes of nutrition, they could invaginate food particles, photosynthesise or absorb free floating organic nutrients in their environment. The theory argues that cells able to respire aerobically were incorporated into cells that invaginate their food. The resultant symbiosis had the capacity to respire. Maintenance of the symbiosis required the two cell types to reproduce at about the same rate, and that one not digest the other.

Plant (and algal) cells arose when photosynthetic prokaryotic cells were incorporated into one line of the respiring cell. The result, given the same proviso, is that the three – way symbiosis could now fix its own carbon using light energy, and respire.

 View animation of eukaryotic evolution (170 Kb).

Eukaryotic cells originated more than 0.6 billion years ago. The mitochondria (and plastids) of extant eukaryotic cells are remarkably similar suggesting aerobic bacteria (or cyanobacteria) were incorporated on only one occasion each. We might assume that the evolution of the Eukaryotic cell was a most unusual event because an amazing array of other cellular characteristics has evolved over the same period.

The theory has been propounded by many people since Darwin, popularized by L. Margulis, and is now supported by detailed experimental testing.