Even the simplest cells are structurally and functionally complex. So how could such
complexity arise through the process of evolution?
Answers
At present, the most widely accepted theory is that mitochondria derive from a bacterium that was engulfed by an archaeon (plural = archaea), a kind of prokaryote that looks similar to a bacterium but has many molecular differences. The eukaryotic membrane systems, including the nuclear envelope, then formed within the boundaries of this archaeal cell through the invagination of the outer membrane. This fits with much current data, but a few problems remain. Most significantly, no archaeal cells are known that invaginate membranes.
Furthermore, it seems unlikely that mitochondria were engulfed since engulfing food requires a lot of energy, which in eukaryotes is provided by mitochondria, and engulfment likely also requires mitochondrial-derived lipids.