Eukaryotic Origins

Section Summary

The oldest fossil evidence of eukaryotes is about 2 billion years old. Fossils older than this all appear to be prokaryotes. It is probable that today’s eukaryotes are descended from an ancestor that had a prokaryotic organization. The last common ancestor of today’s Eukarya had several characteristics, including cells with nuclei and an endomembrane system (which includes the nuclear envelope). Its chromosomes were linear and contained DNA associated with histones. The nuclear genome seems to be descended from an archaean ancestor. This ancestor would have had a cytoskeleton and divided its chromosomes mitotically.

The ancestral cytoskeletal system included the ability to make cilia/flagella during at least part of its life cycle. It was aerobic because it had mitochondria derived from an aerobic alpha-proteobacterium that lived inside a host cell. Whether this host had a nucleus at the time of the initial symbiosis remains unknown. The last common ancestor may have had a cell wall for at least part of its life cycle, but more data are needed to confirm this hypothesis. Today’s eukaryotes are very diverse in their shapes, organization, life cycles, and number of cells per individual.