LIFE × EVOLUTION
The First Eukaryotes
~2.0–1.8 billion years ago
Proterozoic · Paleoproterozoic · Statherian
The eukaryotes, organisms whose cells have a nucleus enclosing their chromosomes, appeared between about 2.0 and 1.8 billion years ago, when everything alive was still a bacterium or an archaeon, two kinds of cell without a nucleus.
Eukaryotes are organisms whose cells possess a nucleus, that is, a membrane-bounded compartment enclosing the chromosomes. Until their appearance, everything alive was a cell without a nucleus, bacterium or archaeon. The eukaryotic cell is larger than these cells. It has a cytoskeleton, a network of internal filaments that gives the cell its shape and moves its contents. It has membrane compartments, among which its chemical reactions are distributed. It can engulf whole cells. The eukaryotic cell arose from a single event, the association of an archaeon with a bacterium. That bacterium became the mitochondrion, a compartment of the eukaryotic cell. Every organism visible to the naked eye, animal, plant, fungus or alga, is built only from eukaryotic cells. The oldest fossils of eukaryotes that are universally accepted are about 1.6 billion years old, which is later than their appearance.
Why it matters
Bacteria have carried out the chemistry of the planet for four billion years, but the largest body they have built is a mat, a thin layer of cells growing together. Every larger organism is made of eukaryotic cells.
Dating & uncertainty
Estimates of the age of the last common ancestor of living eukaryotes range from 2.2 to 1.6 billion years. The spread comes from the methods. Molecular clocks date an ancestor from the differences accumulated between the genes of its descendants. Their models differ, and so do the fossil calibrations, the dated fossils that set the clocks. Researchers also dispute whether ancient microfossils, fossils of microscopic size, belong to eukaryotes.
Earth history as one day
14:29:049 h 31 min before midnight
if Earth’s 4.54 billion years were compressed into a single day