EVOLUTION × GENETICS × LIFE
The Cell That Swallowed a Bacterium
~1.9 billion years ago (order of events debated)
Proterozoic · Paleoproterozoic · Orosirian
A bacterium ended up inside another cell, and stayed. Over generations it handed most of its genome to its host's nucleus, kept a small chromosome of its own, and became the mitochondrion, the organelle in which complex cells burn oxygen. The host was an archaeon; the lodger an alphaproteobacterium, or a bacterium from a lineage that split off just before that group.
Why it matters
One cell entering another, just once, is the reason there are organisms large enough to see.
Dating & uncertaintyDisputed
The merger itself leaves no fossil. Phylogenomics reads it in the branch lengths of the proteins it brought, and disagrees on the order of events: whether mitochondria entered a simple host and drove the rest, or joined a cell that was already complex.
Sources
- Roger et al. (2017), Current Biology · The origin and diversification of mitochondria
- Pittis & Gabaldón (2016), Nature · Late acquisition of mitochondria by a host with chimeric prokaryotic ancestry
- Martijn et al. (2018), Nature · Deep mitochondrial origin outside the sampled alphaproteobacteria
Earth history as one day
13:57:2110 h 3 min before midnight
if Earth’s 4.54 billion years were compressed into a single day
See it on the timeline