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 that burns oxygen in every complex cell alive today. The host was an archaeon, the lodger an alphaproteobacterium.

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 dates it by 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

1. Roger et al. (2017), Current Biology · The origin and diversification of mitochondria
2. Pittis & Gabaldón (2016), Nature · Late acquisition of mitochondria by a host with chimeric prokaryotic ancestry
3. 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