GENETICS × EVOLUTION

The Origin of Sex

~1.2 billion years ago

Proterozoic · Mesoproterozoic · Stenian

Sex is two steps that undo each other: meiosis halves a cell's genome, fertilisation restores it, and the offspring is a reshuffle of two parents rather than a copy of one. The genes that run meiosis turn up right across the eukaryotes, including lineages long filed as celibate, such as Giardia; five of them do nothing else. The machinery was therefore already assembled in the common ancestor of all complex cells, and the earliest eukaryotes were probably clonal most of the time, with sex an episode brought on by stress.

Why it matters

Sex looks like a bad deal: two parents for one offspring, and half of each genome discarded. One answer is that it did not begin as a way of making offspring at all, but as repair. The cell that took in the ancestor of the mitochondrion took in with it a source of oxygen chemistry that damages DNA, and the machinery that now shuffles genomes appears to have grown out of the systems that mended that damage. Every plant, animal and fungus inherited it.

Dating & uncertainty

The core meiosis genes are shared by lineages with no known sexual cycle, so the machinery was already in place in the last common ancestor of eukaryotes; the oldest fossil evidence of sexual reproduction is the red alga Bangiomorpha at about 1.05 Ga. The interval between those two anchors is the uncertainty.

Sources

1. Ramesh et al. (2005), Current Biology · A phylogenomic inventory of meiotic genes: evidence for sex in Giardia and an early eukaryotic origin of meiosis
2. Goodenough & Heitman (2014), Cold Spring Harbor Perspectives in Biology · Origins of eukaryotic sexual reproduction
3. Speijer et al. (2016), Philosophical Transactions of the Royal Society B · What can we infer about the origin of sex in early eukaryotes?

Earth history as one day

17:39:226 h 21 min before midnight

if Earth’s 4.54 billion years were compressed into a single day