GENETICS × EVOLUTION

The Origin of Sexual Reproduction

~1.2 billion years ago

Proterozoic · Mesoproterozoic · Stenian

Sexual reproduction, in which meiosis halves the genome of a cell and fertilisation restores it, is at least 1.2 billion years old, and the genes that run meiosis occur throughout the eukaryotes, the organisms made of complex cells.

Sexual reproduction consists of two steps that reverse one another. Meiosis halves the genome of a cell, the genome being its full set of genetic material. Fertilisation merges two such cells and restores the genome. The offspring is therefore a recombination of two parents and not a copy of one. The genes that run meiosis occur throughout the eukaryotes. They occur even in lineages long classed as asexual, such as Giardia, a single-celled parasite. Five of these genes have no other function, so their presence cannot be explained by another use. The machinery of meiosis was therefore already assembled in the common ancestor of all complex cells. The earliest eukaryotes were probably clonal most of the time, that is, they multiplied by copying themselves, and sex occurred as an episode induced by stress.

Why it matters

Sexual reproduction appears disadvantageous: it takes two parents to make one descendant, and half of each parent's genome is discarded. One explanation holds that it began as a mechanism of repair rather than as a mode of reproduction. The cell that incorporated the ancestor of the mitochondrion, the compartment that uses oxygen to supply a complex cell with energy, incorporated with it an oxygen chemistry that damages DNA. The machinery of meiosis, which now recombines genomes, appears to derive from the systems that repaired that damage. Every plant, animal and fungus inherited this machinery.

Dating & uncertainty

The core meiosis genes are shared by lineages with no known sexual cycle, so the machinery of meiosis was already in place in the last common ancestor of eukaryotes. The oldest fossil evidence of sexual reproduction is the red alga Bangiomorpha, around 1.05 billion years old. The uncertainty on the date is the interval between that ancestor and this fossil.

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