Sexual reproduction did not originate suddenly in complex animals but evolved gradually in early eukaryotes, likely between 1.2 and 2 billion years ago. The process began with simpler genetic exchange mechanisms—such as horizontal gene transfer in bacteria and archaea—and evolved into the meiosis and syngamy (cell fusion) characteristic of modern eukaryotes. [1]

Several complementary hypotheses explain why sex evolved despite its costs. The most prominent is DNA repair: when cells fuse or pair their chromosomes, they can use the homologous copy as a template to repair damage, especially valuable under stress from reactive oxygen species produced by newly acquired mitochondria. [2] Another major driver is mutation purging: sexual recombination creates new genetic combinations, some free of harmful mutations that accumulate irreversibly in purely asexual lineages (Muller's Ratchet). [3] A third advantage is the "Red Queen" hypothesis—genetic variation from recombination helps populations adapt faster to changing environments and parasites. [4]

Fossil evidence supports this ancient origin. The red alga Bangiomorpha pubescens, dating to approximately 1.05 billion years ago, shows differentiated reproductive structures consistent with sexual reproduction, making it the oldest known taxonomically resolved eukaryote with clear evidence of sex. [2] The core molecular machinery of meiosis is highly conserved across all eukaryotic lineages, suggesting a single ancestral origin in the last eukaryotic common ancestor rather than multiple independent origins. [5]

Over evolutionary time, simple mating types in single-celled organisms evolved into specialized gametes—sperm and eggs—and eventually distinct sexes with complex reproductive behaviors. While asexual reproduction is faster and more efficient, the genetic diversity provided by sex became a critical driver for the evolution of complex, multicellular life on Earth.