Life › Complex cells & first multicellular life › Eukaryotes: complex cells with a nucleus
Sex & meiosis
in early eukaryotes; clear by c. 1 billion years ago· Evolutionary innovation· Microbes & cells
Strong evidenceGood evidence; dates or details still being refined.
Reproduction by mixing genes from two parents, built on meiosis, a cell division that halves the chromosomes.
See it on the Life map →What happened
In meiosis a cell copies its chromosomes, swaps segments between matching pairs, then divides twice to make cells with half the usual set, such as eggs, sperm or spores. Fertilization joins two of these to restore the full set. The core meiosis genes occur across all major eukaryote groups, so the last common ancestor of eukaryotes probably already had sex. The oldest fossil evidence is Bangiomorpha, about 1.05 billion years old.
Why it matters
Shuffling genes every generation creates variety for natural selection and helps clear out harmful mutations. It also produces the inheritance patterns Mendel discovered.
The evidence (fossils, genes, rocks)
Shared meiosis genes (such as SPO11) across eukaryotes, sexual life cycles in living groups, and distinct reproductive cells in Bangiomorpha fossils.
Still debated
Sex is costly: an asexual female can produce twice as many daughters. Whether parasites, mutation-clearing or something else keeps it worthwhile is debated.
Connections (3)
▸Eukaryotes: complex cells with a nucleus evolved Sex & meiosisStrong evidence
The connection
Meiosis appears to be a modified version of ordinary eukaryotic cell division, made possible by eukaryotes' nucleus and linear chromosomes. Because its core genes occur in nearly all eukaryote groups, sex probably evolved once, in their common ancestor, rather than separately in each group.
When and how
In early eukaryotes, likely well before about 1.05 billion years ago; the shared-gene evidence comes from genome surveys since the 2000s.
How sure are we?
Meiosis-specific genes such as SPO11, DMC1 and HOP1 turn up even in single-celled eukaryotes once thought asexual, such as Giardia.
Debates and open questions
Whether meiosis grew out of ordinary cell division, out of DNA-repair systems inherited from microbes, or both, and why early eukaryotes adopted it.
▸Bangiomorpha: the oldest firmly identified red alga is the oldest clear evidence of Sex & meiosisStrong evidence
The connection
Bangiomorpha filaments contain different kinds of reproductive cells, resembling the separate spore types that living red algae make during sexual life cycles. That makes it the oldest fossil evidence of sexual reproduction, about 1.05 billion years ago.
When and how
Hunting Formation, Somerset Island, Nunavut; Nicholas Butterfield's 2000 study gave it the species name 'pubescens' for its apparent sexual maturity.
How sure are we?
Comparison with living Bangia, whose life cycle is well known; the interpretation rests on cell shapes, not on preserved chromosomes.
Debates and open questions
Fossils cannot show meiosis directly, so sex here is inferred. Sex itself is probably older, dating back to the first eukaryotes.
▸Gregor Mendel found the inheritance rules produced by Sex & meiosisWell established
The connection
Mendel found that each plant passes one of its two 'factors' per trait to each offspring, at random, and that different traits sort independently. In 1902–1903 Walter Sutton and Theodor Boveri saw why: in meiosis, the cell division that makes eggs and sperm, paired chromosomes separate, one copy to each cell, and different pairs separate independently.
When and how
Mendel's pea crosses in Brno, 1856–1863; Sutton (Columbia University, grasshopper cells) and Boveri (Würzburg, sea urchins) linked them to chromosomes in 1902–1903.
The evidence
Thomas Hunt Morgan's fruit fly work from 1910 tied specific traits to specific chromosomes; today genes can be tracked through meiosis directly.
Debates and open questions
Genes close together on one chromosome do not sort independently; whether Mendel's seven traits avoided that by luck or careful choice is debated.