Sex 1.2 bya

Eukaryotic cells invented sex about 1200 million years ago. During the simplest form of sexual production, two cells fuse together to create a single cell with two sets of chromosomes.

These chromosomes swap some genes to create new chromosomes which have the genes of both parent cells mixed together.

The two pairs of then chromosomes split apart to make two individual cells each with one set of chromosomes. Evolution selected the best of these cells to go on to make the next generation.

By swapping genes in this way to make cells with new (and probably random) mixtures of genes, sex helped beneficial mutations to spread quickly, and speeded up evolution because it provided new combinations of genes which might confer new benefits.

Diploid

The most advanced eukaryotes kept two pairs of chromosomes through most of their lives

A cell like this is called diploid. This let evolution speed up even more.

Fertilization

In later cells, two types of sex cells evolved. Some were small, could swim, but contained little food. These are called sperm. Other sex cells were large, carried food but could not swim. These are called eggs. The sperm swam to the egg. When a sperm enters an egg it is called fertilization.

Fertilization of sea urchin egg