Every animal begins with a partnership between two single cells — and from that tiny union comes a body, a mind, and a life.

Eukaryotes are the plants, fungi, and animal direct-line common ancestor. That’s what makes Eukaryotes so special when studying modern life. We even have their single-celled descendants in protists today!

LECA is the name for the Last Eukaryotic Common Ancestor. That’s the ancestral population from which all modern eukaryotes descended. And this is the group that includes sexual reproduction. This is when and where the core machinery evolved. This is a time long ago:  1.75 billion years ago. We know because of the meiotic genes across widely separated groups, including red algae, green plants, fungi, and animals. This is when their shared ability to recombine and reshuffle DNA evolved.

Bacteria and viruses also replicate, and bacteria can exchange genetic material, but they do not reproduce through meiosis and the fusion of specialized reproductive cells. The eukaryotic sexual cycle introduced something more systematic: genetic material from compatible cells could be combined, recombined, and be passed into new lineages. This marked a major evolutionary transition from copying DNA to reshuffling it.

These early sexual systems were not divided into male and female. There were, and are, many more sexes than just two. Many single-celled eukaryotes used mating types, which are genetically defined compatibility groups. A species may have two mating types, but it may also have three, seven, dozens, or even thousands of compatible combinations. However, there might be a tendency for evolution to evolve toward male and female. The familiar male-female system evolved later in some lineages, especially animals and many plants.

Fish, wolves, and humans as well as many plants, all begin the same way: two tiny cells, and then the long miracle of becoming.