Common ancestor

A common ancestor is a species, population or individual that two or more descendant lineages share somewhere in their past. Humans and chimpanzees, for example, both descend from an ape species that lived in Africa roughly six to eight million years ago. Going further back, humans share common ancestors with mice, with fish and eventually with every living thing, since all known life is thought to trace back to a single early population of cells, often called the last universal common ancestor, or LUCA.

On a phylogenetic tree, each branch point, or node, stands for a common ancestor at the moment its lineage split. The more recently two species shared an ancestor, the more closely related they are. A common ancestor together with all of its descendants forms a clade, the basic unit of modern classification. A frequent misunderstanding is that one living species descended from another, as in the claim that humans evolved from chimpanzees. In fact both are modern descendants of an extinct ancestor that was neither.

Evidence of shared ancestry comes from homologous structures, from fossils and above all from DNA, because related species carry similar genes inherited from the ancestor they have in common.