Definition
The last common ancestor (LCA) of two lineages is the most recent population from which both descend. It is a node on the phylogenetic tree, not a single individual — an entire interbreeding population, whose descendants subsequently split into the two daughter lineages.
In human evolution, the most important LCA is the one shared with chimpanzees and bonobos, often called the CHLCA. It lived 6–8 million years ago, somewhere in Africa, and was neither a chimp nor a hominin — it was the common ancestor of both. Every claim about what early hominins evolved from implicitly references this node.
Why it matters
How it works
The position of an LCA on the tree comes from cladistic and molecular analysis — it sits at the node where two lineages diverge. Its date comes from molecular clocks calibrated against older fossil LCAs. Its anatomy is reconstructed by identifying traits shared (and therefore likely ancestral) between its descendant lineages, then constrained by what the earliest known fossils on each side actually show.
For the CHLCA, this reconstruction yields a chimp-sized ape, probably partly arboreal, with some terrestrial knuckle-walking or alternative quadrupedal locomotion, large canines, and a small brain. It was not a forerunner of either chimps or humans in any teleological sense — just a population from which both lineages happened to descend.