Humans evolved from monkeys or apes.

The phrase entered biology classrooms as a compressed summary of Darwin's argument and became, in the decades after 1859, one of the most productive sources of confusion in science education. Humans evolved from apes. The statement seemed to follow directly from the evolutionary framework: if all life shares common ancestry, and if humans are genetically and anatomically similar to great apes, then apes must be the ancestors of humans.

The reasoning was flawed at the level of what ancestor means. An ancestor is an organism from which a lineage descends. A cousin is not. Modern chimpanzees and modern gorillas are our evolutionary cousins, not our parents. We share with them a common ancestor, a creature that lived roughly six to eight million years ago, and from that common ancestor, two separate evolutionary trajectories diverged. One led toward Pan troglodytes and Pan paniscus, the chimpanzee and the bonobo. The other led through a sequence of hominins, Sahelanthropus, Ardipithecus, Australopithecus, and eventually the genus Homo, toward us. Neither lineage descended from the other.

The distinction matters because it changes the question evolution is answering. The question is not why chimpanzees did not become humans, which implies that chimpanzees represent an inferior or incomplete version of humanity waiting to achieve its potential. The question is what selective pressures produced the particular adaptations of each lineage from their shared starting point. The two lineages have been evolving independently for six to eight million years. Chimpanzees have adaptations suited to their environments and social structures. So do humans. Neither is more evolved than the other; they are differently evolved.

Thomas Henry Huxley made this case with characteristic force in his 1863 work Evidence as to Man's Place in Nature, arguing from comparative anatomy that humans shared more structural features with gorillas and chimpanzees than gorillas and chimpanzees shared with other primates. Huxley's work preceded any substantial fossil evidence of human ancestors, he was working from skeletons of living species, and yet the anatomical argument was powerful enough to establish the African great apes as our closest living relatives.

Darwin himself had been circumspect in On the Origin of Species in 1859, barely mentioning human evolution, and treating it more fully only in The Descent of Man in 1871, where he situated human origins within Africa on the basis of anatomical similarity to the African great apes. Darwin said the common ancestor, he acknowledged it was unknown, must have lived in Africa, because that is where our most similar living relatives still lived.

The fossil record gradually filled in. Eugene Dubois discovered Homo erectus remains in Java in 1891. Raymond Dart described Australopithecus africanus from a skull found at the Taung limestone quarry in South Africa in 1924, a creature with a brain larger than any ape but smaller than any human, with teeth consistent with the hominid lineage. The discovery pushed the likely origin of human evolution firmly into Africa and suggested a lineage that was neither human nor any living ape, but something that had branched from a shared starting point and followed its own path.

The misstatement persisted partly because it was rhetorically useful, for those who supported evolution as a way of emphasizing human continuity with the rest of life, and for those who opposed it as a way of making the claim seem absurd. Both sides shared an interest in the compressed phrasing. The correct version, humans and modern apes share a common ancestor, which was itself an ape but not any species alive today, was harder to fit on a banner or use as a punchline.

The actual tree of life does not march; it branches. Humans and chimpanzees are terminal branches on different limbs of the same tree, both equally distant from their last shared ancestor, both equally adapted to the worlds they now inhabit.