Commander Riker, turned into an australopithecine
An Australopithecus was a proto-Human species, one of the earliest Earth hominids. Compared to Humans, they had thicker cranial plates and a smaller brain.
Commander William T. Riker mutated into an Australopithecus after contracting Barclay's Protomorphosis Syndrome in 2370. (TNG: "Genesis")
According to the Star Trek Encyclopedia, 4th ed., vol. 1, p. 52, Australopithecus lifeforms – which included the species A. afarensis, A. robustus, and A. boisei – inhabited Earth 4 to 5 million years ago.