"Nutcracker Man" (OH-5), Australopithecus boisei (formerly Zinjanthropus)
Discovered by Mary Leakey in 1959 at Olduvai Gorge, Ngorongoro, Tanzania. Estimated age is 1.8 million years. It is an almost complete cranium, with a brain size is about 530 cc. This was the first specimen of this species. Louis Leakey briefly considered this a human ancestor, but the claim was dropped when Homo habilis was found soon afterwards.
A. boisei existed between 2.1 and 1.1 million years ago. It was similar to robustus, but the face and cheek teeth were even more massive, some molars being up to 2 cm across. The brain size is very similar to A. robustus, about 530 cc. A few experts consider A. boisei and A. robustus to be variants of the same species.
Credit: photo and (edited) text courtesy of Talk Origins.