Origin of the genus Homo: a Paranthropus link?

Reconstruction of a Paranthropus head (Credit: Jerry Humphrey, Pinterest)

Paranthropoids had large, broad teeth and pronounced cheekbones plus a bone crest on the top of their skulls that were the attachments for powerful jaw muscles, much as in modern gorillas. Unlike gorillas they were definitely bipedal and were more similar to australopithecines. They have been called ‘robust’ australopithecines but they were not significantly taller or heavier. The first to be unearthed at Olduvai, Tanzania in 1959 (Paranthropus boisei) was dubbed ‘Nutcracker Man’ by its finder, and many have implied that paranthropoids’ teeth and powerful jaws were signs of a vegetarian diet that needed a lot of chewing. Yet their teeth do not show the microscopic pitting associated with living primates that eat hard plant parts and nuts, or the heavy wear that results from eating grasses. They probably ate soft plants, such as semi-aquatic succulents or tubers, but meat-eating that causes little dental wear cannot be ruled out. Some specimens are associated with long bones of other animals whose ends are worn, suggesting that they may have used them as tools for digging. Plant remains found at paranthropoid sites suggests that they inhabited woodland, together with coexisting australopithecines. They were around in the form of three successive species from 2.9 to 1.2 Ma, outlasting australopithecines. The later paranthropoids coexisted with Homo habilis and H. erectus: they were clearly just as successfully adapted to their surroundings as were early humans.

In early 2023 evidence was published that associated Oldowan stone tools with remains of Paranthropus, together with deliberately defleshed and cut bones (see also): though association is not proof of a direct link. Interestingly, the hand of a P. robustus found in the Swartkrans cave system in South Africa is consistent with a human-like precision grip, i.e. it had an opposable thumb. Swarkrans also yielded the earliest evidence for the deliberate use of fire about 1.5 Ma ago, associated with remains of both P. robustus and H. erectus. All this suggests that a case could be made for paranthropoids’ being human ancestors – supporting evidence has just been published (Braga, J. et al. 2023. Hominin fossils from Kromdraai and Drimolen inform Paranthropus robustus craniofacial ontogeny. Science Advances, v. 9, article eade7165; DOI: 10.1126/sciadv.ade7165).

Fossil-bearing breccias beneath the floor of the Kromdraai cave in the Cradle of Humankind World Heritage Site 45 km NW of Johannesburg, South Africa yielded the first near-complete P. robustus skull in 1938, another being found in cave breccias at the nearby Drimolen quarry. These deposits also contained remains of four infants assigned to the species, whose teeth and cranial parts were at different stages of juvenile development (ontogeny). José Braga of the University of Toulouse, France and co-workers from South Africa and the USA compared this growth sequence with those teased out from immature specimens of Australopithecus africanus and early Homo.Their tentative conclusion is that Paranthropus robustus is more closely related to early humans than to australopithecines of the same stratigraphic age.

Skull of a probable adult female P. robustus (left) with that of H. habilis (centre) and A. africanus (right). Credits: all from Wikipedia pages

So, it now seems possible that paranthropoids are not ‘robust’ australopithecines in an acceptable, taxonomic sense. Their closer resemblance in early development to early humans, together with their association with early stone tools used for defleshing prey animals, together with evidence for possible their use of fire, further strengthens their candidacy for an ancestral link to humans. The best preserved skulls of Homo habilis and a female P. robustus (males of that species show the distinctive saggital crest) do show close similarities, that of a roughly contemporary A. africanus having distinctly wider cheeks than both. All three species were in life probably of much the same weight and stature (30 to 40 kg and 110 to 130 cm) but H. habilis had a significantly larger brain volume (500 to 900 cm3) than the other two (each ~450 cm3). However, this isn’t proof that the genus Homo evolved from a paranthropoid ancestor. That would require genetic evidence, unlikely to be extracted from specimens because DNA seems to degrade more quickly under the conditions of the tropics than at high latitudes. Debate on ultimate human origins will probably be endless. Perhaps it would make more sense simply to accept that early humans weren’t the only ‘smart kids on the palaeoanthropological block’, one of which left no issue after 1.2 Ma ago.

See also: Handwerk, B. 2023. Who made the first stone tool kits? Smithsonian Magazine, 8 February 2023, article 180981606