First known hand fossils of extinct human relative reveal ‘surprising’ features

The long thumb and straight fingers would have allowed Paranthropus boisei to form a powerful grip, similar to how modern humans might grasp a hammer. Other features, such as the broad shape of the finger bones, closely resemble those of a gorilla. (Louise Leakey via CNN Newsource)

The first known hand fossils from an extinct human relative have been unearthed in Kenya, revealing a species with unexpected dexterity and a gorilla-like grip. The hand bones, which were discovered alongside skull and teeth fossils, are leading researchers to believe these early humans may have been able to use stone tools.

Paranthropus boisei was previously identified only by its distinctive skull and large teeth, with molars up to four times bigger than those of living humans, so researchers didn’t know what the rest of the body looked like or how the hominin interacted with its environment. They did, however, theorize about the huge chewing muscles its jaw would have contained and its eating habits, which earned it the moniker Nutcracker Man.

The remarkably well-preserved hand bones comprise a long thumb, straight fingers and a mobile pinkie finger that would have allowed the species to form a powerful grip, similar to how modern humans might grasp a hammer. Other features, such as the broad shape of the finger bones, closely resemble those of a gorilla, however.

The partial skeleton, unearthed at Koobi Fora, a site on the eastern edge of Lake Turkana, is estimated to be slightly more than 1.52 million years old. The teeth and skull fossils matched previously studied P. boisei specimens, while the hand and foot bones proved to be unique among previously studied hominins, a term referring to all species that emerged after the genetic split from the ancestors of the great apes six million to seven million years ago.

“This is the first time we can confidently link Paranthropus boisei to specific hand and foot bones,” said Carrie Mongle, a paleoanthropologist and an assistant professor at Stony Brook University in New York. Mongle is the lead author of a study on the fossils that published in the journal Nature on Wednesday.

The hand was “quite unexpected,” according to Tracy Kivell, director of the department of human origins at the Max Planck Institute for Evolutionary Anthropology in Leipzig, Germany.

“It is clearly the hand of (a) human ancestor, but also has features that are remarkably similar to gorillas, which is surprising,” Kivell said via email.

“No other hominin that we know of has hand morphology that is so gorilla-like, which greatly broadens our perspective on what is ‘possible’ within (the) human evolutionary story of hand use,” she added. Kivell coauthored a commentary published alongside the study but was not involved in the research.

Key question

P. boisei lived in eastern Africa from 1.3 million to 2.6 million years ago, coexisting with at least three other hominin species: Homo habilis, Homo rudolfensis and Homo erectus.

Some researchers hypothesized that only species within the genus Homo had the ability to make stone tools, although recent discoveries have undermined that assumption. Stone artifacts unearthed in Kenya dating 2.9 million years ago suggest that tool use was more widespread in the hominin family tree than once thought.

Hominins include species within the genus Homo, such as our own Homo sapiens; more recently extinct species such as Neanderthals, which disappeared 40,000 years ago; early Homo species like Homo erectus; and more distantly related species such as Australopithecus afarensis, represented by the famous Lucy skeleton in Ethiopia, which is 3.2 million years old.

Mongle said the proportions of P. boisei’s hands would have allowed it to manipulate stone tools just as well as the other Homo species living in Africa at the time. “This paper is careful in not claiming that Paranthropus made and used tools, but instead they say that there is essentially nothing in the hand anatomy that would prevent that,” Ryan McRae, a paleoanthropologist at the Smithsonian National Museum of Natural History in Washington, DC, told CNN in an email.

“Without a smoking gun of stone tools found in a fossilized hand, or stone tools found at a site with only (one) hominin species represented, we may never know for sure who was and was not making these tools, but this paper is a huge step in the ‘Paranthropus the tool maker’ hypothesis.”

Later humans such as Neanderthals and Homo sapiens had different wrist anatomy, and P. boisei, along with its contemporaries, likely would not have been able to precisely pinch its fingers together, the study noted.

The hand fossils also suggest that P. boisei shared grasping capabilities with gorillas that would have allowed it to grab and strip tough-to-eat plants, removing the indigestible parts with its hands, the study found.

While its powerful hands indicate it would have been an adept climber, the hominin’s feet had arches, which allowed for efficient movement. This means it was unquestioningly adapted to walking upright on two legs, Mongle said.

“Because of the combined morphology of the hand and foot, the authors suggest that this species was likely not arboreal (climbing in trees), but that any convergence with gorillas in the hand is likely due to how they used their hands for processing of tough foods. This makes sense,” McRae, who was not involved in the study, added.

Multigenerational fossil finds

The fossils were found during excavations between 2019 and 2021 by a team led by coauthor Louise Leakey. In the 1950s, her grandparents, the renowned paleoanthropologists Louis Leakey and Mary Leakey, found the first P. boisei skull in what is now Tanzania and gave it the nickname Nutcracker Man. However, wear marks on the species’ teeth indicate that rather than cracking hard food like nuts, it chewed and ground tough foods like tubers and roots to survive.

The latest fossils emerged from a layer of sandy silt just above an extraordinary trackway of hominin footprints made public last year.

Pressed into soft mud, the footprints were attributed to P. boisei and Homo erectus, leading researchers to believe the two species crossed paths and were able to live as neighbors, not competitors, in the same habitat.

Mongle said the two species would have occupied different ecological niches, but based on what’s known of the hominin’s face, teeth, jaws and now hands, P. boisei likely ate a specialized diet of plant foods such as grasses.

By Katie Hunt, CNN