What a fossil pinky reveals about human evolution

Sometimes, the littlest finger can tell a big story.

Researchers say OH 86, a fossil specimen found in Tanzania, could be the oldest example of a "modern human-like hand." The fragment belonged to an unidentified member of the human family tree who lived over 1.84 million years ago, alongside Homo habilis and Paranthropus boisei. According to a study published Tuesday in Nature Communications, the specimen could even belong to a yet-unknown human ancestor – one who adapted "modern" features earlier than its cousins.

OH 86, identified as part of a pinkie finger or "proximal phalanx," was excavated from the fossil-rich Olduvai Gorge. According to lead author Manuel Domínguez-Rodrigo, even incomplete fossil specimens like OH 86 can be quite informative.

"By looking at a single proximal phalanx, we can understand the overall morphology of fingers,"  says Dr. Domínguez-Rodrigo, who is a co-director of the Institute of Evolution in Africa. "By understanding the morphology of fingers we can understand the morphology of hands."

Researchers use the term "modern human-like hand" to mean "a hand that shows a morphology adapted for precise manipulation in a wide range of grips, and which lacks any adaptation related to arboreal climbing or locomotion." In other words, modernity is directly linked to function – and in particular, tool making.

"For human evolution, the new discovery shows the oldest hominin adapted to terrestrial [life] completely," Domínguez-Rodrigo says. "This implies a creature using tools more frequently. This modern morphology is also documented in a hominin that is bigger than the other hominins previously known."

Hands of tree-climbing primates are optimized for arboreal life, he explains. Their short thumbs and long, curved finger bones are excellent for grasping branches. By comparison, humans have longer thumbs and straighter fingers, which allows us to produce a wide range of different grips.

"Among modern primates this is only observed in our species, Homo sapiens," Domínguez-Rodrigo says.

Although OH 86 is nearly two million years old, analysis suggests that the complete hand had modern proportions. A great deal of fossil evidence found at Olduvai Gorge has been attributed to the smaller and more primitive H. habilis, but researchers say their yet-unnamed hominin may be their true source.

Small anatomical discoveries have a way of shaping our understanding of human evolution. Earlier this year, researchers were able to estimate the brain capacity of an early hominin using only a partial mandible. And according to Domínguez-Rodrigo, recently discovered pelvic and occipital bones, similar in age to OH 86, also show evidence of modernity.

"This implies our genus has a longer history than previously documented," Domínguez-Rodrigo says, "and its origins must be before two million years ago."