A fossil specimen unearthed at the Philip Tobias Korongo site, Olduvai Gorge, could be the oldest ‘anatomically modern’ human hand bone, says an international team of scientists led by Dr Manuel Dominguez-Rodrigo, of Complutense University in Madrid, Spain.

The specimen, labeled Olduvai Hominin 86 (OH 86), is part of a little finger or ‘proximal phalanx.’

“Assuming that the assignment of OH 86 as a fifth proximal phalanx is correct, it must also derive from a left hand,” Dr Dominguez-Rodrigo and co-authors wrote in a paper published in the journal Nature Communications.

The remarkable thing about the bone is that it belonged to a previously unknown hominin species that lived over 1.84 million years ago, alongside hominins Paranthropus boisei and Homo habilis.

“OH 86 represents the earliest human-like hand bone in the fossil record, of a size and shape that differs not only from all australopiths, but also from the phalangeal bones of the penecontemporaneous and geographically proximate OH 7 partial hand skeleton (part of the Homo habilis holotype),” the scientists wrote.

The overall size of the bone is within the range of modern humans and chimpanzees, as it is the case of other hominins except Australopithecus sediba. In terms of relative length, it is in the midrange of humans and the upper range of gorillas, but below chimpanzees and monkey.

According to the scientists, these results lead to the conclusion that OH 86 represents a hominin species different from the taxon represented by OH 7 (Homo habilis), and whose closest form affinities are to modern Homo sapiens.

“However, the geological age of OH 86 obviously precludes its assignment to Homo sapiens, and ambiguity surrounding the existing potential sample African Homo erectus hand bones also prohibits its confident assignment to that species at this time.”

OH 86 adds to previous 1.9 – 1.8 million-year-old evidence that indicates that several key aspects of modern human body morphology emerged very early in human evolution.

“For example, the KNM-ER 3228 hominin pelvis (Homo erectus, Kenya) resembles those of modern human males, and the Dmanisi postcranial remains (Republic of Georgia) demonstrate that Homo erectus limb proportions were similar to those of modern human.”

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Dominguez-Rodrigo, M. et al. 2015. Earliest modern human-like hand bone from a new >1.84-million-year-old site at Olduvai in Tanzania. Nature Communications 6: 7987; doi: 10.1038/ncomms8987