The collarbone of an unknown hominid that lived in East Africa around 1.8 million years ago is remarkably similar in length and curvature to that of a barrel-chested modern man. Discovered in Tanzania in 2005, the ancient fossil provides evidence that our ancient ancestors may have been as broad as we are, despite differences in height and overall size.

Also known as the clavicle, the collarbone provides intriguing insights into the biomechanical forces that act on the forelimbs during different types of locomotion. As such, analyses of ancient hominid clavicles have been used to draw conclusions regarding climbing ability, manual dexterity, and throwing power, among other things.

Given the considerable differences between humans and apes, one would naturally expect the oldest hominid collarbones to differ considerably from those of modern humans. To test this theory, the authors of a study – which has been posted as a preprint and has not yet been certified by peer review – compared the ancient bone from Tanzania with the clavicles of modern humans, gorillas, chimpanzees, and baboons, as well as several ancient species including Neanderthals, Homo erectus and the iconic Australopithecus afarensis.

Despite ethical concerns, the researchers obtained modern human clavicles from the remains of people who died in the Cleveland area more than 100 years ago and “whose bodies were unclaimed from hospitals and morgues.”

“These people therefore disproportionately represent low-income groups and, given the standards of medical ethics at the time, most likely did not agree to donate their remains to science,” write the authors apologetically.

Overcoming their moral hesitations, the researchers discovered that the overall length of the ancient collarbone was similar to that of a large human male, while the bone’s curvature was also “relatively human-like relative to its length.” They therefore concluded that the fossil “derives from an individual with a shoulder breadth similar to a large male today.” 

Despite the fact that archaeological records indicate the presence of at least two distinct hominid species in Tanzania around 1.8 million years ago, the study authors were unable to confirm which species the collarbone might have belonged to. However, given the striking similarities between the ancient clavicle and modern collarbones, the researchers speculate that “the evolutionary and biomechanical forces acting on the arms and shoulders of 1.8-million-year-old hominids are not entirely dissimilar to those acting on modern humans today.”

“This finding indicates that there has been little morphological change in the hominid clavicle in the last ~2 million years,” they write. “It also suggests that shoulder breadth (though not necessarily body size) may have been similar to modern humans as far back as 1.8 [million years ago].”

The preprint has been posted to BioRxiv.