More than 1500 fossils unearthed in South Africa could mean the discovery of a missing link in the human evolutionary chain. Aisling Brennan sheds light on this recent discovery.

Findings unearthed in a South African cave have opened up a new chapter of human evolutionary history, revealing the existence of a previously unknown hominid species: Homo naledi. This groundbreaking discovery comes from the richest African hominid fossil site so far and presents not only new data on the evolution of human physiology, but could also possibly cause a major shift in how we understand the evolution of human behaviour.

Discovered accidentally in 2013 by amateur cavers exploring the Rising Star cave system near Johannesburg, H. naledi became the focus of the Rising Star Expedition. At the start of a 21 day exploration, 60 scientists and volunteer cavers only hoped to recover remains from a single skeleton, but quickly found that the site was an unprecedented trove of fossilised individuals. It was “something different and extraordinary” as described by research leader Lee Berger. A total of 15 skeletons from the single species were found in a single chamber, 90 metres from the cave entrance. The team believes that this is only the beginning and that there are thousands of similar remains still untouched.

The site has already provided fossils of almost every skeletal element from a variety of ages, ranging from those of infants to adolescents and adults, stretching to the elderly. H. naledi appears to have been a distinctively slim hominid, with muscular joints supporting an approximately 5 foot frame. Though slender and light in body, their skulls are still unexpectedly small, less than half the size of our own.

The site became even more unusual upon the finding that all of the individuals unearthed were remarkably similar. More similar, in fact, than modern identical human twins. This has led to the belief that these individuals were closely related and could perhaps have represented multiple generations of a family.

It is not yet known precisely how old these fossils are. The possibilities range from maybe two or three million years to as recent as 100,000 years old. While the evolutionary age of the specimens is of obvious importance, no matter how old they are, the mosaic of physiological forms found in the skeletons are just as, if not more, intriguing.

The structure of H. naledi is considered unusual because it mixes old and new evolutionary skeletal structures. The shoulders and pelvis of H. naledi are primitive, and resemble the ape-like Australopithecus which appeared in Africa approximately four million years ago. But the limbs and extremities are incredibly human-like. The hands are almost modern, with fingers suited to gripping, not to climbing trees. These skeletons, compared to many other remains linked to human evolution, prove to us that we are far from fully understanding our history, and that there are many more questions to be asked and answered.

The possibility exists that the discovery of this new species could merely be a tangent of human evolution and not one of the missing links hoped for, but this would hardly diminish its scientific importance. Even if H. naledi is a localised species, and not as big a contributor to modern human evolution as we might think, it is a remarkable new addition to the human fossil record.

In addition to their morphology, this new species could vastly impact on current theories about the evolution of human behaviour. One of the most important questions is one of the simplest: how did so many of these individuals come to rest in the Naledi chamber? After all, the discovery of more than 1500 fossilised hominid bones and teeth in one place is extremely unusual.

One of the strangest aspects of the site isn’t what has been found there, but actually something that’s missing. Aside from a few rodent fossils and the more recent remains of an unfortunate owl, there are no other vertebrate species present. One suggestion was that a carnivore could have hunted the species and dragged the remains into the cave, but this is implausible for a variety of reasons. It would be very rare for a predator to target a single species so specifically, and no bite or gnaw marks were found on the bones that might indicate predation by a large carnivore.

There are also far too many individuals of varying ages to support the idea that they could have stumbled across the small chamber entrance deep inside the cave system and fallen to their deaths accidentally. While a fall down the 12 metre shaft leading to the chamber could’ve been fatal, it would be incredibly unlikely for infant, adult and the elderly alike all to make the same mistake.

Once probable scenarios were ruled out, scientists were left with the astonishing belief that this is evidence of the species deliberately and repeatedly disposing their dead in a protected area. This raises all sorts of varied questions about the behaviour of H. naledi, including how they navigated the treacherous caves in pitch darkness over and over again. More significantly, however, are the questions that arise surrounding Homo naledi’s skull size. The depositing of their dead in a protected place is not unprecedented among extinct hominids, but it has only ever been suggested in species with larger brains that looked and behaved more similarly to modern humans.

The possibility of a species with a brain nearly half the size of our own having even a semblance of recognition of their own mortality is incredibly significant to current theories about the origins of human behaviour and cognitive development. Furthermore, Berger’s team produced a cast of H. naledi’s small brain, which remarkably appears to hint at intriguing features close to a brain region that’s associated with speech in modern humans. According to Berger himself, it’s possible that for the first time ever, we’ve found another species that though not that closely related to us, could have a cognitive ability “different but essentially equal to ours”.