In 2015, scientists announced the discovery of a series of bones in a South African cave that revealed an entirely new species of hominin. Dubbed Homo naledi, this previously undiscovered species was initially thought to be around two million years old as it shared traits with other primitive human ancestors from that period. But recent discoveries of more Homo naledi bones, and subsequent comprehensive fossil dating, has shown the remains to be much more recent than expected. The revelation dramatically alters our view of human evolution and suggests these primitive humans may have lived side-by-side with our more modern Homo sapien ancestors.

Dating the fossils accurately proved to be quite a challenge for scientists as traditional carbon dating techniques result in destruction of the bones as well as having reasonably large error margins. To come up with an accurate date for the fossils the scientists utilized a variety of techniques, from studying samples of Homo naledi teeth to analyzing the sediments found in the chamber where the fossils were found.

"To get the final date range we used 10 different labs and six different techniques which also involved double-blind testing," explains Associate Professor Eric Roberts from James Cook University.

The scientists finally aged the Homo naledi fossils as somewhere between 335 and 236 thousand years old. This striking revelation alters previous assumptions that only Homo sapiens existed in Africa across this age known as the late Middle Pleistocene.

While the Homo naledi lineage is still estimated to have originated between one and two million years ago, this new dating discovery has significant implications on how we previously interpreted other fossil data. Not only does it mean that more primitive hominids persisted in Africa for much longer than previously thought, but it places the Homo naledi in an environment where many ancient tools have been discovered, bringing into doubt the belief that it was just Homo sapiens who were developing tools at the time.

"The new dating puts it on the landscape at a time from which we find lots of tools in Africa in the middle stone-age," says Dr Roberts. "One of the implications of the new dates is that it's no longer automatically possible for us to assume that [it was] early Homo sapiens that were making these tools."

No tools have been found alongside the Homo naledi remains, but studies of the bones do indicate that the species did have hands capable of tool usage. Features of the Homo naledi hands indicate several characteristics similar to that of modern humans but interestingly the species' fingers have been found to be curved in a way that suggests they were well suited to climbing.

The more recent discovery of new hominin specimens at the site reveals some of the most complete ancient skeletons ever recovered, giving scientists an excitingly comprehensive insight into this new species. One particular adult skeleton, nicknamed "Neo" offered up a skull that the team described as "spectacularly complete."

"The skeleton of Neo is one the most complete ever discovered, and technically even more complete than the famous Lucy fossil, given the preservation of the skull and mandible," says Professor Lee Berger of The University of the Witwatersrand in Johannesburg, South Africa.

A comparison between the famous "Lucy" skeleton (on the left) and the recently unearthed "Neo" (right)(Credit: Wits University/John Hawks)

Notably, the Homo naledi lineage was found to have a very tiny brain about the size of an orange and stood only about five feet (1.5 m) tall. While its teeth and skull point to more primitive relationships with our genus, its feet and slender legs are similar to modern humans, suggesting the species was able to comfortably walk long distances.

Professor Paul Dirks from James Cook University notes that this discovery highlights the rich complexity of human evolution and points to a degree of interactivity between different branches on our family tree that were never previously thought possible.

"The new dating of the fossils opens up all sorts of possibilities for an interchange of tool use, cultural activities and behaviours between Homo naledi and homo sapiens," he says.

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