In 2013, Lee Berger at the University of the Witwatersrand in Johannesburg and his colleagues made an extraordinary discovery – deep inside a South African cave system they found thousands of bones belonging to a brand new species of early human — and now we finally may know when this species lived and how it fits into our evolutionary tree.
By 2015 it was becoming clear that the new species, which was named Homo naledi, was unlike anything researchers had discovered before. Although parts of its skeleton looked identical to our modern human anatomy, it had some features that were strikingly primitive – including a skull that was only slightly larger than that of a chimpanzee.
But Berger and his colleagues had trouble establishing how old the H. naledi fossils were. Without that piece of information, most other researchers agreed that the true significance of H. naledi for understanding human evolution was unclear. Guesses have varied from as old as 2 million years to as young as 100,000 years.
Today, news broke that Berger’s team has finally found a way to date the fossils. In an interview published by National Geographic magazine, Berger revealed that the H. naledi fossils are between 300,000 and 200,000 years old.
“This is astonishingly young for a species that still displays primitive characteristics found in fossils about 2 million years old, such as the small brain size, curved fingers, and form of the shoulder, trunk and hip joint,” says Chris Stringer at the Natural History Museum in London.
Here, we address some of the implications of the announcement, as we wait for the full publication of the results.
Why has it taken so long to establish the age of the fossils?
It can be surprisingly difficult to work out how old fossil bones are. Many of the techniques researchers can use require the isotopic analysis of bone samples. Berger and his colleagues are reluctant to use these techniques, because they involve destroying small samples of precious fossil material.
Another option is to date the rock or sediment that blankets the layer in which the fossils are found. Ancient lava flows, in particular, contain chemical signatures that are perfect for isotopic dating. But the H. naledi remains were found in a cave in which there were no easily dated sedimentary layers covering the fossils.
Researchers can also work out the rough age of the fossils by looking at the fossil remains of other species found alongside them, if the age of those other species has already been established. The cave in which the H. naledi fossils were found contains virtually no bones from other species, though, making this approach a nonstarter.
So how did Berger and his colleagues work out the age of the fossils?
We don’t know yet. The scientific papers in which this information will be revealed haven’t been published. The National Geographic interview mentions that Berger and his colleagues have found a second cave chamber containing more H. naledi remains – perhaps these additional fossils were preserved in a context that made dating less challenging.
If the fossils are 300,000 to 200,000 years old what does that mean?
Our earliest hominin ancestors lived at least seven million years ago. The first species to look a little like modern humans appeared between about two and three million years ago.
But our own species – Homo sapiens – evolved about 200,000 years ago.
So, if H. naledi lived 300,000 to 200,000 years ago that’s a remarkable discovery.
It means that a species of human with some surprisingly primitive features – including a tiny skull and brain – survived into the relatively recent past. Conceivably, H. naledi might even have met early members of our species, H. sapiens. One could even speculate we had something to do with it going extinct.
Does the age help us to work out where H. naledi fits in the human evolutionary tree?
It probably depends on whom you ask. Based purely on its strange anatomy, H. naledi seems to belong somewhere near the very base of the “true human” family tree – an idea suggested in some studies of the fossils.
But we know that the first early humans appeared more than two million years ago. If H. naledi is just 300,000 years old, some researchers might argue that it can’t belong to the base of our family tree. It’s too young. Perhaps it even had a modern-looking ancestor and later evolved primitive-looking features.
But it is, in fact, still perfectly possible that H. naledi really does belong somewhere near the base of our human evolutionary tree.
The species might have evolved more than two million years ago, as one of the earliest “true” humans, and then survived, unchanged, for hundreds of thousands of years.
“It could lie close to the origin of the genus Homo, suggesting that this is a relic species, retaining many primitive traits from a much earlier time,” says Stringer.
Berger has previously talked about this possibility. He says H. naledi might be like a human version of the coelacanth – a primitive fish with ancestors that first appeared 400 million years ago but that is still found in oceans today.
Is there any precedent for that idea in the human fossil record?
Yes – potentially. About a decade ago researchers working on the opposite side of the world, in Indonesia, made another astonishing discovery: they found remains of another ancient human species with a tiny chimp-sized head that also lived just a few hundred thousand years ago. It is named Homo floresiensis – although it is better known by its nickname: the “hobbit”.
Researchers have been arguing about H. floresiensis’s place in the human family tree for years. Last week, one paper revived the idea that H. floresiensis can trace its roots back to a very early species of human called H. habilis that we know lived in Africa more than two million years ago.
The idea is that a population of H. habilis left Africa about two million years ago and gradually moved across Asia, ultimately reaching Indonesia. If this idea is correct, H. floresiensis falls on one of the lowest branches in the “true” human family tree despite its young age, because it evolved directly from the primitive H. habilis.
In other words, species of evolutionarily primitive humans might, in some circumstances, be able to survive for hundreds of thousands of years.
“There are obvious parallels with the late survival of H. floresiensis in Indonesia, but in that case island isolation probably accounts for its longevity,” says Stringer. “How did a comparably strange and small-brained species linger on in southern Africa, seemingly alongside more ‘advanced’ humans?”
What happened to H. naledi in the end?
There are no answers to this question yet. But if the fossils really are just 300,000 to 200,000 years old there is at least one possible scenario. Our species, H. sapiens, evolved in Africa about 200,000 years ago. If those early H. sapiens reached southern Africa shortly afterwards, they might have contributed to the extinction of H. naledi.
Again there is precedent for this. The fossil record elsewhere in the world shows that H. sapiens left Africa and gradually spread across Eurasia. As it did so, H. sapiens arrived in areas already populated by ancient humans – species like the Neanderthals. Within a few thousand years of H. sapiens arriving in these new areas, the indigenous species of ancient humans disappeared, apparently outcompeted by H. sapiens.
Even the hobbit, H. floresiensis, seems to have suffered this fate. The most recent information suggests it went extinct 50,000 years ago – about the same time that H. sapiens arrived in this part of Indonesia. H. naledi might have the dubious honour of being the earliest ancient human species to have been driven to extinction by the spread of our species. But this is still speculation at the moment.