400,000 year old teeth could rewrite the evolutionary history of our species

Eight recently discovered teeth are very similar to those of modern humans and date back 400,000 years... 200,000 years older than our species is supposed to be. To explain this mystery, we must retrace human evolution.

The Teeth in the Cave

The finding sparks a major mystery in the story of human evolution. All previous evidence - and there's an awful lot of previous evidence - had suggested the first anatomically modern humans emerged from archaic Homo sapiens about 200,000 years ago, and then only in Africa. So what are teeth that apparently belong to an anatomically modern human doing in an Israeli cave 400,000 years ago?

400,000 year old teeth could rewrite the evolutionary history of our species

That's the mystery that paleontologists will now have to deal with. Tel Aviv University's Avi Gopher led the team that made the find. He explains just what they discovered in the cave:

"Our cave was used for a period of about 250,000 years — from about 400,000 years ago to about 200,000 years ago. The teeth are scattered through the layers of the cave, some in the deeper part, that is to say from 400,000 years and through all kinds of other layers that can be up to 200,000 years. The oldest are 400,000 years old. It is accepted at the moment that the earliest Homo sapiens that we know is in east Africa and is 200,000 years old — or a little less. We don't know of anywhere else where anyone claims to have an earlier Homo sapiens."

It's a dramatic discovery, and it's always good to be skeptical about something that seems to rewrite everything we know about a given field. (Just look at NASA's recent purported discovery of arsenic-based life.) But Gopher points out that they discovered the teeth back in 2006, and it's only after four years of testing and analysis, not to mention locating additional samples, that the team has published their findings. The researchers dated stalagmites, stalactites, and other material in the cave to arrive at the most accurate possible date for the teeth.

Although Gopher and his team were cautious about publishing their data, now that it's out there he isn't coy about what this could mean for the study of human evolution. As he explained in an interview with the BBC:

"This conclusion may be of great importance because it may be a first hint at changing some of the paradigms we are used to use in human evolution."

The story of human evolution is a complex one, and we still don't know all the details. Still, if we're going to understand what this discovery means, we need to understand a little of the story of the Homo genus. So here's a short version of how we get from the earliest hominids to modern humans.

Erectus and Ergaster: The First African Migration

The Homo genus, of which humans are the only surviving members, diverged from our more basic australopithecine ancestors about 2.4 million years ago with the emergence of Homo habilis. This species, the most primitive of our Homo ancestors, originated in Africa and is not thought to have left the continent during its roughly one million years of existence as a species.

400,000 year old teeth could rewrite the evolutionary history of our species

We don't know exactly how all the different Homo species fit together - we don't know, for instance, whether Homo habilis is a direct ancestor of humans - but roughly two million years ago, two new species emerged: Homo ergaster and Homo erectus. Again, we don't know a lot of the finer details of these two species; in fact, we're not even 100% certain that these are even separate species.

Ergaster and erectus went down very different paths. While Homo ergaster remained in Africa, Homo erectus spread out across Eurasia, reaching from Spain to China and Indonesia. Both species are thought to be a major step forward towards many of the traits we associate with modern humans, being the first hominid species to use complex tools, hunt in coordinated groups, and take care of the weak in their groups. These species might even have had some basic language abilities.

400,000 year old teeth could rewrite the evolutionary history of our species

Homo erectus and Homo ergaster were arguably the two most successful species in the history of our genus, living for nearly two million years and only dying out less than 100,000 years ago. Indeed, it might be even more recent than that, if Homo floresiensis, the so-called "hobbits" of Indonesia that died out as recently as 13,000 years ago, really were erectus descendants.

Homo heidelbergensis and the Second Migration

But from the African ergaster came a new species, Homo heidelbergensis, which first emerged in Africa about 600,000 years ago. Although heidelbergensis is now one of the more obscure hominids, they were a very advanced species, with brain capacity very similar to ours and some shockingly modern customs, including perhaps burial of their dead. They stood an average of six feet tall, and they were more muscular than we tend to be, making Homo heidelbergensis one of the most powerful members of our genus.

400,000 year old teeth could rewrite the evolutionary history of our species

Homo heidelbergensis probably supplanted Homo ergaster and then split off into a couple different main groups, similar to how ergaster and erectus had split apart about a million years previously. This spawned the second African migration, in which part of heidelbergensis left Africa and spread out into Europe and western Asia.

By about 300,000 years ago, Neanderthals had emerged from these heidelbergensis migrants. Neanderthals were close relatives of their heidelbergensis ancestors. Although Neanderthals were shorter, they retained the muscular build of their forebears, with similarly large brow-ridges and protruding face.

400,000 year old teeth could rewrite the evolutionary history of our species

It's not known exactly what happened between the erectus groups that had already colonized Eurasia and the incoming heidelbergensis and Neanderthal groups, as the fossil record isn't precise enough to reveal what, if any, overlap there was between the two waves of migration.

It's possible that Homo erectus had already died out, that the new arrivals simply outbred their predecessors, or even that Neanderthals actually actively wiped out erectus, although that possibility should probably be considered the least likely in the absence of any evidence to back up such a dramatic idea.

The Emergence of Homo sapiens

In any event, while Neanderthals emerged in Eurasia, Homo heidelbergensis gave rise to another successor species back in Africa. Although we still don't know all the exact steps in between, around 200,000 years ago Homo sapiens emerged.

According to the current scientific consensus, known as the Recent African Origin model, humans only left Africa around 60,000 years ago, an idea backed up by studies of mitochondrial DNA and recent anthropological findings. (We can save a more detailed explanation as to how we got to this theory, particularly the role of DNA studies, for another time, as that really is its own topic.)

Most scientists favor this model to the competing multiregional origin model. The latter theory holds that modern humans actually evolved from whichever group of Homo erectus had colonized their part of the world, creating a single continuous species with a global evolutionary history. This theory doesn't have many proponents, particularly because it's thought to have trouble explaining just how different groups of humans could have maintained their ability to interbreed while pursuing parallel evolutionary lines.

So, let's recap. Modern humans are only 200,000 years old, although we do have closely related archaic ancestors that are significantly older. Indeed, there's still plenty of debate about whether heidelbergensis, Neanderthals, and other species are really distinct species at all, or in fact just subspecies of a single species. This can very quickly get terrifically complicated.

400,000 year old teeth could rewrite the evolutionary history of our species

Although humans and almost all other members of the Homo genus originated in Africa, there were at least three periods of hominid migration, one by erectus over a million years ago, one by heidelbergensis/Neanderthals about 600,000 to 300,000 years ago, and one by modern humans about 60,000 years ago. Importantly, the current consensus is that all humans are descended from those who lived in Africa 60,000 years ago, and that any overlap between erectus or Neanderthals was trivial. So now, with all that in mind...

How do these teeth in the cave fit into all this?

That's a very good question. Cambridge University expert Sir Paul Mellars says he accepts this discovery as reputable, saying the teeth really are 400,000 years old. However, he remains skeptical that the teeth are actually human. As he explains:

"Based on the evidence they've cited, it's a very tenuous and frankly rather remote possibility."

Mellars suspects these teeth are actually related to Neanderthals. Part of the problem is that teeth, while they are the most likely part of an animal to be preserved, aren't reliable indicators of what species they belonged to. If we could find testable skull fragments in the cave, Mellars suggests that would provide a much better opportunity to pin down which species these teeth really did come from. In his BBC interview, Gopher did mention the discovery of skull fragments, but so far they have been too small to be of any real use.

If these teeth really did belong to 400,000 year old modern humans, particularly humans who lived well outside Africa, then that is going to mean some serious reconsideration of our current theories. After all, these date back to 100,000 years before even Neanderthals showed up on the scene, let alone our modern ancestors.

Such a confirmed finding could mean any number of things. It might lend some unexpected credence to the multiregional origin theory, although we will need a lot more than a few teeth to lend that theory much weight. It might mean modern humans actually did evolve far earlier than we had previously suspected, and some of the earliest members had taken at least some tentative steps out of Africa into nearby regions. Or - and this is the most likely possibility - precursor species like heidelbergensis were a little more human-like than we had thought.

Whatever is gong on here, it's an important finding, and it will help to further flesh out the most important story in our species's history: how we got here. And, if our little tour of our evolutionary past has shown you anything, hopefully it's that there's still far, far more that we don't know about human evolution than what we do know. The basics of the theory are firmly in place, but there's plenty of crucial details left to uncover.

[American Journal of Physical Anthropology]