Mar 29 2010

A New Human Species?

First there were several species of humans, at least from the perspective of some Western naturalists who considered Africans a separate species. But it soon became apparent to biologists that all people of the earth are one species. Darwin raised the possibility that we evolved from other species, and this was apparently confirmed with the discovery of Neanderthal man. It turns out Neanderthals are our cousins, not our ancestors, but with them there have been at least two species that can be called “human” on this planet.

And that was the status of things for a while. Homo erectus and Homo habilis extended the number of species in our genus (it’s debatable whether habilis should really be an Australopithecus), but they are not close enough to us to be considered human cousins. More recently several species have been identified at the root of humans and neanderthals. Homo heidelbergensis is probably our direct ancestor from about 650,000 years ago. But there is also Homo antecessor, which (along with heidelbergensis) likely evolved from Homo ergaster in Africa.

Then a third recent close human cousin was discovered – Homo floresiensis, or the Hobbit. This is still controversial, but the data is leaning in the direction of concluding that a diminutive species of human lived until about 12,000 years ago on the island of Flores. We missed by a geological hair’s breath there being as many as three distinct human species occupying the earth. Given the problems we have had with race relations, I have often wondered how our society would deal with an actual distinct species.

Now, as announced in Nature magazine, there could be a fourth species of human to have occupied the earth as recently as 30,000 years ago. The specimen is called Woman-X and it was found in Siberia, at a site which also contains modern humans.

The new evidence is mitochondrial DNA from the pinky bone of Woman-X. The analysis shows features in common with modern humans – the specimen is clearly human – but also differences between both modern humans and neanderthals. So if Woman-X was human, but not modern and not neanderthal – what was it? That is the exciting news – perhaps it was a new species. If so then there were at least three human species co-mingling around Europe and Asia 40 thousand years ago or so (Homo floresiensis was isolated on the island of Flores.)

We discussed this news item on the SGU this week and have had some interesting feedback. One listener pointed me to John Hawkes blog where he discusses the alternate interpretation that Woman-X may simply be from a branch or clade of neanderthals with a separate mitochondrial lineage that was uncommon. He makes the point that genetic lineages are not necessarily the same thing as “species”, to which I would add that the very notion of “species” is fuzzy.

Of course this is a preliminary analysis – one specimen and mitochondrial DNA only. Somatic DNA analysis is apparently underway. This may clarify the issue, but we may also need more specimens before we can confidently conclude that Woman-X was a member of a distinct human species.

Part of the analysis is about divergence time – when was the last common ancestor between two species. Humans share a last common ancestor with neanderthals about 500,000 years ago, but other estimates put it at 660,000 years. The Woman-X analysis suggests a last common ancestor with modern humans about 1 million years ago. If true this would definitely make it a distinct species, and suggest a separate and earlier migration out of Africa. But, as Hawkes points out, divergence time conclusions are highly dependent on sampling size – how many different genes are you looking at among how many individuals?

There are a few take-home messages from this research:

The evolution of humans is far more complex than previously imagined, and it is very likely that our picture of human evolution over the last million years will get much more complex. There apparently were multiple migrations out of Africa to Europe and Asia, with multiple early branchings. This is likely further complicated by various amounts of time in which different branches continued to interbreed when they overlapped. The more detailed a picture we get, the more complex and fuzzy it is likely to become.

This new evidence is very preliminary, and only raises the possibility of a new distinct species, but other interpretations are still viable. Clarifying this will have to wait for further analysis of this specimen, but also likely the discovery of new specimens.

I have not mentioned this point yet, but this research also raises the question of how much we can use culture as a proxy for species. In other words, when we find certain tools that tend to go along with a certain human species, can we be confident that that species in fact made and used them? How much cultural and technological contamination was there among the overlapping human species?

Also, when reporting on a new science item it is almost universally true that when the scientific community starts to take a close look at the research new interpretations will emerge, perhaps even errors will be pointed out. That is the cycle of science. But this means that early reporting of the new science publication is likely to be followed by more expert analysis that calls the original conclusions into question. This is why we often say that we will “follow this story” – we need to follow the process of science as it plays out. So take science news items with a grain of salt – new information is by its very nature tentative.

In fact what I find most interesting about these stories is watching the process of science – reading about how different scientists argue their cases tells us a great deal about the state of the evidence, and also about the kinds of arguments scientists use and how they interpret the evidence – how we know what we know.

No matter how this particular story pans out, it is likely to be highly interesting.

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