Fun with Liquid Nitrogen

We’ve discussed previously how archaeological and mitochondrial DNA (mtDNA) evidence both suggest that all modern humans on the planet arose from migrations out of Africa. This is more or less easy to understand: we all came from a population numbering in the low tens of thousands and went forth from there as modern Homo sapiens. But did you know that the Homo genus has had no fewer than three major migrations out of Africa, our own being the most recent?

It’s true. First you’ve got that classic representation of early human species and the perpetual cause of snickering amongst junior high school students, Homo erectus, who stepped off of Africa sometime around 1.9 – 1.6 million years ago. Though we may have descended from H. erectus, it would be because we descended not from these wayward early homonids but because we most likely descended from Homo ergaster, who may or may not be simply H. erectus who stayed behind in Africa. That said, it’s more likely that H. erectus is a more direct descendant of the still-controversial Homo floresiensis, who inhabited Indonesia as late as only 13,000 years ago.

Confused yet? Good. The next migration we know about happened around 500,000 – 300,000 years ago when Homo heidelbergensis – presumably another descendant of H. ergaster – migrated toward Europe via the Middle East and – we think – eventually gave rise there to our old friends Neanderthals, the only other homonid species we know for absolute certain lived and interacted with modern humans.

Speaking of which, always the late-comers, Homo sapiens seems to have finally left Africa somewhere in the ballpark of 50,000 years ago, spreading across Europe, Asia, and – for the first time (as far as we know), Australia and the Americas.

Thereby giving rise to Encino Man

As if the debate as to whether H. floresiensis is a unique descendant of H. erectus or a genetically isolated offshoot of H. sapiens wasn’t enough, just this past month evidence was published of another potential species around in the early days of modern humans in central Asia that arose from yet a fourth migration. This is based on the discovery of a finger-bone in Siberia appearing to date to between 30,000 and 50,000 years ago. Mitochondrial DNA samples (recall that mtDNA is particularly durable and can last over 100,000 years under good conditions) from this bone placed its species lineage at around an even 1 million years ago: too late to be H. erectus but too soon to be H. heidelbergensis in origin. The researchers dubbed the homonid from which this sample was found the “X-Woman.”

The results of this study – the 2008 archaeological dig in central Russia led by Michael Shunkov and Anatoli Derevianko and the mtDNA analysis performed by a team lead by Svante Pääbo – were published in Nature late last month and have sparked a storm of controversy (get it? X-Woman? Storm? Eh? Eh? Eh?).

Xavier is not amused.

Is the X-Woman a unique species? Can we say that based on a single bone and some mitochondrial DNA evidence? If it’s not a new species, then what is it? For his part, Pääbo is not jumping the gun on the magic finger [sorry, couldn't find that video] and declaring the X-Woman as a unique species (with a unique migratory pattern previously unseen), but he does say that this evidence certainly warrants further investigation into the matter. And Pääbo – a man so Swedish his name has two consecutive umlauts – doesn’t take this sort of thing lightly, either. His resume reads like a history of early human archaeology itself over the past 20 years. So this is not exactly an upstart post-doc fresh out of grad school making wild assertions to garner some notoriety. His analysis of the mtDNA (below, red) places its lineage as outside of both modern human (gray) and Neanderthal (blue) ancestors:

Of course, not everyone thinks this means its a new species. As Carl Zimmer points out, while that’s one possibility, the mtDNA patterns might have also arisen from an inter-species pairing between humans and Neanderthals or Neanderthals and another pre-human species, though there’s a number of problems with this, mostly surrounding maintaining a stable population that carries this inter-species mtDNA. As Zimmer points out, it’s been known to happen in primates on occasion when two similar but distinct species share the same geographical area (the “any port in a storm” hypothesis, though one has to wonder – if it did happen in early human species – if rape isn’t more likely), but that doesn’t mean it’s the case here. Furthermore, it could be that the mtDNA is giving a misleading read-out as it could have arisen from a unique, central-Asian population of Neanderthals that broke off from their European ancestors long before the ~40,000 year-old X-Woman was born. Once again, this falls under the heading of “unlikely, but possible.”

The trouble is that our record from practically any time before modern humans is so woefully incomplete due to the number of conditions that need to be in place for tissue to be preserved or well-fossilized that we’re perpetually playing Wheel of Fortune with what we have and making our best educated conclusions. Lacking a time machine, we can’t definitively prove any of these. Whether or not the X-Woman is a unique species, it shows what we’ve always suspected: early human lineage is far, far more complex than that steady, linear march of progenitors leading toward early-90s Brendan Fraser seen above. There is almost certainly far more stuff we don’t know than stuff we do know.

The X-Woman’s discovery is particularly intriguing because it could show that, while we previously thought there was only one species of homonid that co-existed with modern humans – Neanderthals – there now might be no fewer than three in the form of H. floresiensis in Indonesia and now this one in central Russia. It begs the question as to how many more distinct homonid populations existed before modern humans took over the globe.

And that’s the thing, isn’t it? We’re not sure yet just who lost out in terms of species preservation, but we do know that we won, or rather, our ancestors did. Whoever was there other than ancestral modern humans in the range of 50,000 years ago, we know almost for certain that by no later than 15,000 years ago, only we were left. That was before we started farming and therefore started forming the permanent civilizations that allowed us to boom in population from the hundreds of thousands to nearly 7 billion, too. So something happened in those intervening 35,000-or-so years that enabled modern humans to persevere and to later flourish that resulted in the extinction of every other extant homonid population (that we know of).

Was it the environment? Was it us, by force? Was it us through competition for resources? Was it some combination of all of those? That will remain an area for much debate for many years to come, I think. And, in that regard, you might as well have your own opinion on the matter, too. After all, if it weren’t for the perseverance of ancestral H. sapiens, you wouldn’t be here.

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