During the week of Christmas there was an exciting paper released that found that Melanesians have, on average, 4 to 6 percent of their genome derived from a third, previously unknown archaic human species, the Denisovans. This news has been written about in several mainstream publications and numerous blogs, so this post is more than a little belated, but I wanted to add my thoughts about this important discovery.
Who are the Denisovans?
Denisovans is the name now given to a human group that was discovered in Denisova cave in southern Siberia. Earlier in 2010, the mitochondrial DNA of a pinky bone was sequenced that showed a separate and divergent type of human was living in Siberia up until at least 30 to 50 thousand years ago. The sequencing of the mitochondrial DNA showed that these humans had diverged from living humans at least a million years ago. This person was dubbed the “X-woman.” Some anthropologists playfully called her the Yeti, which set off all kinds of internet speculation that this human was in fact the origin of legends of the abominable snowman and sasquatch. A molar belonging to another individual was found in the cave, so now we have a confirmed population.
The sequencing of the nuclear DNA from that same finger bone has shown that the Denisovans shared a common ancestor with Neanderthals and split off from them 50-100 thousand years after Neanderthals split off from us. This conflicts with the mitochondrial DNA study, which showed the Denisovans breaking off from us at least a million years ago. Why is this? This could be evidence that the Denisovan population had bred with another archaic human species and acquired the mtDNA from them. Or the Denisovans could actually be a much older group and share alleles with the Neanderthals because of inter-mixture.
Since we only have a finger bone and a tooth, we can only speculate as to what they looked like. The molar is fairly large, which would suggest a large jaw, but beyond that it’s anyone’s guess.
Why is this important?
A year ago, the out of Africa model of human evolution was so set in the minds of most that it was practically the consensus theory. Those that promoted the alternate multi-regional model were generally dismissed and any evidence suggesting that there was regional continuity between archaic non-Africans and modern humans was mostly ignored. The out of Africa model was neat and tidy and fit nicely into the “We are all one” meme now prevalent in Western leftist thought. Then last year, proof of Neanderthal introgression into modern non-Africans shocked much of the scientific community.
This was vindication for many that promoted the multi-regional model. Paleo-anthropologists have pointed to fossil evidence that supported this view for years, but geneticists had dominated the debate and in essence had told the world, “Our science is more sound than theirs,” and everyone believed it.
After the Neanderthal buzz settled, there seemed to be so much attachment to the simple out of Africa model that, unsurprisingly, some downplayed the discovery, stating, “Well, we are all still mostly out of Africa.” Instead of smashing the out of Africa model, the Neanderthal news seemed to just make a small dent to it.
Then the Denisovan paper was published. Now we have definitive proof of at least two archaic human species contributing to the modern human gene pool. If the Neanderthal discovery smacked the pompous smirk off out of Africa’s face, then the Denisovan discovery gave it a swift kick in the nuts.
The Denisovan story hasn’t gained as much traction in the popular mind as the Neanderthal one, which is to be expected. After all, Neanderthals are a widely known human species that have been portrayed for years in books and film. Finding out that most people in the modern world have Neanderthal genes is going to blow a lot more minds than news that an unknown species of humans passed some genes onto Melanesians, a group of people that the average person doesn’t care about.
What makes this discovery so important is that it shows the evidence previously used to make the claim that all humans have a recent common origin was wrong. This assertion was made by looking at mitochondrial DNA and the Y chromosome, which pointed back to Africa and we now know paints a misleading picture. The idea was so entrenched in mainstream science that the numerous studies showing otherwise were treated with skepticism and/or ridicule. Now it’s the out of Africa model that should be treated with skepticism. Evidence for the multi-regional model that was brushed aside will now need to be reexamined and reinterpreted. It now becomes not just possible, but probable, that modern humans are the result of the combination several extremely divergent human lineages.
Do Melanesians have the most archaic admixture?
With 4 to 6 percent Denisovan genes plus the 4 percent Neanderthal contribution, that would make Melanesians almost 10 percent “other human.” I think these results are actually a bit lower than the actual percentage. While Melanesians are shown to share genes with the Denisovans, considering that they were found in Siberia and the Melanesians are located at the equator, it may be that Melanesians received the genes indirectly from another group that had mixed with or diverged from the Denisovans. Melanesians may actually have more archaic lineage than we currently find.
Other human populations are also likely to have introgression into their gene pool from ancient human species. Another study has shown evidence of archaic admixture in over 50 percent of Chinese, with an origin in eastern Asia that is 2 million years old. And another study shows 13 percent archaic lineage in some sub-Saharan African populations.
What’s important to remember is that fossil evidence is always sparse and discoveries of new species is often by accident. There are likely human species that we may never know about and without having the sequenced genome of these extinct species it becomes impossible to draw a direct comparison between these species and modern human populations.
What’s for certain is that the out of Africa model has been forever altered. It is on a trajectory to become the new fringe theory. The origin of homo sapiens is far more complex than that simple model and we should look forward to more discoveries of verifiable admixture in humans.
A note on species
There’s a general misunderstanding about the taxonomic definition of species. It’s often assumed that if two populations can mate and produce fertile offspring, then they’re part of the same species. This is often true, but not always. Many different recognized species can and do mate. Wolves and coyotes, for example, have produced a new hybrid species called the coywolf(I prefer wolfyote) that has become a menace in the eastern United States. The two species of orangutan, the Bornean orangutan and the Sumatran orangutan, also can mate and produce healthy offspring and they don’t even have the same number of chromosomes.
I bring this up, because some have stated that the Denisovans are not a separate species but a branch of homo sapiens. Species are generally classified by morphological differences, yet there is no set measure to delineate species. We barely know anything about how Denisovans may have differed from us physically, but we do know that they probably shared a common ancestor with Neanderthals, who are usually considered a separate species. If we use the mitochondrial DNA, which we have seen can be problematic in making these assessments, then they split off from us even before the Neanderthals did.
Given that evidence, I’d say the Denisovans should definitely be labelled as a different species from us. This causes pangs for some because of the implications. This would mean that some groups of humans(most likely all) are hybrids of multiple species. I think that this is the more sensible conclusion, rather than the incredibly simplistic idea that humans just appeared suddenly in Africa some 200 thousand years ago and somehow avoided mixing with any other human species they encountered. If history has shown us anything, it’s that humans will have sex with practically everything, human or otherwise.
See also The Denisova Genome FAQ at John Hawks’ blog.