Denisovans and the species problem

A few weeks ago I blogged on the Denisovans, a new group of human relatives discovered through genetic analysis of two bones from Denisova in Siberia (Reich et al. 2010, Nature 468:1053). Fascinatingly, the Denisovans seem to have made about a 5% contribution to the genome of living Melanesians.

I mentioned that this new discovery did not seem compatible with a young-earth creationist framework, and awaited with interest a creationist explanation of the findings. Answers In Genesis (AIG) has now commented on the Denisovans. No explanation, merely a one-sentence handwaving solution:

Answers in Genesis wrote:

But the most interesting twist (from the evolutionary perspective) is that modern humans from New Guinea have Denisovan DNA. While an evolutionary perspective interprets this as meaning that Guineans’ ancestors “interbred” with Denisovans, a biblical perspective interprets this as simply meaning that the descendants of one of the people groups leaving Babel eventually settled in what is now New Guinea.

It’s not clear what this even means. After all, their ‘biblical perspective’ had exactly the same interpretation (that the descendants of a group leaving Babel settled in New Guinea) even before we knew about the Denisovan genetic contribution. This ‘explanation’ fails to address a key point: how did the Denisovan genes get into Melanesians, if not by interbreeding with Denisovans?

And the above scenario doesn’t resolve any of the other problems with a young-earth framework.

Why is the Denisovan genome so different from all modern humans, if they were descended from the same eight people on the Ark? (The Denisovans would, presumably, have to be another group of people who left Babel, since Babel happens after the global flood in the Bible.) Why does Africa have the greatest genetic diversity in modern humans but no Neanderthal or Denisovan genes? Why are Neanderthal genes found in all non-Africans, but not in Africans? The genetic diversity of modern humans, Neanderthals and Denisovans is much greater than that of modern humans alone. How could there be so much genetic diversity so soon after disembArking from the Ark? How did all that extra genetic diversity disappear? Genes can disappear quickly in population bottlenecks, or slowly through random processes like genetic drift in stable populations, but it’s very unlikely for genes, let alone large numbers of genes, to disappear in a rapidly expanding population (and going from 8 to 7 billion in under 10000 years is definitely rapid expansion).

On to the question of what species the Denisovans should be assigned to.

Cautiously, and commendably IMO, the scientists declined to classify the Denisovans taxonomically, given that we know almost nothing about their anatomy. Answers in Genesis, of course, considers them all humans:

Answers in Genesis wrote:

Writing for BBC News, Clive Finlayson, director of the Gibraltar Museum, emphasizes that both Denisovans and Neanderthals belonged to our species, Homo sapiens. (Indeed, given the original definition of species as referring to organisms that could interbreed successfully, treating them as separate species doesn’t make sense. However, that definition is no longer observed.)

There’s a reason why scientists have struggled since before Darwin to define what a species is. It’s called the ‘species problem’, and it’s why so many species definitions and concepts have been proposed. Basically, the world is a complex, messy place. Scientists would love it if they could use AIG’s definition. They can’t and don’t, because it doesn’t work in the real world. It may be simple, but it’s also simple-minded. Do we really want to put lions and tigers in the same species? Dogs and wolves? The fact that lions and tigers can interbreed is less important to biologists than the fact they they differ in many other important ways. There are other problems. For example, AIG’s definition doesn’t handle cases where species A can interbreed with B, and B with C, but A can’t interbreed with C. It’s an inherently difficult problem that will probably never be fully solved because the complexity of life defies easy categorization.