(Apologies to Current Biology).
Something like this probably happened, but with sex, too. (Photo from Science.)
The genome of Neandertals was just sequenced (reference below).
For those who don’t want to plow through the long (but informative!) explanatory posts about the Neandertal genome that have appeared on several science websites, here’s a quick guide:
- What’s the deal? A large group of scientists from several countries have determined the DNA sequence of most of the Neandertal genome.
- Is it “Neandertal” or “Neanderthal”? Either is correct. “Neanderthal” is most common but the new Science paper uses “Neandertal.” The name is taken from the German “Neander Tal”, or “Neander Valley,” where the first bones of these individuals were found.
- So who were Neandertals? They were an extinct group of humans who lived in Europe from about 400,000 years ago until about 30,000 years ago, when they seem to have gone extinct without leaving any descendants. Their lineage diverged from that of all modern humans several hundred thousand years ago, so they were not our direct ancestors. Our real ancestors appear to have spread out from Africa to Europe and Asia in a migration event beginning about 100,000 years ago. Current Biology published a nice “quick guide” to Neandertals in 2006.
- Were they members of species different from ours? Neandertals are usually classified as being a subspecies of ours, Homo sapiens neanderthalensis, so they’re usually considered members of our own species. Most biologists regard species as groups that are not capable of exchanging genes with other such groups. Since Neandertals now appear to have hybridized with “modern” H. sapiens and produced fertile offspring, this might suggest that they’re in our species. My own species concept, however, allows for some small amounts of gene flow between species, and given the low level of gene exchange between the two groups and the fact that they appear to have lived side by side in several areas without fusing into one species, I’d say that Neandertals were members of a different species, or at least of an “incipient” species. This is to some extent a semantic issue. The two groups may have had cultural aversions to cross-mating, which count as “reproductive isolating barriers” similar to those separating species of birds who don’t hybridize because they have different appearances or songs.
- What about their genome? Sequencing the genome of this group is a stupendous achievement. The workers took DNA from the bones of three individuals (all females) from a cave in Croatia; these females lived about 40,000 years ago. Although the DNA extracted from their bones was highly contaminated with bacterial DNA and slightly contaminated with modern human DNA, the researchers managed, with many controls, to sort out the Neandertal DNA and sequence it. Good sequence was obtained for about 2/3 of the genome, and was compared to sequences from modern humans living in Africa (2 samples), France, China, and Papua New Guinea.
- What does the DNA show? First, that Neandertals seem to be a bit older than we thought: their lineage diverged from ours about 825,000 years ago (this is a very rough estimate based on a “molecular clock”). Second, there are some interesting genes that appear to have evolved faster in our lineage than in the Neandertal lineage (we can tell this because we can compare both genomes to that from our closest living relatives, chimps). These 78 faster-human genes include those involved in skin pigmentation, sweat glands, sperm motility, as well as genes that, in modern humans, carry mutations associated with Down syndrome and schizophrenia. Perhaps selection leading to modern humans, then, acted on skin traits and cognitive abilities.
- What else? The finding that has gotten the most attention is that some “old” genes from Neandertals still persist in human populations. These shared human-Neandertal genes are found in the French, New Guinea, and Asian samples of modern humans but not in the African ones.
- What does this DNA sharing mean? Most likely that there was some hybridization between Neandertals and “modern” humans after our ancestors left Africa but before they had spread throughout Asia and the Pacific. In other words, the genome of those modern humans not from Africa carries a trace (about 1%-4%) of DNA that “introgressed” from Neandertals. This is surprising because, based on earlier sequencing work of Neandertal DNA from the cellular organelle mitochrondia, there was no evidence of mixing between H. sapiens and H. sapiens neanderthalensis populations. This led to the idea that Neandertals went extinct without leaving any descendants or DNA. In light of the new findings, that’s probably wrong.
- Hybridization! Does that mean that Neandertals interbred with the ancestors of modern humans? Probably! But there’s one other explanation that we can’t rule out: perhaps the “Neandertal” genes in modern non-African humans are simply genes that are very old and were present in the common ancestors of both modern humans and Neandertals. These then could have been transmitted to Neandertals and non-African humans, while the ancestors of modern Africans simply did not get a sample of those genes. (Note that this does not mean that, evolutionarily speaking, modern non-Africans are more closely related to Neandertals than to modern Africans!) The “differential sorting” scenario seems unlikely to me because it involves special pleading about a non-random distribution of genes between proto-human populations that occurred well before our lineage diverged from that of Neandertals.
- Why the attention about hybridization? People are fascinated by the possibility that several different species (or subspecies) of humans could have successfully mated with each other. We already knew that, over our evolutionary history, there have been times when several hominin species lived at the same time and roughly at the same place (as I recall, there could have been as many as three or four species of hominins living at the same time in some parts of Africa). But in none of these cases do we have evidence for interbreeding. The new work shows that this probably did occur between two hominin “species”. This fascinates people for one reason, I think: sex. It conjures up pictures of hairy, beetle-browed Neandertals shagging individuals that looked much more like us. And indeed, that’s probably what happened. This scenario evokes all sorts of primal emotions, and I predict a new genre of internet porn involving human/Neandertal encounters! But that shouldn’t completely distract us from what is a truly remarkable achievement: our own ability to reconstruct the genome of our extinct relatives by grinding up their bones, extracting the DNA, and using really sophisticated methods to determine the proper sequence of millions and millions of DNA nucleotides. Think about how amazing that is!
- Now that we have a Neandertal genome, can we make them by cloning? No. We don’t have enough DNA, and even if we did we don’t have the technology to package it into chromosomes, or put it in the proper order with the proper surrounding elements. And if we could do those things, we’d still have to inject the Neandertal chromosomes into the egg of a modern human female, and that’s both hard and ethically dubious.
- Where can I find out more? You’ll find longer but not-too-technical descriptions of the Neandertal work in pieces by Ann Gibbons at Science and Carl Zimmer at The Loom.
Green, R. E. et al. A draft sequence of the Neandertal genome. Science 328:710-722.