There have been many branches on the hominid tree, but now a lone species walks the earth. We had company once, though, at least in Europe and West Asia - the Neanderthals.
Until recently, the scientific consensus was that they were sufficiently different from Homo sapiens that no interbreeding took place. We knew that they controlled fire; constructed tools, shelters and garments; took care of their weak, injured and elderly; and buried their dead with grave goods. But until two decades ago it was widely believed that they had attributes which disadvantaged them to such an extent that competition with modern humans led to their extinction (for example, lack of capacity for complex language, almost exclusive dependence on hunting for sustenance... to say nothing of the Tarzanist view that their doom came about because -- horrors! -- they allowed women to join the hunt).
This idea of Neanderthals as grunting, shuffling dead-end cavemen began to change as our techniques allowed us to examine Neanderthal fossils with more precision and depth. In the mid-eighties, bone and genetic analysis proved that their ability to hear and produce sounds was almost identical to ours (including a human-like FOXP2 gene, whose function is critical for language). Sequencing of their melanocortin gene indicated that some might have red hair and light skin.
Finally, the just-published draft sequence of their genome (headed by Svante Pääbo's team from the Max Planck Institute) showed that up to 4% of the genes of non-Africans may come from them. If confirmed, this means that Neanderthals and Cro-Magnons indeed interbred when the latter first came out of Africa - most likely in the Middle East, before further migration outward.
The two branches of humanity share 99.7% genetic identity. They show differences in genes involved in cognitive development, skeleton structure, energy metabolism, skin physiology. They also differ in regulatory regions and microRNAs. This information will eventually help us answer the question of what makes us uniquely "human" - perhaps even what has made us so adaptable that we now threaten to overwhelm the earth.
When I read about the conclusions from the draft sequence analysis, tears sprang to my eyes, just as they do at spaceship and planetary probe launches. It moved me inexpressibly to think that they haven't vanished but are with us still, a thread in our fabric, a whisper in our song.
Note 1: The best fictional depiction of the interaction between Neanderthals and Cro-Magnons is Dance of the Tiger by Björn Kurtén, a distinguished vertebrate paleontologist who was Swedish -- as is Pääbo.
Note 2: This article is also on the author's blog, with a nifty graph that shows the genetic interweaving.
If you go to their paper (available online, http://www.sciencemag.org/cgi/content/full/328/5979/710) and look at Figure 6, you will see all the possible scenarios for their results.
There is of course the possibility that Human females were simply more attractive or produced a lower infant mortality rate. But, among current primates only the Human and Bonobos species females have a menstrual cycle while others have a estrus cycles. The difference is sexual receptivity. A primate with a estrus cycle would be receptive only during ovulation, which may be only a few days monthly or even only a few days a year depending on the species. A primate with a menstrual cycle is typically receptive at all times.
Since Neanderthals appear to have developed in a climate of sever climatic variation, it is likely that a seasonal estrus cycle would have been beneficial for the survival of offspring. Conversely, Humans evolved in a climate where that would not have been a benefit.
Rather than out thinking, out hunting, or our farming Neanderthals, is it possible that the ever-present "girl next door" was tearing the Neanderthal's family and tribal cohesion apart?
Farming came into existence a lot later, so it doesn't enter the equation at all. One possibility is that the Neanderthals' apparently heavier reliance on game made them less adaptable than the Cro-Magnons, who depended a lot on gathering -- done mostly by women. So I think the women played a constructive role, not a destructive one.
The menstrual theory doesn't hold up to close scrutiny. For one, if bonobos have menstrual cycles, all hominids -- not just Cro-Magnons -- also would, since they came from a common ancestor shared with the bonobos. For another, if we judge by what we know about today's few left gatherer-hunters, women of those groups are lean enough that they're fertile only very intermittently, regardless of their sexual receptivity.
I've read elsewhere that, based in physiology, Neanderthal would have been the more likely gatherer, the hip structure and muscularity being more suited to the rapid short movements of gathering, while the human running gait would be more adaptable to following game and even outrunning large game over long distance. Apparently you disagree. I'm curious why.
BTW, thank you ever so much for engaging with your readers like this!!!!
the Neanderthals rode dinosaurs 6,000 years ago!
The possibility that either we or primate relatives would branch into additional species is not formally excluded. However, for such a thing to happen, we need isolated groups (so-called founders) that will be isolated long enough to start diverging to the point of eventually becoming a new species -- defined by the inability to interbreed with the rest.
The problem is that humanity now occupies the entire planet. This means that 1) there are no isolated human populations and 2) we have disturbed the habitat of related primates to the point that they are about to become extinct. This configuration leaves little room for branching into new species.
However, there is a scenario that will create speciation, and do so much faster than the usual time frame: if we embark on long-term space travel or planetary colonization, the resulting isolation and the pressures from planetary parameters that differ greatly from terrestrial ones will result in distinct human species. Each spaceship will effectively be a founder population.
It is also possible that settlers on other planets will have to resort to genetic engineering to meet the challenges of their new home. This, too, may result in speciation if they engineer the germline. I discuss this more extensively in Making Aliens, a six-part artlcle starting at http://www.starshipreckless.com/blog/?p=24.
If you go to their paper (freely available online, http://www.sciencemag.org/cgi/content/full/328/5979/710) and look at Figure 6, you will see all the possible scenarios for their results.
Two years later...voila! We see neanderthal genes identified not just in Geico adds but by illustrious statesmen like Henry Kissinger (seriously...make the comparison!)
The list of prior assertions now proven false are troubling. So Neanderthals could speak, make stone tools better than us (we?) at times, had 10% larger brains, used complex mixtures to form make-up, buried their dead with flowers, and mated with humans and produced fertile offspring? We’re clearly getting short changed by academia on this topic.
And we're still saying they were a different species? I'm confused. The mating of social politics and science usually makes my head spin that way.
Also, conclusions in science change as more data come in and as techniques to observe and interpret them improve. That's why it's called science, not religion.
Since earlier studies of mitochondrial DNA had seem to show that our modern populations are not genetically related to Neanderthals, had figured that the two species must not have been inter-fertile, or that they produced only sterile hybrids, as horses and asses do. It was nice to see this new study of the full Neanderthal genome (first draft) showing that there must have been viable interbreeds after all!
I must admit the the first thought that crossed my mind upon reading this was to laugh my a$$ off at the thought of all those white supremacists who have been calling African Americans sub-humans discovering that the opposite is true. *giggle*
Who says Karma has no sense of irony?
Yet think of the Elf-Human couplings in Tolkien. Granted, there are only three of them, and the human partners supposedly were la crème de la crème of their species. But unlike Neanderthals and Cro-Magnons, who were very similar, Elves are presented as far superior to Humans. Yet nobody cringes at the idea of Arwen mating with Aragorn.
Why is the image horrifying? They looked almost identical -- as different (or as similar) as members of different races. A great book about how an equivalent romancing might have happened and looked is Elizabeth Marshall Thomas' The Animal Wife.