Connexion
When the history of our ancestry is written, the fact that we've interbred with some of our closest relatives, the Neanderthals and Denisovans, will have a central role. And it will be tempting to write it as a very tidy story: Once we got genomes from these other groups, it was possible to identify the sequences in our genomes that we shared with them.
But in reality, as scientists started poking large enough collections of data, there were regular hints of some strange ancestry in our genomes. It was hard to pin down, though, at least in part because the 2 percent on average of Neanderthal DNA found in many populations does not guarantee that any two individuals will have the same 2 percent. So having the genomes of those two groups made sense of some things researchers had already been seeing.
But knowing what we do about Neanderthal and Denisovan DNA is now allowing researchers to answer a somewhat different question: Is there anything else? Using recently developed analytical techniques, they find evidence of a third lineage that we apparently interbred with before any modern humans left Africa. Again, there were hints of this earlier, but so far, there has been no genome from a modern human relative to help us understand the details—the source of this DNA remains a "ghost lineage."
Humanity's ancestry has grown far clearer thanks to our ability to obtain ancient DNA. We now know that, as humans left Africa, they interbred with the groups they met there, Neanderthals and Denisovans. Evidence from the Denisovan genome also suggests that this was nothing new; the Denisovans had apparently interbred with an even earlier group. But the identity of that group remained a bit of a mystery.
Now, some evidence from ancient proteins suggests that the mystery group was Homo erectus, a species that left Africa over a million years ago and spread throughout Eurasia. And, thanks to the Denisovans, it appears that modern humans inherited some of that Homo erectus DNA.
Without access to all the repair enzymes made by living cells, DNA rapidly degrades. The double helix fragments, and bases change identity or fall off entirely. While cooler, drier environments slow this process, it sets a hard limit on how far back in time we can obtain DNA sequences. So far, it seems that Homo erectus remains on the far side of that time limit.
If you look at a Neanderthal skull and a Homo sapiens skull, they're visibly different: Neanderthal skulls are lower and longer, whereas ours tend to be rounder. However, those differences probably don’t say much about the brains within them, according to a recent study, which compared MRI scans of modern people’s brains with casts of the inside of Neanderthal skulls.
The results suggest that there’s more variation in brain size among modern people than between Neanderthals and Pleistocene Homo sapiens. And because brain size is actually a terrible way to predict cognitive capability, Neanderthals could have been a lot more like us than some previous studies have claimed, which definitely fits what the archaeological record tells us about how they lived. It would also mean that our species probably didn't out-compete the Neanderthals by being smarter or more adaptable.
Years after you die, the inner vault of your skull will hold the shape of your brain; if future archaeologists make a cast of that inner space, they’ll get a neat resin model of the outer contours of your brain, called an endocast. (Sediment that filled the skull of an Australopithecus africanus child who died 2.8 million years ago did this naturally, creating an endocast that’s half rocky brain-sculpture and half sparkling crystal.) For years, researchers have studied endocasts of Neanderthal skulls, trying to piece together how their brains were different or similar to ours. And that has been a matter of some debate.
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