We know that as ancient humans expanded into Eurasia, they began interbreeding with our Neanderthal cousins. But it now appears that the fun didn't start there - our ancestors also reproduced with precursors like Homo erectus and Homo habilis.
Thanks to DNA evidence, it's now well-established that humans interbred successfully with Neanderthals and another contemporaneous group, the Denisovans - so much so that it's now arguable whether Neanderthals should even be considered a separate species. Unfortunately, it's a lot harder to know about possible interbreeding that happened before Homo sapiens left Africa.
The reason for that is quite simple - DNA evidence is easily preserved in cool climates, and generally destroyed in warm ones. That means ancient DNA evidence is almost exclusively found in Europe and Asia, but not in the warmer climes of Africa. As such, we can't rely on direct DNA evidence to prove African interbreeding in the same way we did with Neanderthals. But researchers at the University of Arizona have found a way around this challenge.
The trick is to look for instances in the modern human genome that appear out of place, as those could be evidence of genetic inheritance from other groups. Admittedly, that's still a tricky process - after all, we don't have any Homo erectus or Homo habilis genomes to compare, so we can only guess which parts of the modern human genome come from them.
The researchers zeroed in on modern Africans. This is because they're mostly closely related to the earliest human populations, and there's less genetic drift because they haven't moved around as much. They took genome samples from six populations in Africa and then went on the hunt for potentially archaic sequences.
Researcher Michael Hammer explains:
"We can simulate a model of hybridization between anatomically modern humans and some archaic form. In that sense, we simulate history so that we can see what we would expect the pattern to look like if it did occur. Then we asked ourselves what does the general pattern of variation look like in the DNA that we sequenced in those African populations, and we started to look at regions that looked unusual. We discovered three different genetic regions fit the criteria for being archaic DNA still present in the genomes of sub-Saharan Africans. Interestingly, this signature was strongest in populations from central Africa.
"We are talking about something that happened between 20,000 and 60,000 years ago — not that long ago in the scheme of things. If interbreeding occurs, it's going to bring in a whole chromosome, and over time, recombination events will chop the chromosome down to smaller pieces. And those pieces will now be found as short, unusual fragments. By looking at how long they are we can get an estimate of how far back the interbreeding event happened."
The archaic sequences found by the researchers only accounted for about 2 to 3% of the modern genome samples, but the interbreeding might actually have been more extensive than that. If the genetic content derived from interbreeding with Homo erectus and habilis didn't add anything from an evolutionary perspective, it would likely drift back out of the population over time.
"We think there were probably thousands of interbreeding events. It happened relatively extensively and regularly. Anatomically modern humans were not so unique that they remained separate," he added. "They have always exchanged genes with their more morphologically diverged neighbors. This is quite common in nature, and it turns out we're not so unusual after all."
The takeaway from all this, it would seem, is that ancient humans were willing to have sex with any species that looked even vaguely like them, and they had a lot of success with hybridization. Perhaps Star Trek isn't quite as biologically ridiculous as I thought it was. On second thought...