An international team led by researchers at the University of Washington made splashes when their newest article , revealing the sexual escapades of our hominin ancestors, made it into the popular press. See their figure below displaying how ancestral humans interbred.
This led to a flurry of activity on Twitter with speculations about the frequency of matings and how often mating resulted in viable offspring. I found the tweets by Matthew Herper and Carl Zimmer, two science journalists, to be especially interesting.
In thinking about Carl Zimmer's comment about the offspring of human-Neanderthal matings not being mules, several thoughts come to mind. To start, what's a mule? Answer: a mule is the product of interbreeding of two distinct species, horses and donkeys, which have a different number of chromosomes. As a result, mules are sterile. So to say that the offspring of human-Neanderthal (or human-Denisovan) matings weren't mules might imply that they weren't sterile. Indeed, humans and Neanderthals (and Denisovans) have the same number of chromosomes, making our ancestors less like mules. But more than this, Neanderthal and Denisovan DNA survives in us. For this reason, we can infer that, minimally, some of the offspring of the ancient interbreeders were fertile.
Second (and more tangential) thought: what is our closest relative who has a different number of chromosomes? (Perhaps this is something you learned in junior high and high school biology, but I missed this piece of not-so trivial trivia.) Well, if you already know that humans and chimps share most of their DNA, then chimps are a good guess. As it turns out, chimps have 48 chromosomes. We have 46. So how is it that we can have a different number of chromosomes but share so much DNA? Part of the answer is that our chromosome 2 is actually a hybrid of the two more ancient chromosomes existing among members of the Hominidae family (humans, Neanderthals, and Denisovans being the exception). The evidence for this is that human chromosome 2 actually contains a vestigial centromere, the remains of two telomeres joining that resulted in the singular human chromosome 2 [2,3]. The genes on our chromosome 2 and the corresponding ones in chimps match up.
I'm a Public Health Genetics PhD student at the University of Washington and a molecular epidemiology research fellow at the Fred Hutchinson Cancer Research Center. I post (mostly) about topics in epidemiology and genetics.