Despite going extinct tens of thousands of years ago, Neanderthals' influence on humans remains. Every modern human has DNA inherited from when our ancestors mated with this other species of hominid, although some have more than others.
The presence of these genes is no longer surprising, but now researchers have found a peculiar location for some of this genetic material: centromeres. Centromeres are in the middle of chromosomes, where the “pinched-in” bit gives chromosomes their traditional X-shape. These regions are pulled apart when cells divide, so defects can occur during the process, which can lead to cancer, among others.
Since centromeres contain many repeating features, they are difficult to map in detail. Despite these difficulties, the researchers decided to track haplotypes within them. These are a group of genes inherited together and can be used to track evolution.
Centromeres are not affected by the “crossover” process that can happen between chromosomes when egg and sperm cells are created, so the team thought they may hold genes from a long time ago. The team tested this on Drosophila fruit flies, demonstrating that they could indeed track centromeric haplotypes.
They then looked for these in humans by using centromere sequences from the 1000 Genomes Project, the largest public catalog of human genetic variation, collecting the genomes of 2,504 individuals from 26 populations.
As they report in an upcoming issue of the Journal eLife, they found that some Neanderthal haplotypes are found in the centromere of chromosome 11 in non-African genomes, which could play a role in one of our senses. Humans have roughly 400 genes for smell receptors and 34 are located within the chromosome 11 haplotype. For some people, their sense of smell might be partially thanks to Neanderthals.
Another finding was the presence of a much older haplotype that appears to be derived from an unknown hominid. Maybe this is the same mysterious hominid that Neanderthals mated with before Homo sapiens came out of Africa. Or maybe it is a completely different population that we are yet to discover.
On the X chromosome, the team found several major centromeric haplotypes extending back almost half a million years. Peculiarly, one of the oldest was only present in modern humans from the African continent.
