Thanks to advances in DNA technology, we now know we're all a little bit Neanderthal. Just how their genes continue to affect us has become a topic of wild speculation, with anything from red hair to sleeping habits a potential legacy left by our extinct cousins.
With more research, many of these claims have been put to rest, but such suggestions do remain, contesting that many Neanderthal genes still hold sway over our physiology, behaviour, and even susceptibility to disease.
A new study suggests at least some of this might be a little overblown, failing to find more than a handful of traits that could be influenced by our ancient genes.
Researchers from Aarhus University in Denmark and a number of Icelandic research institutions took a close look at some 28,000 genomes to work out if possessing DNA from ancient relatives made us more or less likely to have certain biological traits.
A process like this might typically involve hunting down sequences from the two Neanderthal genomes we've managed to collect a decent amount of data on. It's a simple concept, but relies on genetic material dug out of a few old bones to stand in for the vast diversity of genes of a long-extinct population.
Instead, the team analysed a further 286 genomes representing individuals from sub-Saharan Africa to serve as a comparative baseline, assuming that their own DNA would contain little – if any – intruding Neanderthal or Denisovan genes.
Whittling out sequences likely to belong to modern humans, the researchers were left with tens of thousands of ancient fragments scattered all over the genomes of this fraction of the Icelandic population.
Not all of the uncovered codes evolved in Neanderthal populations, though. Around 3 percent of it was found to be associated with another family of ancient relative, the Denisovans.
This also left around 12 percent with no clear heritage, potentially having evolved in long-lost hominids we're yet to discover.
To work out if any of this antique software clunking around in Icelandic bodies makes any real difference to our biology, the researchers measured their relationship with more than 270 physiological traits, including everything from age of menopause to body mass to thickness of the outer layer of the brain.
Differences in virtually every single one of them could be explained better by more modern variants of our genes. So much for you pinning your insomnia and freckles on Great, Great Uncle Grug the First.
There were, however, five notable exceptions. Three were associated with qualities such as blood clotting and haemoglobin measurements. One reduced the risk of men developing prostate cancer.
That left one sequence that just might make itself known by exerting a slight influence on your height.
"Whether individually or collectively, our genome enables us to learn more about who we are by telling us where we come from," says neurologist Kari Stefansson, CEO of the Reykjavik-based biopharmaceutical company deCODE genetics.
"This paper is a kind of ancestry report for one branch of our species, and it's telling us that in this particular neighbourhood we are not just Homo sapiens but also the descendants of ancient archaic humans – cousin species whose lineage is thus not entirely extinct."
There could still be a number of characteristics unaccounted for by the study. For example, past research suggests the shapes of our skulls could have Neanderthal underpinnings, a trait that wasn't included in this latest investigation's list of traits.
Then there's recent research that warns against being too hasty in presuming DNA among modern African communities isn't quite as clear of Neanderthal input as we might expect. When it comes to flinging our genetic material around the globe, our ancestors were pros.
Just how humans evolved – and continue to evolve – is turning out to be a seriously complex story, one that will only become more complicated as we expand our databases of archaic genomes.
To the researchers, there are lessons to be learned in the qualities that set us apart from our ancestors.
"What we know is that in the 50,000 years from their time to this, our adaptability and diversity have enabled us to mix and move, settle and thrive in every corner of the planet as they did not," says Stefansson.
"In these dark days we would do well to remember that our differences are literally the mark of our success, and so to help each other as best we can."
This research was published in Nature.