Although Neanderthals are often portrayed in drawings as swarthy, in fact they arrived in Northern Europe thousands of years before modern humans, giving time for their skin to become paler as their bodies struggled to soak up enough sun.
When they interbred with modern humans those pale genes were passed on. Likewise genetic mutations which predispose people to arthritis also came from our Neanderthal ancestors, as did the propensity to be a night owl rather than a lark, as northern latitudes altered their body clocks.
A raft of new papers published in the journals Science and the American Journal of Human Genetics has shed light on just how many traits we owe to our Neanderthal ancestors.
Scientists also now think that differences in hair colour, mood and whether someone will smoke or have an eating disorder could all be related to inter-breeding, after comparing ancient DNA to 112,000 British people who took part in the UK Biobank study.
The Biobank includes genetic data along with information on many traits related to physical appearance, diet, sun exposure, behavior, and disease and helps scientists pick apart which traits came from Neanderthals.
Dr Janet Kelso, of the Max Planck Institute for Evolutionary Anthropology, in Germany, said: “We can now show that it is skin tone, and the ease with which one tans, as well as hair color that are affected.”
When modern humans arrived in Eurasia about 100,000 years ago, Neanderthals had already lived there for thousands of years and would have been well adapted to lower and more variable levels of sunlight than the new human arrivals from Africa were accustomed to.
"Skin and hair color, circadian rhythms and mood are all influenced by light exposure," added Dr Kelso.
"Sun exposure may have shaped Neandertal phenotypes and that gene flow into modern humans continues to contribute to variation in these traits today."
Based on new genetic material taken from a Neanderthal female who lived in Croatian around 50,000 years ago, scientists now estimate that Europeans owe up to 2.6 per cent of their DNA to Neanderthals, not the 2.1 per cent which was previously thought.
A separate study by Cambridge University also gave clues as to why Neanderthal populations died out, while modern humans thrived.
Early humans seem to have recognised the dangers of inbreeding at least 34,000 years ago, and developed surprisingly sophisticated social and mating networks to avoid it, the researchers found.
Genetic studies of Upper Paleolithic skeletons from Sunghir in Russia show no inbreeding in modern humans, unlike Neanderthals where inbreeding can be seen in genetic mutations. Objects and jewellery found at the site, linked to different tribes, also suggest that modern humans selected partners from a wider network of group, and may have even held rudimentary marriage ceremonies, swapping precious objects.
“What this means is that even people in the Upper Palaeolithic, who were living in tiny groups, understood the importance of avoiding inbreeding,” said Professor Eske Willerslev, a fellow at St John’s College, Cambridge.