Throughout history, humans have used changes in skin colour to support the idea that people belong to different races, but a new study says it's beyond time to challenge that thinking.
Research into genetic variants associated with skin pigmentation in Africa helps to explain why African peoples embody a wide variety of different skin colours, and shows the factors that control our complexion are more complicated than concepts of race allow.
"One of the traits that most people would associate with race – skin colour – is a terrible classifier," geneticist Sarah Tishkoff from Penn University told The Atlantic.
"The study really discredits the idea of a biological construct of race. There are no discrete boundaries between groups that are consistent with biological markers."
Tishkoff's team recruited 1,570 African volunteers from ethnically and genetically diverse populations in Ethiopia, Tanzania, and Botswana.
The researchers measured the light reflectance of the participants' skin, which indicates levels of the skin pigment melanin, and took a sample of their DNA.
The skin colour of the volunteers ranged greatly in colour, with the darkest being shown in Nilo-Saharan pastoralist populations in eastern Africa, and the lightest in San hunter-gatherer people in southern Africa.
"[W]ithin Africa there is a huge amount of variation," Tishkoff explains in a press release, "ranging from skin as light as some Asians to the darkest skin on a global level and everything in between."
When the team analysed the participants' DNA, they sifted through some 4 million sites in the volunteers' genomes looking for nucleotide variations that could create differences in skin colour.
They identified six key genes – known as SLC24A5, MFSD12, DDB1, TMEM138, OCA2, and HERC2 – featuring variants that are significantly associated with skin pigmentation, and which altogether are responsible for 29 percent of skin colour variations among people in Ethiopia, Tanzania, and Botswana.
But these genes don't just affect the appearance of Africans. Previous research on pigmentation has identified some of the same markers being behind the generally lighter skins of Europeans and Asians.
The analysis also suggests most of the variants have been around for more than 300,000 years – and some are almost 1 million years old – meaning they long predated modern humans, and would have helped control pigmentation in our primitive ancestors.
The team's hypothesis is that some of the skin-lightening variants, including those around HERC2 and OCA2, may have arisen in Africa almost a million years ago, before being spread to Europe and Asia.
If they're right, that means our ancient ancestors might not have had dark skin, but lighter skin, before adapting to new conditions.
"If you were to shave a chimp, it has light pigmentation, so it makes sense that skin colour in the ancestors of modern humans could have been relatively light," says Tishkoff.
"It is likely that when we lost the hair covering our bodies and moved from forests to the open savannah, we needed darker skin."
While this is the largest study looking at the genes behind African skin pigmentation to date, the researchers acknowledge there's still a lot we don't know about these variants, and some 70 percent of the biological markers affecting the colour of African skin are yet to be identified.
But even so, the researchers say their study shows there's more in our skin that unites us than divides us, so racist views and historically ill-informed conjecture about traits associated with the colour of skin aren't just immoral – they're scientifically wrong.
"[L]ight skin pigmentation, and likely other 'European' traits, are not unique to Europeans," biological statistician Jedidiah Carlson from the University of Michigan, who wasn't involved with the study, told The Atlantic.
"Human populations have been interbreeding for as long as we have existed as a species."
The findings are reported in Science.