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And so is the past. Migration and mingling are essential to human success in the past, the present and into the future
In the future, a lot of people might look like Danielle Shewmake, a 21-year-old college student from Fort Worth, Texas. Shewmake has dark, curly hair, brown eyes, and an olive skin tone that causes many to mistake her heritage as Mediterranean. Her actual pedigree is more complex. Her father is half-Cherokee and half-Caucasian, and her mother, who was born in Jamaica, is the child of an Indian mother and an African and Scottish father.
‘My sister and I are just a combination of all that,’ she says, adding that she dislikes having to pick a particular racial identity. She prefers the term ‘mixed’.
Differences in physical traits between human populations accumulated slowly over tens of thousands of years. As people spread across the globe and adapted to local conditions, a combination of natural selection and cultural innovation led to physical distinctions. But these groups did not remain apart. Contact between groups, whether through trade or conflict, led to the exchange of both genes and ideas. Recent insights from the sequencing of hundreds of thousands of human genomes in the past decade have revealed that our species’ history has been punctuated by many episodes of migration and genetic exchange. The mixing of human groups is nothing new.
What is new is the rate of mixing currently underway. Globalisation means that our species is more mobile than ever before. International migration has reached record highs, as has the number of interracial marriages, leading to a surge of multiracial people such as Shewmake. While genetic differences between human populations do not fall neatly along racial lines, race nevertheless provides insight into the extent of population hybridisation currently underway. This reshuffling of human populations is affecting the very structure of the human gene pool.
Archaeological evidence suggests that Homo sapiens came into existence roughly 200,000 years ago in east Africa. By 50,000 years ago (but possibly earlier) people had begun to spread out of Africa, across the Arabian Peninsula and into Eurasia, perhaps driven by a changing climate that necessitated a search for new food sources. They made their way across now flooded land bridges to reach Australia and the Americas, and eventually came to inhabit even the most remote Pacific islands.
Evidence of these ancient migrations can be found by examining the DNA of living people as well as DNA recovered from ancient skeletons. In some cases, the genome studies corroborate archaeological and historical records of human movements. The Mongol Empire, the Arab slave trade, the spread of Bantu-speaking peoples across much of Africa and the effects of European colonialism have all left a predictable record within our genomes. In other cases, the genetic data provide surprises and can help archaeologists and historians settle controversies. For example, until recently, it was thought that the Americas were settled by a single wave of nomads who travelled across a land bridge spanning the Bering Strait. But recent genome analyses, which include samples from a wide range of indigenous groups, suggest that the Americas might have been colonised by at least four independent waves of settlers.
We are a restless species, and our genomes reveal that even the most intimidating geographical barriers have managed only to somewhat restrict human movements. Today, international migration is increasing at 1 to 2 per cent per year, with 244 million people in 2015 living in a country other than the one in which they were born. The biological implications of this massive experiment in interbreeding we are now witnessing will not be known for generations. But applying what we know about genetics and evolution can help us predict our future, including whether humans will be able to continue adapting to the constantly changing conditions on Earth.
Biological adaptation is a result of natural selection, and natural selection requires diversity. Think of natural selection like a sieve separating one generation from the next. Only the genes from those individuals that are well suited to their environment at that time will reproduce, passing their genes through the sieve to the next generation. Changing conditions alter the shape of the sieve’s holes and thereby which genes can pass through. The more variation there is in the population, the better the chances that some genes present in a generation will be able to pass through the sieve and be inherited by future generations. Unfortunately for us, humans are not very diverse…