by Meave Leakey
When our ancestors reached Eurasia 70,000 to 60,000 years ago, the Neanderthals and Denisovans were there. We did not know the Denisovans existed until their genome was extracted from unidentifiable fragments of bone, but there are dozens of Neanderthal skeletons, some of them found in the same caves that were later occupied by H. sapiens. For 150 years, scientists have pondered and argued about what their relationship to humans would have been, and geneticists have now given us some of the answers. When a human population first invaded Eurasia, they met a group of Neanderthals, and from that encounter, some of the children in the human population had a Neanderthal parent. From those children, every non-African person alive today—some 6.5 billion people—carries small fragments of Neanderthal DNA that make up 1 to 2 percent of their genome. And in the process of expanding further into Asia, some people met Denisovan populations, and a few modern groups—Aboriginal Australians, Papuans, and groups of hunter-gatherers such as the Aeta in the Philippines—can trace a percentage of their DNA to the Denisovans. Given that the different groups obviously could have children together, the real mystery is not that they interbred, but that the admixture was so rare. But we had been evolving separately from Neanderthals for half a million years during which each lineage acquired its own unique features, some of which made us genetically incompatible. Geneticists have been able to show that there was strong selection against some Neanderthal genes in those hybrid children, most of whom probably did not survive. Indeed, the ancient genome of a 38,000-year-old fossil from Romania shows that one of his great-great-great-great-grandparents was a Neanderthal, but those Neanderthal genes are not found in anyone alive today.
Pulling together the information from ancient and modern genomes, scientists have been able to map the dispersal of humans out of Africa. Remember, this was a time of extreme cold. Sea levels were low, and northern Eurasia would have been blocked by arctic deserts while Europe had the resident Neanderthals. Early Eurasians (after mixing with Neanderthals, presumably in the Near East) split into several small groups of hunters. One of these ventured east right away, probably following the coastline around India to the land bridges that emerged to connect many of the Indonesian islands to the continent of Australia. It is thought that they arrived in Australia about 60,000 years ago, and this is corroborated by the earliest evidence of human occupation in Australia at this time found at Madjedbebe in the Northern Territory. This small founding Australian population included several individuals whose ancestry was part Denisovan as the genome of today’s Aboriginal Australians and Papuans has 6 to 8 percent Denisovan genes (as well as the 1 to 2 percent Neanderthal genes that all non-Africans have). Additional genetic mutations and differences arose through time and allow many details to be added to the times and routes that explain how people spread around the globe.
It’s been an honor to participate in the National Geographic’s Genographic Project—an exciting endeavour that began in 2005 and combines the number-crunching power of IBM with the financial muscle of the Waitt Family Foundation under the direction of Spencer Wells. I have already learned much from my association—including my own genetic trail. My ancestors belong to mtDNA haplogroup H. Everyone in my haplogroup has an H marker—we are a single branch of the human family with a common ancestor who first exhibited a new mutation labelled H. Tracing the H marker backwards, I can reconstruct much about my ancestral migratory route. It begins with the group of people who first moved out of Africa some 60,000 years ago, probably following an expanding food source much as we have speculated that the first Homo erectus migrants did over a million years previously. We don’t know the exact route: once again, the Nile must have played an important part.
These early African ancestors moved into the Middle East, where they had children with Neanderthals and hung out for several thousand years. At this time, the ancestral African mtDNA mutated, creating two new lineages, M and N, which in turn gave rise to new mtDNA lineages that spread through most of Asia. My own early ancestors were a sturdy bunch, for among them are humans who traversed the Caucasus Mountains of Georgia and southern Russia and colonised Europe. These were the first Cro-Magnons, and their arrival heralded the end of the Neanderthals. Ancient DNA from Cro-Magnon fossils shows a history of successes and failures shaped by the extreme climate of glacial Europe. Early Europeans carried mtDNA haplogroups M and N as well as already differentiated N lineages such as U and R, which would eventually spawn my own haplogroup H. However, a new and brutal cold spell pushed these ancient hunters into small refugia, and only some of them survived. My ancestors waited out the cold spell in southern Europe, most likely on the Iberian Peninsula. As the current warm, balmy weather set in some ten thousand years ago, they recolonised land that had previously been swathed in thick sheets of ice. But they soon had to compete with newcomers—farmers and Bronze Age peoples from Western Asia. I share the H marker with some 40 to 60 percent of the gene pool of most European populations.
The New World was the last to be populated. Southern Siberia was invaded about 40,000 years ago, but these early migrants were blocked from reaching the New World until the last glacial maximum lowered sea levels sufficiently to provide a bridge across the Bering Strait. Humans probably first arrived in North America some 15,000 years ago before they migrated south. There are few fossils to corroborate the genetic findings because these migrations took place when the world was swathed in ice and the continental shelves were exposed. Most of the physical evidence of their epic trek has long been submerged under the oceans.
The genetic makeup of people today gives us critical clues about our ancestral histories. However, the details of those histories—when and where particular populations lived or changed—can only be inferred from where people live today. We know that our very recent past is characterized by massive population movements as farmers and pastoralists expanded throughout vast parts of the world in the last 10,000 years. Ancient genomes give the time-and-place stamp for these movements as well as giving us extraordinary insights into the history of humans in Eurasia and the Americas. Unfortunately, there are extremely few human fossils in Africa from 200,000 to 10,000 years ago—only about ten! On top of that, DNA preservation is affected by heat and humidity, and attempts to retrieve genetic information from those few precious fossils have failed. And yet the most revealing information has to come from Africa.
Africa is a vast continent—30.37 million square kilometres, large enough to hold three Europes. But it is not only large, it is also where humans have lived some 150,000 years longer than anywhere else. It is not surprising that the people of Africa today have the greatest genetic diversity since they have had the longest time to build up mutations. Most African populations descend from the same group that expanded out of Africa 70,000 years ago and also from small groups of their descendants who expanded as farmers and herders in the last few thousand years. But a few groups of hunter-gatherers, such as the San of southern Africa, the Bayaka pygmies of Central Africa, and the Hadza and Sandawe of Eastern Africa, trace their genetic ancestry deep into the human past. These hunter-gatherer peoples have languages that include unique clicks that are believed to have been initially used when hunting to avoid alerting animals to their presence. They are also considered to be the most closely related to the ancient people who left their beautiful paintings on the rock faces of Southern and Eastern Africa, and through their genetic uniquenesses, they provide us with insights about our remote past. Yet, we must await the joint effort of fossil and ancient gene hunters to tell us where in Africa our ancestors lived, when they spread throughout the continent, whom they met while doing so, and what selective pressures shaped our shared humanity.
Tracing the myriad paths through the genetic footprints of our ancestors has taken on a new urgency because these DNA trails are rapidly becoming tangled as people move from their ancestral homes in search of economic opportunity. Teams of geneticists from all over the world are racing to analyse the DNA of indigenous populations to
trace the genetic markers that characterise them. Collecting samples from as many indigenous people as possible is literally a race against time. More and more people are abandoning their traditional lifestyles for the melting pot of the big city, and the genetic trail that has persisted in their bloodlines for generations will soon be removed from its geographical context and forever lost in subsequent generations.
While many of these markers are hidden clues to our past buried in intricate codes in our cells, other mutations—those rare beneficial ones—have led to tangible differences between the peoples that have populated different parts of our planet. One of the most visible of these is skin colour. This highly visible identifying feature has historically been loaded with tragic implications, falsely signifying differences in superiority that have had—and continue to have—devastating consequences for those seen to be lower down the racial power ladder. The kaleidoscope of skin colours we exhibit ought to be seen as a celebration of our collective versatility and adaptability in an unforgiving world.
We have already suggested that the first move towards our modern cooling system—proficient sweating through bare skin—began when Homo erectus became a successful predator through persistence hunting. Being upright in hot sun and coping with higher body temperatures during marathon runs necessitated this. But it is unlikely that our ancestors permanently lost all their fur very early because we know they survived frigid weather when they migrated northwards. It is impossible to tell when exactly humans lost their protective fur, but I believe it was relatively recently and had to do with the acquisition of clothing.
Like the tapeworm that hitched a ride out of Africa in the gut of Homo erectus, another parasite, the lowly louse, latched on to Homo sapiens and gives us some clues as to when we might have abandoned the naked state. African lice are the most genetically diverse in the world, so we know that the louse originated in Africa. Of the two most common kinds of human lice, the head louse (Pediculus humanus capitis) exhibits the most diversity in its genes, suggesting that the body louse (Pediculus humanus humanus) evolved from the head louse. Mark Stoneking and his colleagues at the Max Planck Institute for Evolutionary Anthropology have hypothesised that this new kind of body louse must have evolved when a new habitat became available—clothes (or probably furs at first). They looked at the sequences of mtDNA and segments of nuclear DNA from the lice, and their molecular-clock analysis suggests that body lice originated 72,000 years ago —right before Homo sapiens started to move around. Their date has a huge margin of error of 42,000 years. But this is certainly around the time we would expect the advent of clothing if Homo sapiens was to survive the northern cold. The first clothes were almost certainly unshaped skins and furs, but it is unclear whether the first body lice could have lived in furs as they infest woven fabrics today.
If you aren’t a naked mole rat—living a protected subterranean existence far from the reach of the sun’s harmful rays—being a terrestrial animal with a lot of exposed skin is a risky business. Skin is easily damaged by the ultraviolet radiation in sunlight, and this damage can eventually lead to skin cancer that can sometimes be fatal. Chimpanzees and other apes have pale skin beneath their protective fur, and the areas of exposed skin—the face, hands, and feet—darken over time with progressive exposure to sunlight (those kept in cages out of the sun never darken). It is generally believed that the common ancestor of humans and other apes had light skin covered with dark hair. It was long assumed that darker skin evolved in humans to protect against skin cancer or conversely, later in our evolutionary history, lighter skin evolved as those in less sunny climes had a reduced need for this UV protection.
Darker skin does offer better sun protection because it has more of the pigment melanin, nature’s sunscreen. Melanin works both physically and chemically to filter the harmful UV radiation in sunlight. It absorbs UV rays so they lose their powerful energy, and it neutralizes the cancer-forming free radicals that develop in sun-damaged skin. Nina Jablonski, whom I first met many years ago through a common passion for monkeys, is an anthropologist with an abiding interest in all things skin. Nina is an intelligent, focused, and grounded person. I knew that she had long puzzled about how changes in skin colour came to be selected for, because skin cancers only develop later in life—after women would have had the chance to have babies. Different amounts of melanin could not have been selected for their protection against cancer. There was something else going on.
Nina stumbled on part of the elusive connection quite by accident in a paper written in 1978. This paper reported that when very fair-skinned people were exposed to large amounts of strong artificial sunlight, they had abnormally low levels of folic acid in their blood afterwards. Sticking a container of human blood serum in the same artificial sunlight reproduced the same result within an hour: the folic acid in the blood serum was reduced by an astounding 50 percent. A second clue turned up in the late 1980s when some of Nina’s colleagues at the University of Western Australia discovered that folic acid deficiency in pregnant women can lead to abnormalities such as spina bifida, a neural tube defect that prevents the spinal vertebrae of the foetus from closing around the spinal cord. They then discovered the huge role that folic acid plays in any process of cell proliferation because it is essential for the synthesis of DNA. It has since been found that folic acid treatment can boost the sperm count of men with fertility problems. Dark skin likely evolved to protect vital B-complex vitamins.
If dark skin is so much better than light, why did those migrants to higher latitudes lose it? This was the next conundrum for Nina and her colleagues. Nina knew about another relationship between the pigment melanin and UV light. It has long been known that the body needs vitamin D for various essential processes ranging from calcium absorption for bone development to maintaining a healthy immune system. It had also been established that the shorter wavelength UV radiation, UVB, helps the skin to manufacture vitamin D. Nina’s brilliance was in putting all these parts of the story together.
Nina is married to George Chaplin, a British expert on global information systems. George and Nina combined their considerable intellectual talents and produced some illuminating maps. First, they constructed a map of the intensity of UV radiation levels on the earth’s surface based on data from NASA satellites equipped with specialized spectrometers that were measuring ozone values. From this map, they produced a second map predicting the skin pigmentation of indigenous people from all over the world based on UV radiation levels. Then they looked at how close actual pigmentation was to their predictions. They found that the skin colour of indigenous people in the Old World closely matched their predictions. But the skin colour of long-term residents in the New World tended to be lighter than expected, which they attribute to more recent migration and factors such as diet.
Nina and George found that the world could be divided into three vitamin D zones. For dark-skinned people living in tropical latitudes, enough UVB gets through their protective melanin coating year-round to initiate the vitamin D synthesis their bodies depend on. For light-skinned people in the second zone, the subtropics and temperate regions, there is insufficient UVB radiation for them to synthesize vitamin D for at least one month of the year (for example, vitamin D production by the skin cells in light-skinned people begins in mid-March in Boston while dark-skinned people who move to these latitudes are unable to synthesize vitamin D for many months of the year). For the third zone, the polar regions above 45 degrees north and south, there is insufficient UVB year-round to prompt vitamin D synthesis. This led Nina and George to conclude that there was a strong natural selection for adequate vitamin D synthesis, which was the primary driver for the development of lighter skin colour in higher latitudes. You might be thinking, as I did when Nina first told me about her work, that the Inuit are not particularly light-skinned considering their polar environment of Alaska and northern Canada. Nina explains this as a likely result of two factors. First, they are relatively recent migrants, hav
ing arrived only about five thousand years ago. Second, they consume lots of vitamin D–rich fish and marine mammals, and this dietary supplement offsets the selective pressure for lighter pigmentation.
Women all over the world tend to have lighter skin than men by a margin of between 3 and 4 percent. Nina again struck down a conventional explanation for this that has strong racist overtones: that men prefer having sex with lighter-skinned women. Remember the role that vitamin D plays in calcium absorption. Women have a higher calcium requirement over their lifetime than men because of the heavy toll that pregnancy and lactation take on the mother’s calcium levels. The reason for women’s skin to be paler has to do with the need to allow more UVB rays in and thereby increase their ability to produce the life-giving vitamin D. “In areas of the world that receive a large amount of UV radiation, women are indeed at the knife’s edge of natural selection, needing to maximise the photo-protective function of their skin on the one hand, and the ability to synthesize vitamin D on the other,” Nina and George conclude. Their eye-opening work has been widely cited and translated into a multitude of languages, and it strikes down some of the worst prejudices that divide us to this day.
One element fuelling our epic migration out of Africa cannot be traced through mutations in our genes or in the physical remains of our fossil bones. This is the mysterious event that led to the evolution of the single feature that most separates humans from every other creature alive on the planet: our tremendous brain power. Our brains reached their present size with the birth of our species some 200,000 years ago. These early humans crafted tools and used fire—but these modest accomplishments are only a small step away from our nearest cousins, the chimpanzees—who have also been known to fashion basic tools from sticks and stones, and who can develop an amazing repertoire of signals for communicating. The leaps and bounds ancestral hominins took—from their first tentative steps as bipeds to the development of an opposable thumb and manual dexterity to the breakthrough of persistence hunting—certainly improved the odds for humans. But there was a quantum leap forward somewhere in the last 40,000 years, long after all these other evolutionary advantages were first united in our species, that led us to become the technologically savvy and supremely dominant species on earth that we are today.