By 200,000 years ago, Homo sapiens had emerged somewhere in Africa. By 14,000 years ago, our species had spread to every continent except Antarctica. What happened in between—the pattern of where humans went and when—is still being worked out. To reconstruct the peopling of the world, anthropologists rely on several types of clues.
Fossils: The most obvious way to track our ancestors’ movements is to look for their physical remains. Researchers sketch out travel routes by mapping where the oldest human fossils are found. The earliest Homo sapiens bones outside of Africa come from a cave site in Israel called Qafzeh. Here the skeletons of both adults and children date to as far as 125,000 years ago. This first foray out of Africa didn’t last long. Humans disappeared from the fossil record outside of Africa for many tens of thousands of years, perhaps because the climate became too harsh. Fossils tell us humans made a successful, sustained exodus by at least 50,000 years ago. Human fossils found at Australia’s Lake Mungo site, for example, have been dated to between 46,000 and 50,000 years ago (PDF).
The problem with relying on skeletal remains to map early migrations is that the timing of our ancestors’ travels is only as good as the methods used to date the fossils. Sometimes scientists find bones in places that are not easily dated by geological techniques. And in some areas, fossils aren’t prone to preservation, so there are probably huge gaps in our knowledge of the paths early humans took as they spread around the world.
Artifacts: Archaeologists also look for the items people made and left behind. For example, stone tool discoveries suggest an alternative route out of Africa. For decades, scientists assumed humans left Africa via the Sinai Peninsula, but in the last several years some researchers have favored a “southern” route: leaving from the Horn of Africa, crossing the narrowest part of the Red Sea and entering into southern Arabia. Last year, archaeologists reported finding stone tools in Oman dating to roughly 106,000 years ago. At that time, the Arabian Peninsula was a much more hospitable place than it is today, home to numerous freshwater lakes. As the region became drier, people might have moved east into Asia or returned to Africa.
Of course, when the only remains at an archaeological site are tools, it’s hard to say with absolute certainty who made them. The researchers working in Oman noted that the tools they found in Arabia match the technology of modern humans found in eastern Africa about 128,000 years ago. The team made the case that the tool makers on either side of the Red Sea belonged to the same cultural group—and therefore the same species. But as anthropologists discover more species, such as the Hobbit or the Denisovans, that lived alongside modern humans outside of Africa up until a few tens of thousands of years ago, it becomes harder to say stone tools alone indicate the presence of Homo sapiens.
DNA: Genetic data can help fill in the holes in the human migration story that fossils and artifacts can’t address. Anthropologists collect DNA samples from different ethnic groups around the world. Next, they count up the genetic differences caused by mutations in certain sections of the genome. Groups that are more closely related will have fewer genetic differences, which implies they split off more recently form each other than they did with more distantly related groups. Scientists calculate when in the past different groups diverged from each other by adding up all of the genetic differences between two groups and then estimating how often genetic mutations occurred. Such analyses not only give a sense of when different parts of the world were first inhabited, but they can also reveal more intricate patterns of movement. For example, genetic data suggest North America was colonized by three separate waves of people leaving Siberia across the Bering Strait.
Genetic data are not foolproof, however. The estimated divergence times are only as accurate as the estimated mutation rate, which scientists still debate. In the early days of DNA studies, scientists used either mitochondrial DNA, passed down only by the mother, or the Y chromosome, inherited only from father to son. Neither of these types of DNA presented the full picture of what people were doing in the past, as mitochondrial DNA only tracks maternal lineages while the Y chromosome only follows paternal lines. Today, whole genome sequencing is beginning to allow researchers to trace entire populations.
Languages: Anthropologists use languages in methods analogous to studying DNA; they look for patterns of similarities, or differences, in vocabularies or other aspects of language. Earlier this year, researchers compared different languages within the Indo-European language family to determine where these languages arose. After assessing the relationship between the languages, the researchers considered the geographic ranges where those languages are currently spoken. They concluded that the Indo-European language family originated in what is today Turkey and then spread west into Europe and east into southern Asia as people moved into these areas. But such linguistic analyses may only track relatively recent migration patterns. For example, H. Craig Melchert, a linguist at the University of California, Los Angeles, told Science News that the Indo-European languages can only be traced back about 7,000 years.