Over the past 20-30 years, scientists have sampled DNA from millions of people around the world, and mapped the variation, or mutations, in their Y-DNA and mtDNA. More recently, as the techniques for extraction of DNA from ancient bone material has improved, these have been supplemented with DNA from skeletal material dating back many thousands of years. Combined with linguistic and archaeological data, this information allows us to draw a picture of how and when mankind has spread across the globe.
We turn first to the history of our close relatives, now extinct, the so-called Neanderthals and Denisovans. The Neanderthals are named from the Neander valley in Germany where the first fossil skeleton of this new ‘species’ was discovered in 1856. Since then skeletal material has been excavated in dozens of sites throughout Europe, south west and central Asia.
Copyright Neanderthal Museum, Mettmann
The Neanderthals were shorter and stockier than modern humans but their brains were larger in proportion to their body size. They were well adapted to living in the arctic conditions prevailing in the region during the Ice Age. They used a large variety of sophisticated tools, wore clothing and ornaments, and buried their dead.
DNA analyses indicates that the Neanderthals have contributed about 2% to the genetic signature of that of most humans. (People from Africa form an exception, the reason being that Neanderthals appear only to have lived in Europe and Asia).
In other words, the two groups must have interbred. They co-existed in Europe and Asia from around 70 000 to 30 000 years BP. Then the Neanderthals died out and homo sapiens reigned.
The reason for their extinction has been the subject of much debate – disease, war, a failure to adapt to a changing climate – have all been postulated. Or possibly the two populations merely merged into one. Whatever the cause, most of us are a small part Neanderthal. Some of the traits we have inherited have been beneficial, others not so much.
As noted above, while the Neanderthals migrated from the Near East into Europe, their relatives the Denisovans moved into Asia. From there they seem to have migrated into south Asia and the Pacific islands, as well as north into Siberia.
The Denisovans are a mysterious race for which a complete skeleton has yet to be found. They take their name from the discovery of a finger bone in the Denisova cave in the Altai mountains of southern Siberia. Evidence for their existence is restricted to the DNA analysis of this finger bone plus a few teeth discovered later.
Russian archaeologists excavating the Denisova cave in 2008 came across a finger bone of a young woman, dated to between 50 000 and 30 000 years BP. DNA analysis of the fragment was performed at the Max Planck Institute in Leipzig. It turned out that the bone had belonged to a completely new member of the hominin race, unlike either humans of Neanderthals. This mysterious race was given the name Denisovan after the site where its remains were first found. To date, no complete skeleton of a Denisovan has come to light. The finger bone and a few teeth are all we have to go on.
In recent years, extensive DNA analyses of living populations have been screened for traces of the Denisovan genes. It transpires that the gene is most prevalent in the indigenous people of south-east Asia, reaching 4.8% in Melanesia.
The current thinking is that the ancestors of the Neanderthals (and their contemporaries the Denisovans) split off from a common lineage around 600 000 years BP. In turn the Neanderthals and Denisovans separated into two groups ca 400 000 years BP. Modern day humans, homo sapiens, evolved from this common lineage around 200 000 years BP, and about the same time the Neanderthasls and Denisovans went their separate ways.
To summarise, we now know that modern humans did not evolve in a straight line from our ancient ancestors the apes, but that many branches and hybrids were involved. More and more evidence is emerging that interbreeding between the three groups, homo sapiens, Neanderthals and Denisovans, occurred frequently over the many thousands of years they co-existed. Hence, rather than extinction, the genes of the latter groups have been absorbed into modern day human populations.
It is now recognised that the species to which we belong – Homo sapiens – arose in Africa somewhere around 200 000 years ago. Between 70 000 and 50 000 years BP a small group of these people migrated from East Africa across the Red Sea and into Arabia. Some 40 000 years BP ago our ancestors had spread across Asia, Australasia and throughout Europe. However, it was not until 15 000 years BP that the establishment of a land bridge across what is now the Bering Strait allowed the migration of peoples from Asia into North America (See map).
World map of human migrations based on mtDNA (matrilinear) haplogroups. North Pole at Centre, Africa to the top. The extent of the ice sheet /tundra during the last Ice Age indicated as a blue line. Numbers are thousand years before present.
During this period the Northern Hemisphere was in the grip of the Ice Age, and, with the exception of a few hardy souls who survived the freezing conditions in caves in south France and Spain, Europe was unpopulated.
As the climate warmed around 12 000 years BP the ice sheets retreated and small bands of hunter-gatherers moved northwards to populate northern Europe. These in turn were followed around 8000 years BP by groups of settlers who moved from the Near East and brought with them the practice of agriculture.
Up until recently this was the established view of how Europe was populated. More recently, improvements in analysis techniques have allowed DNA to be extracted from ancient skeletal material and provided a more manifold picture of how Europe was populated. For example, the DNA of Øtzi, the mummy of a hunter which emerged from a glacier in the Alps in 1991 and who lived ca 5 000 years BP, indicates affinity with the population of present day Sardinia.
Analyses of bone material from burials ranging from 24 000 to 8 000 years BP suggest that Europe was settled by at least three groups of people:-
- The original ‘resident’ hunter-gatherers
- A nomadic people who migrated westwards from Siberia
- The early European farmers who migrated from the Near East
To this mix was added a fourth component in the Bronze age, some 4500 years BP. There is now evidence that around this time Europe was invaded by a nomads from the Eurasian steppes, the so-called Yamnaya people. They were a horse-riding people, who also had developed bronze and copper tools. Some of their skeletons showed traces of the DNA of the plague bacteria leading to the speculation that they may have brought with them the disease. This may explain how they were so sucessful. On a more positive note, they probably brought with them the Indo-European language. (see map below where 8 kya = 8000 years ago).
Turning to the America’s, the ancestors of most Native Americans probably migrated across the Bering Straits (which was then a land bridge) into Alaska relatively recently, i.e. between 15 and 13 000 years BP. From Alaska they migrated southward, initially following the region west of the Rockie Mountains. From there they have spread through North, Central and South America.
Evidence for the Asiatic origin comes from several studies, the best known being the DNA analysis of bone material from a 24 000 year old skeleton of a boy, found in a cave near Mal’ta in southern Siberia. Part of his DNA was European while the remainder was unique to modern day Native Americans. It is estimated that between 14-38% of the DNA of Native Americans can be traced back to populations like that living at Mal’ta. The remainder is of European and East Asian origin through admixture of these groups in post-Columbian times.
The ancient inuit population of Greenland seems to have migrated from Siberia relatively recently, some 5 500 years BP.
As described above mutations in both the mtDNA and Y-DNA can be passed on to succeeding generations, giving rise to a ‘clan’ of people living at a particular time and place, so-called haplogroups. These can be plotted on maps to show their distribution, The Y-DNA for present day population of Europe is shown below.