Over the past 20-30 years, scientists have sampled DNA from thousands of people around the world, and mapped the variation, or mutations, in their Y-DNA and mtDNA. Combined with linguistic and archaeological data, these maps allow a picture of how and when mankind spread across the globe from their origin in Africa.
It is now recognised that the species to which we belong – Homo sapiens – arose in Africa upwards of 300,000 years ago. About 70,000 years ago a small group of these people began to migrate north and east across the Red Sea. What triggered this migration is still uncertain. It may have been caused by the onset of the Ice Age some 100,000 years ago. Another theory suggests that about 70,000 years ago there was an enormous volcanic eruption in Indonesia – Mt. Toba. The ash cloud that ensued led to a global cooling period which almost wiped out all human life. Some estimates suggest that only some 10,000 individuals survived this period.
Whatever the reason, it now appears that as this small group of Homo sapiens first migrated into Europe they came across and mated with another hominid species, the Neanderthals. The ancestors of this last group split off from our lineage around 500,000 years ago. It seems that between 1 and 4% of the DNA makeup in modern Europeans is derived from these encounters. Ultimately, however, the Neanderthals disappeared and only Homo sapiens survived to spread to all parts of the globe.
Some 40,000 ago our ancestors had spread across Asia, Australasia and Europe. Around 15,000 years BC they had migrated into the Americas. By the birth of Christ most of the Pacific Islands had been colonized.
Remembering that mtDNA survives only in the female line, Mitochondrial Eve is defined as the most recent matrilineal common ancestor (MRCA) of all modern humans. In other words, she was the most recent woman from whom all living humans today descend, on their mother’s side. She is estimated to have lived in South or East Africa sometime around 200,000 years ago, as one of a small group of Homo sapiens, a population distinct from other hominid species living at the time. Even though she lived alongside other females, she is the only one who produced a direct unbroken female line to every woman living today. In other words, her sisters, cousins and so on, although they may be our ancestors, this is through males and females – thus meaning that their mtDNA is not seen today.
Her counterpart on the male side is Y-chromosomal Adam. He is the most recent common ancestor from whom all living people are descended along the paternal lines of their family tree. Recent studies show that Y-chromosomal Adam lived upwards of 300,000 years ago, most probably in west central Africa. In other words, he and Eve could not have known each other (let alone mated!). Y-chromosomal Adam lived alongside other human males, but none of his contemporaries have produced a direct unbroken male line.
As noted above, mutations have occurred during the replication of our DNA. These changes are very rare and infrequent. To take an example, if we take the sequence listed above – A T A T CG C G T A A T G C– one person carrying this sequence might mate with the sequence A T A T C G T G T A A T G C., (see also Fig. 2). This type of genetic change is called a single nucleotide polymorphism (SNP) marker. SNP markers are found in both the mtDNA and the Y-DNA. They form the basis for the human phylogenetic tree.
The main branches of the human phylogenetic tree are called haplogroups or clans. The Y-DNA tree has approximately 18 main clans, classified by the letters A to R. Each Y-DNA haplogroup has many further sub-branches (subclades), classified by numbers and letters i.e. R1a, R1b1, R1b2, etc. The main Y-DNA haplogroups are shown below, while Fig. 4 gives the latest map of the spread of Y-DNA haplogroups in Europe.
Fig.4. Y-DNA (patrilineal) haplogroups in Europe.
The mtDNA tree has approximately 26 main branches “mtDNA haplogroups” classified by the letters”A to Z”. Each mtDNA haplogroup has many further sub-branches (subclades), classified by numbers and letter, i.e. L1a1, L1a2, L1b1a1, etc. All people living today have descended from of the main branches of the human mtDNA phylogenetic tree. See below. Fig. 5 gives a recent map of human migrations based on major mtDNA haplogroups.
Fig. 5 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.