Rebecca Cann, of the University of California at Berkeley, was one of the
first to use DNA to uncover the past. In 1987 she examined the mitochondrial
genes we inherit from our mothers and showed that our female line can be traced
back to a single woman who lived in Africa about 200,000 years ago, a "mitochondrial
Eve". Fossil evidence for modern humans goes back at least 500,000 years
so it seems that humanity went through a severe bottleneck when this Eve was
alive - it remains a mystery why of all the females who inhabited the globe
at that time, she was the only one to leave modern descendants.
DNA fingerprinting has since been used to resolve many other contentious issues
- most notably those concerning the origin of culture and national identity.
Early prehistorians tended to imagine waves of invaders who displaced the
indigenous populations to erect new cultures. Modern prehistorians, using
DNA evidence, see a more complex picture.
In 1997, Luigi Luca Cavalli-Sforza, of Stanford University, plotted gene frequency patterns across Europe and the near east in the Neolithic period, and found that they bore a striking similarity to the spread of agriculture. Neolithic farmers travelled with both their crops and their genes to eventually reach the British Isles some 5,000 years ago.
Later migrations coincided with the expansion of the Indo-European languages, including Latin, German, Sanskrit and English. Linguists have long sought a homeland for the Indo-Europeans. The genetic data favours an origin on the Russian steppes with a group of nomadic farmers known as the Kurgan who domesticated the horse and rode out to invade India and Europe. Studies of Irish Y-chromosomes indicate that the British Isles represents the western edge of that migration.
The displacement of Romano-British culture by Anglo-Saxon has been variously
considered to mark a mass invasion or a more peaceful cultural takeover by
a small military elite. Mark Thomas at University College London examined
Y-chromosomes from across central England, Wales, Denmark, Norway and Friesland,
the Anglo-Saxons' home.
Central England and Friesland Y-chromosomes were indistinguishable and distinct
from their Welsh counterparts. The genetic data supports the invasion hypothesis
with either elimination or displacement of the native Celts from central England.
However, the spread of genes and languages do not always coincide. The Lemba
are a tribe of Bantu-speaking black Africans who believe they are descended
from Jews. They practise semitic traditions, such as circumcision and the
keeping of the Sabbath. Thompson's laboratory discovered that in their Y-chromosomes
was a genetic marker found only among Jews. The Lemba tradition that a high
priest named Buba led them out of Judaea may indeed be based on a real event.
But there is a danger in seeing gene migrations as the source of culture and
national identity. The Jewish gene in the Lemba tribe is found in only about
10% of the men, yet the whole tribe practices Jewish traditions. Similarly,
although Kurgan genes may have spread westward from an Indo-European homeland,
the Kurgan swapped lots of genes with the indigenous populations along the
way. By the time they reached the British Isles, their genetic inheritance
was heavily diluted, though the language they spoke remained thoroughly Indo-European.
The differences between populations are nearly always just a matter of the
varying proportions of a few (probably unimportant) genes. Germans, Celts,
Latins, Jews, Polynesians, Africans, Iraqis and Americans are all blends of
genes with only about one DNA base in a hundred that separates any one of
us from our common ancestor. Genetically speaking, there is little to distinguish
one race from another. Cavalli-Sforza's studies have led him to hypothesise
that the invention of modern language was the spark that ignited human expansion.
Perhaps what set Cann's Eve apart was that her people invented language. Language
allowed a new form of heredity - cultural inheritance - whose letters were
not written in our genes.
So although our ancestors left us their DNA, they also left us with language
that allowed us to transmit a culture that is independent of our genes. That
shared skill of communication - a way of settling differences with words rather
than violence - is unique to mankind and as important to us today as it was
to our earliest ancestors. Watson and Crick's discovery allows us to unravel
a bloody past but it also tells us that we can forge our own future.
· Johnjoe McFadden is professor of molecular genetics at the University
of Surrey and author of Quantum Evolution (HarperCollins)