|by Stephen Oppenheimer
In all our cells we have genes. Genes are made up of DNA, the string-like code of life that determines what we are, from our fingernails to our innate potential for playing the piano. By analysing genes, we can trace the geographic route taken by our ancestors back to an ultimate birthplace in Africa, at the dawn of our species. Further, if we take any two individuals and compare their genes, we will find that they share a more recent ancestor - living, in all probability, outside Africa. What is more, I believe that we can now prove where those ancestors lived and when they left their homelands. This remarkable proof has become fully possible only within the last decade, as a result of pioneering work by a number of people.
Many of us have wondered what we would find if we could perhaps board a time machine and travel back through the generations of our ancestors. Where would it take us? Would we find ourselves to be distantly related to some famous or notorious person? How many generations would we pass through before we arrived at the first humans? Does our line continue back to monkeys, and beyond to worms and single-celled creatures, as Darwin maintained? We know from dry biology lessons at school that this ought to be so, but as with the uncertainty of what happens to us after we die, it is hard to fully grasp.
We are now so used to the pace of technical advances that the sense of wonder fades with each new one. Yet, until very recently, geneticists could only dream of using our genes to trace the detailed history of how we conquered the world. The reason for their pessimism was that the majority of the genes they examined shuffled themselves around at each generation and were common to most populations anyway. Their task was like trying to reconstruct a previously played card game from the pack of cards after it has been shuffled. So it was nearly impossible to draw an accurate genetic family tree going back even a few hundred years, let alone back to the beginning of our species. Most human populations look very similar beneath the skin, so where could one start?
The use of gender-specific gene lines, the so-called Adam and Eve genes, has in the last ten years changed all that. In contrast to all other genes, mitochondrial DNA (a collection of genes outside the cell nucleus) is inherited only through our mothers, and the Y chromosome is inherited only by men. These two sets of gender-linked genes are passed on unchanged from generation to generation, with no shuffling, and can therefore be traced right back to our ancestors, to the first Primates. We can thus construct two family gene trees, one for our fathers and one for our mothers. As a result, in any population, of whatever size, we can trace any two individuals through one of these two gene trees back to a most recent shared ancestor on the tree. Such an ancestor may have lived 1,500 or 150,000 years ago, but all ancestors can be assigned a place on the newly constructed Adam-and-Eve genetic trees. These are real family trees of modern human gene lines, with real branches. Each branch on each tree can be dated (although the accuracy of such dating still leaves much to be desired).
Many regional human gene trees have now been fitted together, like a large jigsaw that is started by assembling the edges using certain clear landmarks. In this way, a picture of the Adam-and-Eve gene lines spreading from Africa to every corner of the world has been pieced together over the last decade. It has got to that satisfying point, as with jigsaws, when the whole structure suddenly links up and takes shape; the remaining pieces, though many, are now being placed on the tree and on the map with increasing ease and speed. The pace is now so rapid that people working at the cutting edge on one geographic region may still be unaware of breakthroughs in another region. The whole branching tree can now be laid flat on a world map to show where our ancestors and their gene lines travelled in their conquest of the world.
The new knowledge has resolved some of the apparent paradoxes thrown up by the contrast between the cultural and biological stories of the last 150,000 years. We can now even start to hang the regional human fossil relics of that period in their correct places on the genetic tree of life.
Many questions have been answered. It turns out that, far from the world being a common genetic melting pot with massive to-and-fro prehistoric movements and mixings, the majority of the members of the modern human dispersal have conservatively stayed put in the colonies their ancestors first established. They have dwelt in those localities since well before the last ice age. We can also trace the dates of specific migrations over the last 80,000 years. Thus, from a picture of great diversity and lack of definition, we have the opportunity to move to a highly specific and regional focus on the branching networks of human exploration.
Several other obvious examples of long-standing archaeological questions have been resolved by the new gene trees. One is the 'Out-of-Africa' v. 'Multiregional' controversy.
The Out-of-Africa view is that all modern humans outside Africa descend from a recent movement from Africa less than 100,000 years ago. This exodus wiped out all earlier human types around the world. The multi-regionalists, in contrast, argue that the archaic human populations, Homo neanderthalenis (Neanderthals) in Europe and Homo erectus in the Far East, evolved into the local races we now see around the world.
The Out-of-Africa view now wins the contest because the new genetic trees lead straight back to Africa within the past 100,000 years. No traces of Adam-and-Eve gene lines from older human species remain on our genetic tree, except of course at the root, from which we can measure our genetic distance from Neanderthals. Neanderthals have now been genetically typed using ancient mitochondrial DNA, and it seems that they are our cousins rather than our ancestors. We share with them another common ancestor, Homo helmei.
Current Out-of-Africa proponents have usually hedged their bets, claiming that Australians, Asians, and Europeans came as separate migrations of Homo sapiens from Africa. Not so: the male and female genetic trees show only one line each coming out of Africa. There was only one main exodus of modern humans from Africa - each gender line had only one common genetic ancestor that respectively fathered and mothered the whole non-African world.
Other prejudices have also foundered. Some European archaeologists and anthropologists have long held that Europeans were the first to learn to paint, carve, develop complex culture, and even to speak almost as if Europeans represented a major biological advance. The structure of the genetic tree denies this view. Australian aboriginals are related to Europeans, and share a common ancestor just after the exodus from Africa to the Yemen over 85,000 years ago. Thereafter they moved progressively round the coastline of the Indian Ocean, eventually island-hopping across Indonesia to Australia where, in complete isolation, they developed their own unique and complex artistic cultures. The first Australian rock art has been dated at least as early as the first European one.
Another old archaeological controversy concerns the spread of the Neolithic culture across Europe from Turkey 8,000 years ago. Did the farmers from the Near East wipe out and replace the European hunters, or did the new ideas spread more peacefully, converting the pre-existing Palaeolithic hunter-gatherer communities? The genetic answer is clear: 80 per cent of modern Europeans descend from the old hunter-gatherer gene types, and only 20 per cent from Near Eastern farmers.
Finally, moving to the other side of the world, there has always been colourful speculation over the origins of the Polynesians. Thor Heyerdahl was not the first (in fact, Captain Cook was nearer the mark in arguing for a Polynesian link to the Malay archipelago). For the past fifteen years archaeologists have thought that Polynesians came from Taiwan. The genetic tree discounts this now: the ancestors of the sailors of the great canoes started out further along the trail, in Eastern Indonesia.
We should remember that we are participants in this genetic story, since 99 per cent of the work of reconstruction of our ancient gene trees was carried out using modern DNA given voluntarily by people living in different parts of the world today. This is a story of relevance to each and everyone of us.
Many Anthropologists now say that we came out of Africa, but how do they know? If we all have a single recent origin there, why do there appear to be different races of humans? How closely are these races related? Are we all part of one family, or do Africans, aboriginal Australians, Europeans, and East Asians all have different parallel evolutionary origins? What key forces in our evolutionary history took descendants of apes that had just left the trees to walk the African savannah and catapulted them onto the Moon within a couple of million years?
DNA analysis has led to extraordinary advances in our understanding of the regional biological history of modern humans. The so-called Adam and Eve genes really do allow us to track back in time and space to follow the human family in its wanderings in Africa and then round the globe over the past 200,000 years.
Much of the human history of the past 2.5 million years has been reconstructed by a combination of studies of fossil bones and past climates. All but one human species became extinct, some of them long ago, so we do not have their living genes to study.
To say that there are no genes left over from past human species is, however, not quite true. Most of our nuclear genes are inherited nearly intact from ancestral humans and apes. Some human genes can be found in several forms that split from one another long before Homo sapiens appeared on Earth. Scientists have also extracted short fragmentary stretches of mitochondrial DNA from a number of Neanderthal bones, and are now in a position to answer basic questions about how closely we are related to them and whether there are any of their genes left in modern human populations.
However, the real revolution in understanding human genetic prehistory covers the last 200,000 years, which is what concerns us here. For this period, the new genetics has shone a bright light onto a controversial field previously dominated by collections of European and African stone tools and a few poorly dated skeletal remains.
Within each of the cells of our bodies we all have incredibly long strings of DNA. It is the stuff of the genes. It stores, replicates, and passes on all our unique characteristics our genetic inheritance. These DNA strings hold the template codes for proteins, the building blocks of our bodies. The codes are ‘written’ in combinations of just four different chemicals known as nucleotide bases (represented by the letters A, G, C, and T), which provide all the instructions for making our bodies. We inherit DNA from each of our parents, and because we receive a unique mixture from both, each of us has slightly different DNA strings from everyone else. Our own DNA is like a molecular fingerprint.
The diagram above shows the drawing of
During human reproduction, the parents’ DNA is copied and transmitted in equal proportions. It is important to know that although most of the DNA from each parent is carefully sorted during reproduction, small bits of their respective contributions are shuffled and mixed at each generation. This splicing and mixing is known technically as recombination and makes it more difficult to trace back our genetic prehistory in those genes. Luckily, for the purposes of genetic researchers, there are two small portions of our DNA that do not recombine. Non-recombining DNA is therefore easier to trace back since the information is uncorrupted during transmission from one generation to the next. These two portions are known as mitochondrial DNA (mtDNA) and the non-recombining part of the Y chromosome.