Out of Eden: The Peopling of the World (8 page)

So far I have made little reference to genetics, which is going to figure prominently in this book. 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. This is not to say that genetics has nothing to tell us about the dark ages of our evolution, before modern humans appeared on the stage. In the 1970s, some geneticists began to use crude immunological tools to measure protein similarities between species, and suggested that humans and chimps were even more closely related than had been thought. Their suggestion was met with derision at the time, but as techniques of comparison turned from immunology to demonstrating a basic genetic similarity and then eventually to measuring precise genetic differences, they were vindicated. The realization has grown that we are much closer to chimps than to the other great apes, the gorillas and orang-utans, our common lineage having split not much more than 5 million years ago.
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To say that there are no genes left over from past human species is also 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 we 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 contentious field previously dominated by collections of European and African stone tools and a few poorly dated skeletal remains. Before turning to details of genetic tracking, it may help to look at some of the ideas behind genetic inheritance and how they have evolved. The concepts are mainly simple, being related to our own everyday understanding of and pre-occupation with inheritance, but are often misrepresented, either for reasons of hype or because they are veiled in jargon.

The secrets of the peas

Humans have been aware of some idea of genetic inheritance ever since the first animals and plants were domesticated. Farmers made deliberate attempts to breed out unwanted features such as large size or aggression. Cereal grain was bred for increased size and ease of cropping. However, much of the detail of ‘farmer’s lore’ was fundamentally wrong, though crudely functional. Speculation about the exact mechanisms and rules underlying inheritance increased in the mid-nineteenth century with the publication of Charles Darwin’s
Origin of Species
, but Darwin’s understanding of the principles of heredity was not much more sophisticated than that of his predecessors. Instead it was his contemporary, the nineteenth-century Austrian cleric Gregor Mendel, who first laid down a logical framework for understanding how parental characteristics were transmitted, based on his obsessive mathematical calculations on the inheritance of the colour and shape of pea plants.

Basically, Mendel showed that, for any particular physical feature, or
character
, such as flower colour, each pea plant possessed two genes (although he used another term) that determined the
expression
or outcome of the character. Variation in relative dominance between these two genes determined their expression in the plant. During the process of sexual procreation, only one of these genes would be donated by each parent to each offspring plant. Thus, each offspring inherited a mixture of characters from each parent, with
the combined effect of two genes determining each physical character at each generation. Since either parent could have different functioning gene types, for instance different petal colours, for each (and any) pair of genes, and only one of the two was chosen at random, the proportions of different varieties of offspring formed a pattern that could be predicted from a knowledge of the characters possessed by each parent. In this way, Mendel showed that inheritance of characters took place by the transfer of discrete packets of information. The variation in the offspring was down to the precise but random mix of these packets, or genes, in each individual, whether it be a pea seed or a human.

Mendel was careful to choose simple, common, easily distinguished characters and to study them individually. In reality, the expression of some physical characters is determined by more than one gene, and we all have around 30,000 pairs of functioning genes because we are rather complex organisms. Thus the visible random variation between siblings in the same family is not the result of any vagueness in the process of heredity, but arises because there are large numbers of gene pairs being randomly chosen during sexual procreation. With so many gene pairs in different combinations, there is huge potential for variety. By way of contrast, the extreme similarity of identical twins gives us a glimpse of how precise the conversion of heredity into physical form really is. The extraordinary achievement of Watson, Crick, and Rosalind Franklin was to translate Mendel’s discoveries into biochemistry – or molecular biology, as this aspect of biochemistry came to be known.

Cardboard keys to life

‘We are the products of our genes.’ The secret keys to this Edwardian truism were traced and cut out on bits of cardboard by two adventurer-scientists, Jim Watson and Francis Crick, in 1953.
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Their ‘keys’ were scale diagrams of four chemicals (nucleotide
bases), whose unique interlocking relationship, set in the double-stranded zip-locking deoxyribonucleic acid (DNA), holds the code for life on Earth. Those bits of cardboard unlocked the mechanism linking Mendel’s work to the theory of evolution by natural selection as set out by Darwin in the
Origin of Species
. Watson and Crick explained exactly how thousands of unique characteristics, varying from one individual to another, are passed on intact from generation to generation. In short, it was the greatest advance in biological understanding in the twentieth century.

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.

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 segregated during reproduction, small bits of their respective contributions are shuffled and mixed at each generation. The mixing here is not that of mass random allocation of genes inferred by Mendel, but tiny crossovers, duplications, and swaps between maternal and paternal DNA contributions. This is known technically as
recombination
. Luckily, for the purposes of genetic researchers, there are two small portions of our DNA that do not recombine. Non-recombining DNA is easier to trace back since the information is uncorrupted during transmission from one generation to the next. The two portions are known
as mitochondrial DNA (mtDNA) and the non-recombining part of the Y chromosome (NRY).

Mitochondrial DNA: the Eve gene

To say that we get exactly half of our DNA from our father and half from our mother is not quite true. One tiny piece of our DNA is inherited only down the female line. It is called mitochondrial DNA because it is held as a unique circular strand in small tubular packets known as mitochondria that function rather like batteries within the cell cytoplasm. Some molecular biologists say that, aeons ago, the mitochondrion was a free-living organism with its own DNA, and possessed the secret of generating lots of energy. It invaded single-celled nucleated organisms and has stayed on ever since, dividing, like yeast, by binary fission. Males, although they receive and use their mother’s mitochondrial DNA, cannot pass it on to their children. The sperm has its own mitochondria to power the long journey from the vagina to the ovum but, on entry into the ovum, the male mitochondria wither and die. It is as if the man had to leave his guns at the door.

So each of us inherits our mtDNA from our own mother, who inherited her mtDNA intact from her mother, and so on back through the generations – hence mtDNA’s popular name, ‘the Eve gene’. Ultimately, every person alive today has inherited their mitochondrial DNA from one single great-great-great-. . .-grandmother, nearly 200,000 years ago. This mtDNA provides us with a rare point of stability among the shifting sands of DNA inheritance. However, if all the Eve chromosomes in the world today were an exact copy of that original Eve mtDNA, then clearly they would all be identical. This would be miraculous, but it would mean that mtDNA is incapable of telling us much about our prehistory. Just knowing that all women can be traced back to one common ancestral Eve is exciting, but does not get us very far in
tracing the different geographic lives of her daughters. We need something with a bit of variety.

This is where DNA point mutations come in. When mtDNA is inherited from our mother, occasionally there is a change or mutation in one or more of the ‘letters’ of the mtDNA code – about one mutation every thousand generations.
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The new letter, called a
point mutation
, will then be transmitted through all subsequent daughters. Although a new mutation is a rare event within a single family line, the overall probability of mutations is clearly increased by the number of mothers having daughters. So, within one generation, a million mothers could have more than a thousand daughters with a new mutation, each different from the rest. This is why, unless we share a recent maternal ancestor within the past 10,000 years or so, we each have a slightly different code from everyone else around us.

Using mutations to build a tree

Over a period of nearly 200,000 years, a number of tiny random mutations have thus steadily accumulated on different human mtDNA molecules being passed down to daughters of Eve all around the world. For each of us this represents between seven and fifteen mutational changes on our own personal Eve record. Mutations are thus a cumulative dossier of our own maternal prehistory. The main task of DNA is to copy itself to each new generation. We can use these mutations to reconstruct a genetic tree of mtDNA, because each new mtDNA mutation in a prospective mother’s ovum will be transferred in perpetuity to all her descendants down the female line. Each new female line is thus defined by the old mutations as well as the new ones. As a result, by knowing all the different combinations of mutations in living females around the world, we can logically reconstruct a family tree right back to our first mother.

Although it is simple to draw on the back of an envelope a recent
mtDNA tree with only a couple of mutations to play with, the problem becomes much more complex when dealing with the whole human race, with thousands of combinations of mutations. So computers are used for the reconstruction. By looking at the DNA code in a sample of people alive today, and piecing together the changes in the code that have arisen down the generations, biologists can trace the line of descent back in time to a distant shared ancestor. Because we inherit mtDNA only from our mother, this line of descent is a picture of the female genealogy of the human species.

Not only can we retrace the tree, but by taking into account where the sampled people came from, we can see
where
certain mutations occurred – for example, whether in Europe, or Asia, or Africa. What’s more, because the changes happen at a statistically consistent (though random) rate, we can approximate the
time
when they happened. This has made it possible, during the late 1990s and in the new century, for us to do something that anthropologists of the past could only have dreamt of: we can now trace the migrations of modern humans around our planet. It turns out that the oldest changes in our mtDNA took place in Africa 150,000–190,000 years ago. Then new mutations start to appear in Asia, about 60,000–80,000 years ago (
Figure 0.3
). This tells us that modern humans evolved in Africa, and that some of us migrated out of Africa into Asia after 80,000 years ago.

It is important to realize that because of the random nature of individual mutations, the dating is only approximate. There are various mathematical ways of dating population migrations, which were tried with varying degrees of success during the 1990s, but one method established in 1996, which dates each branch of the gene tree by averaging the number of new mutations in daughter types of that branch,
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has stood the test of time and is the main one I use in this book.