The Origins of the British: The New Prehistory of Britain (63 page)

But in this book I have avoided reference to the more recent period, simply because that is not what it is about. The DNA sample frameworks used in the analysis specifically exclude more-recent immigration, even within Britain. This exclusion is not intended as any jingoistic statement about perceived Britishness, it is simply a practical necessity for looking further back in our history.

I have given a number of percentages relating to individual migration events in this book. The largest figures come from our first hunter-gatherer founders, while figures for more
recent arrivals, such as Anglo-Saxons and Vikings, rarely top 10% locally and 6% overall. Perusing our national census for more recent minority immigration, those figures seem no more dramatic. Recent migrations into Britain have been proportionately minor compared with the pioneer events, so minor that the biggest ‘increases’ are attributable merely to ways of measuring. Without going into the complex analysis of immigration and census, I found a short government statement which was of interest, both for its comment on the statistical effect of changes in categorization and for its relevance to my own family makeup:

[2001] Comparisons with the 1991 Census show: The proportion of minority ethnic groups in England rose from six per cent to nine per cent – partly as a result of the addition of [the category of] Mixed ethnic groups in 2001.
⁵

In spite of my German Jewish name, which might predict a Near Eastern Y chromosome, less than 13% of my ancestry derives from that source. The rest is a collage of more or less ‘British’ ancestors, including a large dose of Scots on both sides, smaller doses of Mancunian and Brummy, and even the possibility of Flemish.

This sort of mobility in marriage is a recent but rising trend. I guess I have taken it a step further. My kids have two cultural and genetic backgrounds, English and Malaysian Chinese, thus finding themselves in the ‘Mixed ethnic groups’ 2001 census category. Occasionally, well-meaning persons ask me in a concerned way whether my children have problems of cultural identification. I pass the question on to them and receive amused
responses. They both feel enriched, having not one but two different cultural and culinary resources to relate to; in other words, the confusion of identification remains in the eye of the observer. I would personally prefer ethnic identity to be more a self-chosen smorgasbord than anything that might be imposed by others.

This lucky example is not smugly intended to underplay the problems of belonging to a minority in the United Kingdom. However, an increase in immigration over the past half-century has not, despite the efforts of political rabble-rousers, resulted in Enoch Powell’s prophecy of ‘rivers of blood’. As with St Gildas, another British prophet before him, there is the faintest suspicion of an agendum of wish-fulfilment in that prophecy. Speaking of the aftermath of the supposed Anglo-Saxon invasion, Gildas tells us of:

fragments of human bodies, covered with livid clots of coagulated blood, looking as if they had been squeezed together in a press

and exhorts the surviving and unrighteous British kings to

seek for the rule of right judgment [on] the proud, murderers, the combined and adulterers, enemies of God, who ought to be utterly destroyed and their names forgotten.
⁶

Should we lightly wish such a fate on our worst enemies? After all, Celts, Angles, Jutes, Saxons, Vikings, Normans and others, we are all minorities compared with the first unnamed pioneers, who ventured into the empty, chilly lands so recently vacated by the great ice sheets.

In the main text I make little reference to the methods used for the genetic tracking that figures prominently in this book. The real revolution in understanding human genetic prehistory covers the last 200,000 years, the most recent 20,000 years of which concerns us here. For a large part of the period before the first farmers, 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. But even for the Neolithic and later periods, archaeology tells us more about cultural spread than human migrations. 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 preoccupation with inheritance, but are often misrepresented, either for reasons of hype or because they are veiled in jargon.

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
(adenine, guanine, cytosine and thymine, represented by the letters A, G, C and T), which provide, in sequence, 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 brilliantly inferred by Gregor Mendel, but tiny crossovers, duplications, deletions and swaps between maternal and paternal DNA contributions. This is known technically as
recombination
. Luckily for genetic researchers, there are two small portions of our DNA that do not recombine. Non-recombining DNA is easier to trace back through previous generations 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).

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. That piece is called mitochondrial DNA because it is held as a unique circular strand in small tubular packets known as mitochondria which function rather like batteries within the cell. Some molecular biologists believe 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, outside the cell nucleus, dividing, like yeast, by simple binary fission with its own DNA. 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 when it enters the ovum the male mitochondria wither and die. It is as if the man has had to leave his guns at the door.

So each of us inherits our mtDNA from our 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- (etc.)-grandmother, nearly 200,000 years ago. (Our descent from a single ancestral maternal line is a result of natural wastage every generation and happens with all genes, so it does not literally mean that we all descend from one woman and one man who lived 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. That 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 it doesn’t get us very far in tracing the different lives of her daughters. We need something with a bit of variety.

This is where DNA 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.
¹
The new letter, called a
point mutation
, is then transmitted through all subsequent daughters. For the large part, these mutations are ‘neutral’– that is, of no functional or health relevance. 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 (i.e. population size). 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 over the past 10,000 years or so, we each have a slightly different code from everyone else around us.

Over a period of nearly 200,000 years, then, a number of tiny random mutations have 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 mutations 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, if we can identify 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 a recent mtDNA tree on the back of an envelope when there are only a couple of mutations to play with, the problem becomes much more complex when we have the whole human race to deal with, sifting through 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
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 (i.e. the earliest in the
tree) took place in Africa between 190,000 and 150,000 years ago. Then new mutations start to appear in Asia, about 80,000 to 60,000 years ago. This tells us that modern humans evolved in Africa, and that 80,000 years ago some of us began to migrate out of Africa into Asia.

It is important to realize that because of the random nature of individual mutations, the dating is only approximate. Various mathematical ways of dating population migrations were tried during the 1990s with varying degrees of success, but in 1996 a method was established which dates each branch of the gene tree by averaging the number of new mutations in daughter types of that branch.
²
This method (estimation of
rho
) has stood the test of time, and is the main approach used to calculate the mtDNA genetic dates I give in this book.

Analogous to the maternally transmitted mtDNA residing outside our cell nuclei, there is a set of genes packaged within the nucleus that is passed down only through the male line. This is the Y-chromosome, the defining chromosome for maleness. With the exception of a small segment, the Y-chromosome plays no part in the promiscuous exchange of DNA indulged in by other chromosomes. Like mtDNA, the non-recombining part of the Y-chromosome thus remains uncorrupted by recombination, with each passing generation, except for random point mutations, and can be traced back in an unbroken line to our original male ancestor.

Y-chromosomes have been used for reconstructing trees for less time than mtDNA has, and there are more problems with using them to estimate time depth. These methods have been
considerably improved over the past few years;
³
for the Y-chromosome analysis presented in this book, I use a variant of the same method mentioned above for mtDNA.
⁴