by Paul Jordan
A potentially very useful dating technique, called Thermoluminescence (TL), makes further use of the fact that naturally occurring radioactive elements can irradiate substances in which they are contained, in such a way that electrons become cumulatively trapped in the crystalline structures of, for example, flint, loess particles and the clays out of which pottery is made. Heat and light drive out the electrons when, say, flint tools are accidentally heated in a camp fire or newly shaped pots are fired in kilns or particles of loess are exposed to sunlight. After the electrons are driven out and the interiors of flints and pots return to relative cool and darkness, or loess particles are buried under deep deposits, the entrapment of radiated electrons commences anew, so that a further, controlled application of heat in laboratory conditions is able to drive them out again, in the form of a light output that can be measured to equate with time elapsed since the previous heating or exposure to light. A technique applicable to an inorganic component of tooth enamel and called Electron Spin Resonance (ESR), refines the process by directly counting the trapped electrons.
Such methods, derived from physics and applied to the geological record, have enabled us to date the fossils that mark the separation of the (old world) monkey and ape lines at 30–25 mya and the emergence of the orang-utans from more generalized ape and human ancestry at 15–12 mya. With a few (more or less) absolute dates like these it has been possible (by comparison of blood group, enzyme and other protein differences) to estimate the likely date for the separation of the line that leads to ourselves from the line that leads to the chimpanzees at about 5.5 mya – which is indeed close to the age of the very earliest fossils we have to hand that mark the incipience of hominid traits. (Another method, which tests the bonding propensities – i.e. genetic closeness – of actual strands of DNA from different species, shows that chimpanzees are closer to humans than gorillas are to either of them, and puts a date of 7 mya on the chimp-human separation.) Such methods of date estimation for episodes of primate and human evolution rest on the assumption that protein and underlying genetic differences between species and genera (most of them neutral in terms of natural selection) accumulate at fixed rates, on the basis of random misreplications of the genetic code in the DNA of living creatures, occurring and piling up at a steady pace that can be timed by the dating of certain instances of evolutionary change. This assumption appears to be largely justified, but subject to complications. It is on the basis of this approach that the recent tests on the mitochondrial DNA remaining in the original Neanderthal specimen have been interpreted to demonstrate a genetic distance between the Neanderthalers and all modern human populations that puts back their last time of common ancestry to the days of Homo heidelbergensis at about 500,000 BP.
Mitochondrial DNA has been favoured as especially useful in tracing the line of human descent – we should more properly say, at the outset, not the line but a line. MtDNA has the advantage, for these purposes, of having nothing like the part in determining our genetic make-up that is played by nuclear DNA. The chromosomes in the nuclei of cells are made of DNA and are replicated at cell divisions as bodies grow and renovate themselves through life. The genes of the chromosomes direct the production of the proteins out of which bodies are made and determine the pattern of development of each living thing, not as rigid blueprints for some finished product but rather as a developmental scheme that must at every second of an individual’s life interact with the environment in which that individual is living. The impact of the environment is, of course, a very complex thing; its effects range from the crudely obvious to the invisibly subtle, and the individual creature is at every stage of its progress from conception to death the result of the ongoing interaction of its genetic developmental pattern and the ever-changing environment in which development takes place. Nuclear DNA is inherited from both parents, and the rearrangement of the chromosomes at conception when egg is fertilized by sperm is the immediate source of human variability in genetically determined developmental patterns. It is upon this variability that natural selection does its ruthless work.
The mitochondria are the energy suppliers of cells, outside the nucleus but also containing DNA. Their DNA is, by comparison with nuclear DNA, only marginally concerned with protein functions and it is thought that its workings are restricted to certain effects on the nervous system which can play a part in, for instance, some forms of eye disease and epilepsy. But, overwhelmingly, the mitochondrial DNA appears to have no bearing on our inheritance of physical characteristics; for this reason, its characteristics are not subject to the pressures of natural selection in the way that some of those of nuclear DNA are and mtDNA mutations occur at a faster rate than they do with nuclear DNA, with far less reconditioning to keep them in line (because it doesn’t matter so much). This makes them easier to track through different populations in the world today.
But the main interest of mtDNA is that it is transmitted very largely, if not entirely, by mothers. In every offspring, the mtDNA of the fertilized egg comes from the mother’s egg cell and what mtDNA was in the tail of the fertilizing sperm is lost. The nuclear DNA of the fertilized egg comes equally from the father’s sperm and the mother’s egg, but the mtDNA comes from the mother and, with cell division as the new individual grows, the mtDNA in all the cells of its body is derived from the mother’s mtDNA.
If the rate at which mutation of mtDNA occurs in primates, including humans, can be determined with sufficient accuracy, then the real possibility exists to trace lines of female-only descent among human populations around the globe, and to put dates to them. We shall not, by this means, be tracing the lines of descent of the nuclear DNA that shapes the bodies of both men and women; rather we shall be tracing the descent of the mtDNA in the power supply cells of the bodies of men and women. Women pass on their mtDNA to all their offspring, male and female, but only their daughters pass it on again – their sons forfeit their own mother-derived mtDNA with each generation, whether they father sons or daughters. It is easy to see that lines of mtDNA descent can come to an end when only sons are born to their carriers, rather as surnames can be lost in modern families when only daughters are born.
Rates of primate and human mtDNA mutation have been estimated on the basis of the generally agreed date for the separation of chimp and hominid lines at around 5.5 mya (though with some arguments acknowledged to put the time at more like 7 mya), together with the dates assigned by archaeological dating techniques to events like the arrival of human beings in Australasia and America. Armed with their estimates of mtDNA mutation rates, geneticists have been able to judge the degree of mtDNA genetic distance between various populations in the world today in terms of years elapsed since they shared the same mtDNA configurations. There have been many arguments about technical details of sampling and statistical methods employed, but the picture that emerges has one or two striking features, of which the most immediate is that today’s populations in sub-Saharan Africa show slightly more mtDNA variation than any other peoples in the world, with the implication that people have been living for longer in Africa than anywhere else with more time to pile up mutations, whereas the rest of the world is populated by more recently arrived people with more similar mtDNA derived from a single (African) source. The geneticists have variously dated the origins of the female lines of mtDNA descent back to between as early as 800,000 and as late as 40,000 BP, though the consensus is that something like 400,000 to 100,000 BP represents the most likely time back to which all lines of mtDNA inheritance through mothers can be traced. Arriving at a reliable date is complicated by the fact that episodes of mutation can be hidden by subsequent mutations and even reversed on occasions, and there are inherent uncertainties about the calibration of the rate from archaeological and evolutionary events. Even the evidence as to the African origin of the mtDNA lines is open to some doubt since the slightly greater diversity in Africa may reflect long-term greater population levels there, in which variations are slower to die out �
� in the same way that surnames are more likely to survive somewhere among large populations. It seems that it is possible, moreover, to construct family trees of mtDNA descent which do not necessarily point to Africa as the source of the oldest roots. There remains the possibility that a small amount of paternal mtDNA can be inherited in addition to the maternal inheritance, in which case the time-scale of mutation would be extended further into the past.
For all that, mtDNA evidence does appear, as things stand at present, likely to support the idea of an African origin for the inheritance through females of humanity’s DNA component in our cells’ mitochondria at somewhere between 400,000 and 100,000 BP. Since the bones of the original Neanderthal Man have yielded mtDNA that shows a divergence from modern samples far outside the modern range anywhere in the world (including Europe where Neanderthal traits might be thought most likely to occur), we may tentatively conclude that the classic Neanderthalers, at least, probably shared a common mtDNA ancestry with ourselves no later than the times of Homo heidelbergensis, and contributed little or none of their own mtDNA inheritance into the pool of modern mtDNA patterns.
Understandably, people have tended to link the idea of mtDNA descent to the entire question of the origin of modern Homo sapiens sapiens, but strictly speaking the mtDNA evidence only argues for a common female ancestry at somewhere between 400,000 and 100,000 BP in Africa. That female ancestry might well not have been Homo sapiens sapiens as we know it; we have seen how few and frequently enigmatic are the fossil remains from Africa in that time-frame. Moreover, the means by which that common mtDNA ancestry was spread across the globe need not necessarily involve, without other evidence, the conclusion that females accompanied by their males spread out of Africa in bands who never interbred with people they found along the way, their children and children’s children doing likewise. We must remember that with mtDNA genetics, we are only tracking mtDNA inherited in the female line. This mtDNA might have been introduced from one group into another by exogamous mating systems operating between neighbouring bands; we have seen that this is part of the standard pattern of the hunter/gatherer way of life of recent times and it may even be that beginnings of this uniquely human behaviour trait (apes do not show it) go back into ergaster/erectus times. By the times of Homo ergaster and archaic Homo sapiens, it may have been women who always left home to go to another family or band, while men stayed at home and received their mates from neighbouring groups. Indeed, there is evidence to show that Y chromosome variants (a sort of male equivalent in terms of genetic tracking of female-inherited mtDNA) are always much more localized than variants of mtDNA. It is possible that the line of mtDNA descent from the females of some small population in Africa was taken all over the world in this way, eventually displaying in different branch lines the small accumulations of mutations (slightly more of them in Africa where they have been going on longer among larger numbers of people) that we see today. (The idea, incidentally, of an ‘African Eve’ from whom all our mtDNA derives is more of a notional mathematical inevitability than a prehistorical fact; it is rather a question of a smallish group of females sharing a common mtDNA pattern back to which all our mtDNA is linked. Any idea of a ‘Mother of Us All’ is even more far-fetched, since as we have been at pains to emphasize, this line of research can only trace the genetic pattern in our cells’ power supplies and not the whole story of our nuclear genetic inheritance.)
The possibility that it was exogamous mating systems operating between neighbouring groups that was able, over a long time from group to group to group, to spread such a near uniformity of mtDNA patterns around the world without whole population treks (and without the expulsion or extermination of indigenous people along the way) could be bolstered by two other considerations. Firstly, the chimps and gorillas show much more mtDNA variation than we do; two chimps from the same group are likely to be more different than two human beings from the opposite ends of the earth, and this is not because chimps live in such large numbers by comparison with ourselves that mtDNA lines do not go out of existence easily. One way in which chimps differ from ourselves is that they do not have exogamy. Secondly, there is the possibility that the mtDNA line that eventually ‘won out’, as it were, over all the others in the world of, say, 100,000 BP was associated with characteristics of the nuclear DNA in the same cells as itself that conferred some crucial survival benefit on its carriers; what if the sons and daughters of exogamously mated mothers were so clever or capable in some valued way that both natural and human selection favoured them highly, making the daughters much sought after as mates by neighbouring groups (where their children outshone, outsurvived their fellows), steadily spreading their mtDNA and their nuclear DNA through many if not most of the human groups in the world? It is thought that it is in just such circumstances, when small and scattered groups keep on exchanging genes across their boundaries, that evolution may proceed at a fast pace for all concerned. In Africa an always greater population density would have allowed for slightly more mtDNA variation to persist down to the present day. Of course, there would still very likely have been places where populations who had become more isolated than most and for longer could be, for some reason, unamenable to mate interchange when its possibility arose – it need not have been speciation to the point of non-interfertility, but simply an overwhelming lack of mutual attraction or some other cultural barrier. The Neanderthalers of Western Europe and some other populations attested by fossil finds around the world may be candidates for such non-participation in the evolution of Homo sapiens sapiens, though some anthropologists are prepared to argue for the survival of some Neanderthal traits into later populations in Central and Eastern Europe.
If only the evidence of mtDNA were available, we might still conclude that the scrappy fossils sometimes claimed to establish the incipience of modern Homo sapiens sapiens in Africa before 100,000 BP had not received sufficient support to advance the case made out for them as the precursors of a conquering exodus from their homeland that would, in a matter of a few tens of thousands of years, see them supplanting by fair means or foul all pre-existing populations in the way of their global spread. (Fair means include ‘outcompeting’ their rivals; foul means are all too obvious. One cannot resist noting that many nineteenth-century anthropologists thought Homo sapiens had exterminated his ‘primitive’ forerunners in the manner of European colonists on the loose in Tasmania and the Belgian Congo, while some later twentieth-century anthropologists see him outcompeting them like a successful businessman. Of course, noting these things does not necessarily make the ideas factually wrong, any more than noting that the idea of global human ascent to Homo sapiens sapiens by peaceful exogamy owes more than a little to today’s progressive, liberal, anti-racist sentiment.)
The study of mtDNA recommends itself as a way of getting into the business of tracing human descent without fossils because its line is clear, though it suffers the vices of its virtues in that its very clarity arises out of the limitation that it does not really track the family trees of evolving humanity but only the thread of mtDNA inheritance through females. It is nuclear DNA that (in interaction with the environment) really makes human beings different, among themselves and through time with evolution from one stage to another. But nuclear DNA is much more complex than mtDNA and inherited in a complicated way from both parents, unlike mtDNA, so its lines are far more difficult to trace, to the point of practical impossibility. Some assistance in tracing at least parts of them is afforded by the strange fact that much of the DNA sequences appears to be quite meaningless, encoding nothing that controls protein manufacture or inheritance of anything at all. Here and there on the sequences are the genes for height, for example, or eye colour or something else about our inherited constitution, but much of the DNA in the nuclei of our cells seems to consist of long lists of various repetitions of chemical bases, differing in detail from individual to individual, whose particular presence or absence has absolutely no bearing on our make-up (or our
survival potential in the face of natural selection). The usefulness of such neutral but recognizable patterns is, of course, that they may be trackable through populations. And it happens that, as with mtDNA, it is in Africa that certain combined patterns of nuclear DNA show more variety, while in the rest of the world the same patterns are very limited in expression. Again, this situation suggests that DNA patterns have been exported relatively recently from a particular African group, out into the rest of the world, and that Africa has gone on displaying more variety with these patterns because it contained, at the time of the export, other groups with different variations on the patterns that have persisted to this day, while in the rest of the world insufficient time has elapsed for the patterns to break down into greater variety. It has been possible to estimate the time of the first appearance of some of these patterns in Africa at between 140,000 and 90,000 BP. This dating is not based on any presumed DNA mutation rate but upon rates of recombination of DNA sequences. Because we are dealing here with nuclear DNA, inherited from both parents and subject to recombination at the fertilization of egg by sperm, the evidence can be seen as strongly indicative that certain genetic patterns, probably appearing first in one particular group of people in Africa some one hundred or so thousands of years ago, were spread so vigorously around the world that they have come to be a feature of virtually all non-African human beings (as well as a feature, of course, of some Africans, but alongside many variations on the themes). The proponents of the ‘Out of Africa’ hypothesis of modern human origins take this judgement to embrace the certainty that these patterns were carried in the bodies of both males and females migrating out of Africa to colonize the world, who went on to have no truck with any pre-existing people they found along the way, to as near as makes no difference the total exclusion of the genetic inheritance of those futureless people. Extensive genetic studies that statistically estimate genetic distances between modern human populations on the basis of frequencies of many different gene types are held to support this view fully. Average genetic distances between populations in modern world times are interpreted to record a separation between Africans and the rest of the world at about 100,000 BP, subsequently between Eurasians and Australasians at about 50,000 BP, and later still between Asians and Europeans at about 30,000 BP, all on the assumption that a single emigration of modern humans took place out of Africa at about 100,000 BP, with further genetic differences becoming apparent among the descendants of those emigrants as time went by as a result of genetic changes within themselves and not as a result of interbreeding with any people they came across in the course of their spread.
