Two other radiometric dating techniques that can be used to date rocks are uranium series and potassium-argon dating. Uranium series dating uses radioactive isotopes of uranium and thorium, which decay to stable lead isotopes. It depends on soluble isotopes being precipitated and then changing to insoluble forms, so it can be applied to speleothem and coral. Potassium-argon (and argon-argon) dating is used to date volcanic rocks. Argon can escape from molten rock, but is trapped in solidified lava. So if archaeological finds or fossils are found between layers of speleothem (in limestone caves) or between layers of volcanic tuff from ancient volcanic eruptions, these techniques can provide a date, or at least a date range, for the discoveries.
A relatively new technique that is proving incredibly helpful in Palaeolithic archaeology is luminescence dating. It is used to pin a date on the last time that grains of quartz or feldspar were exposed to either heat or light. It can be used to date the layers of sediment that an object is buried in, or sometimes even to date an object itself if it was heated – for instance, a piece of pottery or a hearth stone. Luminescence dating is a very powerful tool, useful for pinning an age on objects that are just a few years old, all the way to things that have existed for a few million years.15
The way that luminescence dating works is, I think, quite mind-blowing. When grains made of natural quartz crystals (i.e. grains of sand) are exposed to ionising radiation – from naturally occurring radioactive elements like uranium as well as cosmic rays – electrons get trapped in tiny flaws inside their crystal structure. Light or heat makes the crystal release its electrons. But once a quartz grain is buried, it starts to accumulate electrons again … until somebody comes along and digs it up. Samples for luminescence dating have to be kept completely in the dark when they are collected.
Back in the lab, the quartz grains are sorted out from the sample, under very dim, red light. Then they are exposed either to heat (in thermoluminescence, or TL, dating) or light (in optically stimulated luminescence, or OSL, dating). Then the crystals within them release their trapped electrons – making them glow. By measuring this luminescence, while knowing levels of natural radiation at the place where the quartz grains were buried (from other sediment samples and measures of cosmic radiation), the length of time that the crystal was buried can be estimated.15
Another method that measures levels of trapped electrons, again resulting from bombardment with radiation within sediments, is electron spin resonance (ESR). This technique works well for ageing tooth enamel, which is also a crystalline material – so it is very useful for dating hominin fossils.16
Genetic Studies
Quite recently, another branch of science has begun to provide important clues about our ancestry, about how we are all related to each other and even about the way the world was colonised. This time, however, the evidence is not buried in the ground but inside us, because the DNA contained in each cell of each of our bodies holds a record of our ancestry. Getting samples of DNA is surprisingly simple – and painless. DNA can be collected from volunteers using just a cheek brush or saliva swab. These samples contain cells, and inside those cells is the precious DNA.
While everyone’s DNA is mostly identical, there are some differences. There have to be, otherwise we’d all look exactly the same, clones of each other. Some genes govern our appearance, while others control the machinery of life. There are differences in those genes, too. You can’t tell just by looking at someone, but they might have a different blood group from you, a slightly different enzyme for breaking something down inside their cells. The differences in these active genes and the protein products they make are constrained by natural selection. If a mutation happens in an important gene, it could make the protein product of that gene work better, worse, or perhaps have no effect at all.
If the effect of a mutated gene is deleterious, it may be that the individual carrying it won’t be able to survive at all, or, perhaps, won’t live long enough to pass their genes on. So the mutant gene will disappear from the mix of genes in the population, or ‘gene pool’. If the product of a mutant gene proves advantageous, it could be that the individual with it gains a better chance of survival, and is therefore very likely to pass the new version of that gene on to their offspring. So, gradually, over many generations, a really advantageous gene can spread through a population. If the mutation is neutral, then it is pure chance whether it sticks around or disappears from the gene pool.
But there are also long stretches of our DNA that mean nothing to the cell; they are the bits between genes which are never ‘read’ to produce proteins. Sometimes they contain parts of old, disused genes, or historical bits of genetic material inserted into the chromosomes by viruses. These unused sections are not subject to natural selection like the working genes. Alterations appearing by random mutations in these regions will not be weeded out in the same way. This means that they are quite useful for tracing genetic lineages.
Most of our DNA is coiled up in chromosomes inside the nuclei of our cells; there is also a little bit of DNA in tiny capsules inside the cell. These are mitochondria, the tiny ‘power stations’ of the cell, taking fuel – sugar – and burning it to produce energy. The genes in the mitochondrial DNA have a very specific but incredibly important job, controlling energy transformation inside the cell. To a large extent, because they’re so hidden away, they too are protected from pruning by the grim reaper of natural selection. Mutations accumulate more rapidly in mtDNA than in the nuclear DNA.17 So this means that mtDNA is particularly useful for reconstructing genetic family trees. Geneticists can assume that there is a standard rate of mutation within the mitochondrial DNA, and that, unless they really hamper the work of the mitochondria, those mutations will persist.
The other important thing about mitochondrial DNA is that it does not get mixed up at each generation like the nuclear genes. Gametes (eggs and sperm) contain only half the number of chromosomes contained in every other cell in your body. But when the sperm and the egg are first made, it’s not simply a question of dividing the pairs of chromosomes up – before that happens, each pair of chromosomes swaps DNA with its partner, in a process called recombination. That means that the twenty-three chromosomes left in the gamete contain new mixes of DNA that weren’t there in the father or mother.
Sexual reproduction – with this shuffling of genes at each generation – means that genetically ‘new’ and different individuals keep being created. This in turn creates variability within the gene pool. This variation is incredibly important: it means that, if circumstances change, if the environment changes around us, there will be some individuals who may be able to survive better than others. Biology cannot predict what changes may be needed at some far-off point in the future, but species that have developed this way of ‘future-proofing’ themselves, through sexual reproduction, have been successful in the past – and so we still do it today. But for geneticists trying to trace genes back through generations, it is a nightmare, because the genes keep jumping about.
Mitochondrial DNA, on the other hand, does not get involved in recombination, and stays, chaste and untouched, inside the mitochondria – which we all inherit from our mothers. The sperm from our father contributes just its nucleus with twenty-three chromosomes at fertilisation. The egg also contains twenty-three chromosomes, as well as all the other cellular machinery – including mitochondria. This means that all your mitochondria, and the DNA they contain, are inherited from your mother. And she got hers from her mother, and so on. Geneticists can therefore trace back maternal lineages using mitochondrial DNA (mtDNA). Of the nuclear DNA in our chromosomes, there is actually one bit that doesn’t recombine, and that is part of the Y chromosome (which only men have). So this can be used to trace back paternal lineages.
In fact, other genes, in the nuclear DNA, can be traced back, although the history of these genes is more convoluted and much harder to track back through time than the non-recombining bit of the Y chromosome or mtDNA. Tech
niques for analysing DNA, for reading the sequence of nucleotide building blocks, are getting better and faster, almost by the day. Many labs are not looking at just individual genes from mtDNA or nuclear DNA but are now attempting to read all the DNA – mapping entire mitochondrial and even nuclear genomes. It’s an exciting time.
In terms of investigating our ancestry, it’s those tiny differences in our DNA, nuclear or mitochondrial, that are important. The traditional approach to studying genetic variation was ‘population genetics’, where frequencies of different gene types are compared between different populations. The problem with this approach is that it is particularly subject to distortions, as people migrate and populations mix. The approach of building ‘family trees’ of genes, from mtDNA, the Y chromosome and other nuclear DNA, makes for a much clearer picture of our inter-relatedness and our ancestry. The branch points of the trees correspond with the appearance of specific mutations.18
There are obviously ethical issues involved in the collecting of DNA: it should be done only with the consent of the individual concerned, it should be used only for the purposes initially laid out, and should not be passed on to any third parties. Some people have worried that genetic analyses of human variation might be used for a racist agenda, but these fears can be allayed as this science actually contains a strong anti-racist message. As eminent geneticist Luigi Luca Cavalli-Sforza has put it: ‘Studies of human population genetics and evolution have generated the strongest proof that there is no scientific basis for racism, with the demonstration that human genetic diversity between populations is small, and perhaps entirely the result of climatic adaptation and random [genetic] drift’.18
The Illusion of the Journey and the Folly of Hero-Worship
This book is about several different sorts of journey. There are the physical journeys made by our ancestors as they spread around the world. Then there is a more abstract, philosophical journey, with the gradual transformation of body and mind to something we can recognise as fully modern human. And then, there is my own physical and mental journey. I spent six months travelling around the world, meeting all sorts of experts and indigenous people, and experiencing the huge range of environments that humans manage to survive in today, from the frozen taiga of Siberia to the blazing aridity of the Kalahari.
When we cast our minds back and imagine our ancestors, sometimes surviving against the odds, and managing to make their way into and survive in the most extreme environments, it may inspire us with humility, awe and great admiration. And it certainly is an awe-inspiring story: from the origin of our species in Africa, to the colonisation of the globe.
But it’s all too easy to start thinking of this journey as a heroic struggle against adversity, and to imagine our ancestors setting out with the explicit intention of colonising the world. In fact, this ‘human journey’ is a metaphor – as it wasn’t really a journey at all – and they had no such goal in mind. I think that words like ‘journey’ and ‘migration’ are useful metaphors for describing how populations have moved across the face of the earth over vast stretches of time, but it’s very important to realise that our ancestors were not on some kind of quest to get on and colonise the world. Certainly, they were nomadic and they would have moved around the landscape as the seasons changed, but most of the time they would not have been purposefully moving on from one place to another. It’s just that, as populations (of humans or other animals) expand, they spread out. But I think it’s acceptable to use words like journey and migration to mean something more abstract, a diaspora happening over thousands of years. So there was no quest, and no heroes. We may feel humbled by the survival of our species, though set about by vicissitudes, and we may marvel at our ancestors’ ingenuity and adaptability – but we must remember, through it all, that they were just people – like you and me.
1. African Origins
Meeting Modern-Day Hunter-Gatherers: Nhoma, Namibia
I was sitting at a wooden table, under a thatched roof, somewhere out in the bush, in Namibia. A small but noisy flock of grey louries was flying about in the trees around the camp, calling ‘go-away!’ loudly. Beyond, the landscape of scrub and grassland stretched off, unbroken, into the distance. I was excited about being in Africa: it was the starting point of my journey and where the story of the human colonisation of the world begins.
I had flown into Windhoek and from there taken a small plane to fly out into the Kalahari Desert. We approached our destination, somewhere on the northern edge of the Nyae Nyae conservancy area, and circled, looking for the landing strip: a bare patch of dusty ground in the middle of the bush.
We landed, kicking up great clouds of dust and sending the crowd of inquisitive children that had gathered at the end of the airstrip running, screaming away. They returned to watch as we taxied, came to a halt, leapt out and started to unload. A few of the boys were holding long, straight sticks, and they were trimming off side twigs and whittling them down with knives.
It was extremely hot and very dry. Small trees and bushes were dotted about, with swathes of pale golden grass between. It was a short drive to Nhoma, a Bushman village situated on a ridge looking out across this landscape. There was nothing else for miles around.
I got out at a lodge near the village, and met Arno Oostuysen, who introduced me to field guides Bertus (himself a Bushman) and Theo (a young South African spending a year at Nhoma). We walked through the bush on sandy paths into the village. There were shelters, about twenty of them, around the edges of a clearing. The shelters were simply built, with branches anchored in the sandy ground and bent in to form a dome shape, covered with sheafs of grass.
Theo said there were 110 people living in the village, and that they were mostly members of just two extended families. This was a ‘matrilocal’ society: men married out and went to live with other Bushman bands in neighbouring villages, while women stayed in the village of their birth. He took me to meet one of the older men of the village. The Bushmen do not have leaders as such, but this man had the hunting rights over the land around, and it was courteous to thank him for letting me visit.
These people looked completely different from the black Namibians I had seen in Windhoek. The Bushmen were short and very lightly built, and relatively pale-skinned. They had the tightest curls of black hair on their heads, and open, wide faces, with high cheekbones. I looked at some of their profiles, and, from the side, their faces were quite flat below the nose: they did not have the jutting jaws of other sub-Saharan Africans. They had very narrow shoulders and a very pronounced curve in the lumbar spine which placed the hips far back.
Some people were sitting in groups, gathered in the shade under the trees. One woman was making ostrich-shell beads. Having carefully chipped pieces of shell into tiny discs, she was now drilling holes into them. She had a plank of wood about half a metre long on the ground in front of her. She placed the shell discs into small holes worn into the wood by previous beads, then drilled them by twirling a long, pointed stick between her hands. She drilled into one side, then turned them over and drilled the other. The beads would then be polished before they were ready to be threaded into a necklace or bracelet.
I walked over to where three other women were sitting on colourful cloths spread out on the ground, piles of small glass beads between their legs. They were threading beads to form colourful bands which would make bracelets, necklaces and headdresses. Some of the women had rows of traditional small, black, linear scars on their thighs and faces. Children of various ages were sitting with them, watching the women at their beadwork. I sat down and watched. After a while I indicated that I’d like to try making something, and one of the women started me off, with two rows of yellow beads on a new length of thread, which she then handed over to me to continue. I was given my own small pile of beads and I got to work. It was very calm, almost meditative. Patterns started to emerge in the beadwork. More children came over to see what I was doing and to inspect my slowly growing band of beads. All the t
ime, there were soft conversations going on, and every now and then the children would start up a song that would spread throughout the group. The words sounded very strange to me. There were sounds like vowels and consonants I was used to, but there were also clicks. Some words seemed to be almost entirely made up of clicks.
The language was one of the things that brought me here, to this isolated village in the Kalahari. Click languages are unique to populations in southern Africa and Tanzania – including the Bushmen (San) of Namibia and Botswana, and the Khoi Khoi (Khwe) of South Africa (sometimes these people are collectively referred to as Khoisan). Historically, these populations have had different lifestyles: the Bushmen being traditional foragers (i.e. hunter-gatherers) and the Khoi Khoi traditional herders.1 Although their languages differ, they are bound together by these common sounds: characteristic clicks made by snapping the tongue away from the teeth or the hard palate. For many years anthropologists and linguists have suspected that the languages shared by these now widely separated tribes might indicate a very ancient, common ancestry.2
After a while, one of the little girls spoke to me in English.
‘What is your name?’ she asked, carefully pronouncing the words. I told her, and asked what hers was. ‘Matay,’ she said.
I asked Matay the name of the woman who was teaching me beadwork. She was called ‘Tci!ko’ – pronounced ‘Jeeko’, with a click on the second consonant.
The Incredible Human Journey Page 3