by Tony Joseph
Did we really have to come from elsewhere?
But why do we assume that modern humans arrived in India from elsewhere at all? Why couldn’t they have originated right here? Until a few decades ago, this would have been considered a reasonable question, because the theory that modern humans evolved in different parts of the world separately, from archaic or extinct members of the Homo species such as Homo erectus that had spread out all over Eurasia by about 1.9 million years ago, was still prevalent – even though Charles Darwin had suggested the African origin of modern humans as early as in 1871. The theory was that the later intermingling of very differently evolved populations kept us together as one species, thus preventing us from branching off into different species in different continents.
But this theory has now gone into the dustbin and no serious scientist anywhere puts this forward as a possibility any more (though there may be some isolated holdouts especially in China which, till very recently at least, was wedded to the idea of indigenous, independent evolution of the Chinese people from archaic humans). The reasons why this theory went into disuse are both archaeological and genetic. The fossil record of Africa is rich with the remains of our closest relatives – Sahelanthropus tchadensis 7 million years ago, Ardipithecus ramidus 4 million years ago, Kenyanthropus platyops 3.5 million years ago, Homo habilis 2.4 million years ago and Homo heidelbergensis 700,000 to 200,000 years ago – and there is no other region in the world that comes anywhere close to it. But the clinching argument against multiple origins of humans on different continents is genetic. The DNA evidence has been conclusive that modern humans outside of Africa are all descendants of a single population of Out of Africa (OoA) migrants who moved into Asia sometime after 70,000 years ago and then spread around the world, perhaps replacing their genetic cousins such as Homo neanderthalensis along the way. All recent discoveries have gone on to reaffirm the African origins of all modern humans. As recently as in June 2017 came the news that an ancient skull from a cave in Jebel Irhoud, about fifty kilometres from the city of Safi in Morocco, has been classified as belonging to the Homo sapiens species and was dated to about 300,000 years ago.
Until the Jebel Irhoud fossil was dated and classified, the oldest discovered modern human fossils were two skullcaps dated to about 195,000 years ago, found at the archaeological site of Omo Kibish in Ethiopia. So the Jebel Irhoud discovery takes back modern human origins by about 100,000 years and also removes any remaining doubt about where we came from. Though the skull from Jebel Irhoud looks quite like us in its facial traits, the back of the skull is elongated like that of archaic humans and it also has ‘very large’ teeth, suggesting that the modern human didn’t emerge suddenly and fully formed, but was a work in progress as early as 300,000 years ago.
The logic of genetics
But even if you accept that modern humans arose in Africa, how did the geneticists arrive at the conclusion that all non-African populations descend from a single Out of Africa migration that happened less than 70,000 years ago? One needs to know a little bit of genetics to follow their argument. Genetics can sound somewhat complex to anyone who hasn’t paid attention to it earlier, but it is worth investing a few minutes to get familiar with it. You will be able to follow the story even without a perfect understanding of the mechanics of the science described here, so don’t get hassled if the explanations given here are not clear enough. Once you get more familiar with the vocabulary, you can come back and read this part again. So here we go.
Almost all the genetic code that humans need is packed into twenty-three pairs of chromosomes that we all carry inside the nuclei of our cells. There is one exception and that is the mitochondrial DNA, or mtDNA, which stays outside the cell nuclei. Each person inherits his or her mtDNA exclusively from his or her mother (the father also carries mtDNA passed on by his own mother, but he doesn’t pass it on to any of his children, male or female). The twenty-three chromosomes together with the mtDNA comprise a person’s genome.
Unlike the mtDNA, each of the twenty-three pairs of chromosomes in the cell nuclei has one half contributed by the mother and the other by the father. The two chromosomes that make up each pair are similar to each other, carrying similar codes at similar locations. But they are only similar, not identical. The differences between the chromosomes contributed by each of our parents usually amount to about 0.1 per cent. This is the same as the difference between the genomes of any two individuals, on average. These differences arise because of mutations, or random errors that happen especially during cell division – a necessary part of reproduction in living things. These mutations are then passed down through generations – assuming that, on balance, they are not harmful and, therefore, not weeded out by natural selection.
You could look at a genome as a genetic code written using an ‘alphabet’ of just four chemicals – A (adenine), C (cytosine), G (guanine) and T (thymine) – and if you do that, then each genome is made up of about three billion individual letters.3 A 0.1 per cent difference between the genomes of two people translates to about three million differences between the two genomes. If the two genomes came from people who shared a recent ancestor, then the differences would be smaller (which also means that genetic differences can be used as a measure of how close or distant two individuals are genetically).
Notice that although each person carries twenty-three pairs of chromosomes inherited from their parents, they pass on only twenty-three chromosomes (not twenty-three pairs of chromosomes each) to their children. How does this happen? The genetic term for this is recombination and what this means is that each parent randomly shuffles and divides the twenty-three pairs of chromosomes they inherited from their own parents and then passes on only one set of twenty-three chromosomes to their child. In other words, each parent does not pass on all of the genetic material they inherited from their own parents. They pass only twenty-three chromosomes each, thus together giving their offspring a complete set of twenty-three pairs of chromosomes.
But there is one exception to this rule: the twenty-third chromosome pair, or the sex chromosomes. Sex chromosomes are what makes a person male or female. If a person carries two sex chromosomes of the type XX, the person will be female, and if the person carries two sex chromosomes of the type XY, the person will be male. For a series of complex reasons, the Y part of the sex chromosome that every male carries comes directly from his own father, with no recombination. In other words, in the case of a male, the Y-chromosome he carries in his sex chromosome comes exclusively through the paternal line going back hundreds of thousands of years.
So we could say, up to an extent, that the Y-chromosome – or Y-DNA, as it is sometimes called – is a mirror image of the mtDNA, which is inherited exclusively through the maternal line, going back hundreds of thousands of years. If the Y-chromosome comes from your father and his father and his father and so on, the mtDNA comes to you from your mother and her mother and her mother and so on. Where the parallel breaks is in the fact that while both men and women carry mtDNA, only men carry the Y-chromosome. Since women’s sex chromosomes are of the XX type, they do not have the Y-chromosome at all. There’s a reason for this apparent lack of symmetry. Within every cell, mtDNA performs an extremely critical function – it has the code to convert chemical energy from food into a form that cells can use. No wonder mtDNA is often called ‘the powerhouse of the cell’. So to put it plainly, no man can do without the mtDNA, but every woman can do without the Y-chromosome.
This nature of the Y-chromosome and mtDNA – that they are inherited without recombination and trace the exclusively paternal and exclusively maternal lines of a person – has proved to be of enormous help, especially in the early stages of population genetics, in understanding the migration history of individuals and populations. What made this possible were mutations, or copying errors, as we discussed earlier. If the mtDNA of a person were exactly the same as her mother’s, grandmother’s and so on, or if the Y-chromosome of a man were exactly the sam
e as his father’s, grandfather’s and so on, there would be no substantive information or insight to be had by analysing anyone’s mtDNA or Y-chromosome. But mutations that accumulate over time ensure that the Y-chromosome or mtDNA of a person carries the genetic track record of all that happened in the exclusively paternal or maternal lineage of that person.
For example, if Great-Grandmother had a mutation called PCX on her mtDNA, then she would have passed that on to all her daughters and all her granddaughters born to her daughters and so on. And if you are doing genetic testing of a population in a particular area and come across multiple cases of PCX on the mtDNA, you would be able to create a genetic tree for people with that mutation – and all other mutations that followed since then, if any. In other words, if you have the mtDNA or Y-chromosome of a person, you will be able to locate that person’s maternal or paternal lineage over time. Since global human genetic databases exist for both the Y-chromosome and the mtDNA, it is now possible to locate where in the world people who belong to the same group or mutation are currently widely present.
But that is not all either. Scientists have long noticed that there is a certain pattern or regularity in mutations. This is not an exact science but still, they have worked out mutation rates with large confidence margins for the whole genome, as well as for specific regions of the genome such as the Y-chromosome and mtDNA.
While the track record of mutations as reflected in the mtDNA and Y-chromosome allows us to create genetic family trees, the mutation rate allows us to work out the approximate time that has passed since two branches or sub-branches of a tree diverged.
Population geneticists have given names to the branches of the global mtDNA and Y-chromosome family trees that they have created using extensive genetic studies. The equivalent word in population genetics for a branch is haplogroup – haplo means single in Greek, so haplogroup means single group.4 While a parent branch is called macro-haplogroup, subhaplogroup or clades refers to sub-branches. Some of the oldest branches in the mtDNA genetic tree are haplogroups L0, L1, L2 and M7, while some of the oldest Y-chromosome branches are A, B, CT and D. So by identifying the mtDNA or Y-chromosome haplogroup of a person, you can broadly work out his or her long-term paternal or maternal lineage, and how close or far other lineages are from this. If two people belong to the same mtDNA haplogroup, it means they have a common female ancestor dating from the time that haplogroup originated. And if two men belong to the same Y-chromosome haplogroup, it means they share a common male ancestor dating from the time that haplogroup originated.
A caveat is in order here. Remember that the Y-chromosome or mtDNA that you carry is only a small, less than twenty-third part of your entire genome. So just figuring out your Y-chromosome or mtDNA doesn’t say much about what your entire genetic make-up is: it just tells you who your entirely paternal or entirely maternal ancestors are. And they are just a small part of the people you can legitimately call your ancestors. Your mother’s father, or your father’s mother, or your father’s mother’s father, for example, are all left out in the cold if you go only by Y-chromosome or mtDNA lineages. If you go back ten generations, you will have 1024 people whom you can call your ancestors, but your mtDNA or Y-chromosome would have any connection with only ten of them. If you go back fifteen generations, the number of your ancestors goes up exponentially to 32,768, but your mtDNA or Y-chromosome would be connected to only fifteen of them! This could sometimes lead to odd results.
For instance, it is possible for a person to be almost entirely of Chinese ancestry, but to belong to a Y-chromosome haplogroup that is common only in India. All that would have been necessary for this to happen is for an Indian man to have left behind a son in China, say, ten centuries ago and for this son in turn to have founded a lineage with every generation having at least one son, all of whom lived in China and married Chinese women. A male descendant of this lineage today – the son of the son of the son . . . of the Indian – could still carry the Indian man’s Y-chromosome, but he would be of Chinese ancestry for all practical purposes, because there is only one tenuous, centuries-old link that connects him to India.
So while the mtDNA and Y-chromosome are helpful ways to understand population movements or histories of individuals or groups, they may not be sufficient to grasp a person’s or a population’s entire genetic make-up or its relationship to other populations. For that, we need whole genome sequencing, which studies a person’s entire genome, not just the Y-chromosome or the mtDNA. We cannot create genetic trees out of the twenty-two non-sex chromosomes – which are called autosomes – because recombination, or the shuffling and division of genes, makes that impossible. But whole genome sequencing can clearly help measure the degree of affinity between different population groups. Whole genome sequencing used to be a very costly and time-consuming affair earlier, but with improving technology, it is becoming increasingly common in genetic studies.
Cell
Source: National Human Genome Research Institute, Bethesda
Chromosome
Source: National Human Genome Research Institute, Bethesda
Mitochondrial DNA
Source: National Human Genome Research Institute, Bethesda
Dating ‘Out of Africa’
Now that the basic mechanics of genetics is out of the way, let’s tackle the next question: why do geneticists say that all modern humans outside of Africa come from a single group that migrated out of that continent, and why do they put the time of the exodus to 70,000 years ago or later? The reason is straightforward. When you look at the mtDNA of people outside of Africa all around the world, you will find they all descend from a single haplogroup with deep lineage in Africa, namely, L3. Think about what this means: that all people outside of Africa are descended from a single African woman who originated the L3 mtDNA haplogroup! Africa has about fifteen other, much older, lineages with names such as L0, L1, L1a and L1c, but none of them were part of the group that went on to populate the rest of the world. L3 has two immediate descendant lineages or subhaplogroups today, M and N, with N having its own major subhaplogroup, R. Thus all of the human population in the world outside of Africa carries lineages that follow from M, N or R. While south Asia has all three of these haplogroups, Europe has only two of them, N and R, with M missing.
The picture is much the same when you look at Y-chromosome lineages as well. There are only three haplogroups from Africa that went on to populate the rest of the world – C, D and F, all deriving from a parent haplogroup called CT. Again, this means that all humans outside of Africa are descended from a single man who started the Y-chromosome haplogroup CT. What these facts show is that only a subsection of the modern human population in Africa moved out to populate the rest of the world. Secondly, the fact that all the migrating mtDNA haplogroups descended from L3 and not any of the other haplogroups suggests that the migration event was single and not multiple, because multiple migration events would probably have resulted in present-day populations deriving their ancestry from a larger number of mtDNA haplogroups, not just L3. The likelihood that multiple migrations all happened to have the same L3 lineage is very, very small.
How do we then arrive at a dating of 70,000 years ago or later for the migration event itself? That is also straightforward. By using mutation rates and present-day genome data, geneticists can calculate the time of the emergence of particular haplogroups. They have concluded that L3 emerged approximately 70,000 years ago. Similarly, the N lineage is dated to 61,000 years ago and M to 48,000 years ago. So the Out of Africa event couldn’t have been much later than 61,000 years ago (otherwise there would have been N lineages in Africa, which is not the case), and it couldn’t have been much earlier than 70,000 years ago, because otherwise there would have been no L3 lineage in Africa at all, which is not the case either.5
This may seem like a neat argument that zeroes in on the period of the exodus, but these estimates are based on ‘average’ ages of the haplogroups we are interested in. The actua
l range for each of those calculations could be a few thousand years on either side. So it is more reasonable to say that the OoA event couldn’t have happened later than about 50,000 years ago and earlier than about 80,000 years ago. And out of this rather large range, if we take certain specific climatic considerations into account on top of genetic ones, we arrive at a window of roughly between 50,000 and 60,000 years ago.
The right climate
This is because the period before about 57,000 years ago, up to about 71,000 years ago, was glacial, when the climate was cold and arid and it would not have been the best time for anyone to attempt moving continents. During the ice ages, large bodies of water are locked up in ice sheets, and the cooler weather means there is less evaporation from the sea and hence less rain and more aridity.
Animals expand into new areas mostly during warmer, wetter periods, when new lakes and waterbodies form where there used to be none, and deserts start turning into lush new grasslands. Herbivores such as cows, goats, sheep and deer move into these new areas in search of plenty and safety and, soon enough, the predators, including Homo sapiens, follow the herds looking for abundant food. So if you are trying to identify the time for a major migration, it would be advisable to check the climate cycles – though, of course, critically, there are exceptions in this too, as there are in everything else. Some migrations may indeed have been facilitated by arid, cold climates that shrank the lakes and seas separating continents or regions, and thus made crossing them easier.
Geologists in the 1960s and 1970s discovered a way to figure out the chronology of global climate fluctuations by drilling the deep sea to get sedimentary cores and then looking at the oxygen isotope data that these contained at different depths. High levels of Oxygen-18 represent cold, glacial periods and low levels of Oxygen-18 represent warmer, wetter periods. Based on this, we can now look back at the climatic history of the world for the past many millions of years, which has been divided into periods called Marine Isotope Stages (MIS). Currently we are in MIS 1, a warm, wetter period that began about 14,000 years ago and is still continuing. Odd-numbered MIS stages are all warm and wet, while even-numbered MIS stages denote cold and dry glacial periods.
