Human Evolution On Trial - Mitochondrial Eve

Source:  Human Evolution On Trial - Mitochondrial Eve    Tag:  mitochondrial adam
Mitochondrial Eve

Studies conducted on nuclear DNA shows there is on average less difference between any two living humans than there is within most other species. This presumably shows we have a more recent common ancestry than do most other species. How long ago and in what form is a matter of debate. Beliefs range between just one couple created as recently as 6000 years ago to a population of reasonable size as long ago as two million years.


Research on the human Y-chromosome suggests the male common ancestry dates back to a single Y-chromosome Adam who lived in Africa more recently than 140,000 years ago (Jobling et al 2004). The Y-chromosome types found outside Africa today all diversified even more recently at just 89,000 to 35,000 years ago (Ke et al 2001).


Evidence from the study of mitochondrial DNA indicates we all descend from a woman who also lived in Africa some time before 100,000 years ago. We could refer to this woman as mtEve. The defence mentioned in “First Humans” that she was probably in no way different to her brothers, sisters, parents, cousins and neighbours though. The three people who originally constructed the mtDNA tree (Rebecca Cann, Mark Stoneking and Allan Wilson, a New Zealander) gave the date of their mtEve common ancestor as 290,000 to 140,000 years. So there is a margin of error. But she could have lived twice as long ago as Y-chromosome Adam did.


In “Pedigrees” [Selection] the jury saw that the survival of a single female’s mitochondrial DNA line may be due to no more than the loss of other lines around at the same time. In spite of the popular perception, therefore, the existence of this single ancestor doesn’t necessarily prove we are all descended from just her and her immediate family.


Perhaps there was some selection for her line. But any advantage her descendants had is unlikely to have been simply because of their mitochondrial DNA. Perhaps there was some selection for her genes, her nuclear DNA. There is fossil evidence of fairly modern-looking humans in Africa from more than 100,000 years ago (Olson 2002) and we’ll come back to this in “Out of Africa”. But there is no sudden change in human fossils we can use to follow mtEve’s descendants, and only her descendants, in any expansion around the world. Of course, as we hear at times, “absence of evidence is not evidence of absence” and we may just not have identified the genes yet. Anyway I pointed out in “Pedigrees” [Ancestry] her daughters would have had only half her nuclear DNA, her granddaughters quarter of it and so on. And the jury has also seen that mtDNA can be fairly independent of both Y-chromosome and nuclear DNA. We’ll look at “Culture” in Part V but I mentioned in “Polynesian Origins” [Societies] it is mainly women who transmit culture from generation to generation. Only women transmit mtDNA. I suggest any advantage mtEve’s line may have had is much more likely to have been through a stage in the evolution of culture rather than being genetic.


Agriculture’s development is accepted as being responsible for the modern distribution of many Y-chromosome and mtDNA lines (Jobling et al 2004). Surely similar selection through cultural advantage would be true for their earlier history? As far back as “Conception” [Me] I pointed out that culture, including language, is able to spread independently of gene flow. The defence will explain shortly [Interpretation] how an expanding mtDNA line need not necessarily be closely correlated with a more general genetic expansion.


What cultural advantage might mtEve’s descendants have had? It could have been any one of a number of things; social behaviour or organisation, use of fire, improved language or hunting techniques, religion, the invention of music. But in most regions there is in fact no sudden change in culture or technology we can use to identify her descendants’ earliest presence either. The jury will see that the big change in culture with the development of the Upper Palaeolithic, about 40,000 years ago, is actually much too recent to fit the mtDNA evidence. Besides, as the defence said in “Technology”, the Upper Palaeolithic was very complex in its development and didn’t spring up overnight fully formed in a single group of people. Any evolutionary advantage mtEve’s line had culturally is almost certain to have been slight. We’ll look at what this advantage may have been in Part V (“Culture” [Families]) after we’ve examined evidence from our early evolution and expansion.


The Trees


The following diagrams are based on Jobling et al (2004), along with updated versions freely available on the Internet (and see Behar et al 2008). The splits in the lines show regional variation, just like the ducks we met in “Species” [Kinds]. The jury will see there is no need to assume only members of these lines, or even only descendants of mtEve or Y-chromosome Adam moved. We can be sure that parent lines broke into offspring lines after they’d spread out. And many parent lines moved on with one or more of their offspring lines. Offspring lines have replaced parent lines in some regions. Lines have become extinct.




You will see as we continue with our family history that all branches in the trees make sense if we take it that mtEve lived around 190,000 years ago and Y-chromosome Adam lived about 70,000 years ago. The branches then fit with the periods of cooling and lowered sea level that have occurred at various times over the last 200,000 years. Of course we now have a gap of more than 100,000 years between mtEve and Y-chromosome Adam. This raises the question of who people had mtEve’s line been breeding with to sustain her mtDNA line all those years?


The Branches


The Sahara’s aridity has several times been extreme enough to eliminate human populations from the whole North African coast except for what is now Morocco. This part of Africa has been wet enough for human occupation to be continuous for the last one and a half million years. A drying Sahara has almost certainly actually pushed various migrant waves out of Africa several times during our evolution. In fact Stringer and McKie (1996) mention research that suggests the drying Sahara separated Neanderthals from mtEve’s ancestors about 400,000 years ago. They also write that by about 150,000 years ago the Sahara Desert had once more dried and expanded, separating humans in North Africa from those in Southern Africa, again just like the ducks. The Kalahari Desert in Southwest Africa also expanded. A date of 190,000 years ago for mtEve coincides with the climate cooling beginning around 185,000 years ago that led to this drying (“Neanderthals et al" [Climate]).


Members of the first branch on the mtDNA tree (L0) are concentrated in the southern half of Africa (Jobling et al 2004, and Behar et al 2008). The defence has said many times that selection works most effectively on small isolated populations. It is reasonable to suppose, therefore, that selection for any new culture acted on a population isolated for a long time in a relatively small region. Whatever the new culture was it helped the population survive (see “Culture” [Families]). The mtDNA evidence can be interpreted as showing that by 150,000 years ago a change in culture had spread through the South and East African point of the human star from the southern end.


The next splits in the mtEve line (L1, L5, L2, L6 and L4) seem to simply indicate a slow female expansion from East Africa. So with the global warming and presumably increased precipitation beginning about 130,000 years ago the inbred group with the new culture had expanded north. They interacted with other humans. The cultural innovation and genes spread around Africa. By 90,000 years ago modern-looking humans even began to appear outside Africa (“Out of Africa” [The Middle East]).


The haplogroup diagrams show that perhaps around that time mtEve’s line forked into yet more branches (L3, M and N). In spite of any propaganda M and N didn’t come out of Africa themselves. The lines were almost certainly selected for outside Africa. Their ancestors had come out sometime after the mutation that gave rise to L4, perhaps as long ago as 85,000 years. As I said, modern humans had certainly made it into the Middle East by 90,000 years ago. Neanderthals replaced them there about 70,000 years ago but it’s entirely possible Neanderthals absorbed some of their mtDNA lines. Like chimpanzees and gorillas, females may have been kidnapped or wandered off to join neighbouring groups (“Polynesian Origins” [Societies]). Lines M and N may simply have then been separated by the first obstacle to their expansion, the Zagros and Taurus Mountains (“The Human Star” [Geography]). The Himalayas separated them even more, sending N across the Central Asian plains and M into India. By then Y-chromosome Adam had probably appeared, but before then mtEve’s descendants must have had children with other men as her line had spread. A few of M and N’s descendants moved back into Africa.


After an initial expansion a series of mutations in the M and N mtDNA lines gave rise to regional varieties throughout the world, especially in the east. Line N reached Australia, probably more than 50,000 years ago. A later migration brought M’s line, and N’s descendant line P, to New Guinea (“Into Australia” [Explanation]). Others of M’s descendants reached Central Asia and eventually moved to America (C and D). N’s descendants A and B also eventually reached America, either with or after M’s descendants C and D. We’ll return to America in “North to Alaska”. In Southeast Asia mtDNA line R evolved from mtDNA N. Her descendants, and R herself, spread north and west from there. One of these lines gave rise to H, the most common mtDNA line in Europe.


Now a look at the Y-chromosome line. The first fork in the Y-chromosome Adam line seems to be when a group moved out from the South and East African point of the human star, perhaps a little more than 60,000 years ago. The new Y-chromosome line, B, is especially common in modern day Pygmies. From here on we begin to find non-African Y-chromosome lines.


Line B then split into two: one gave rise to D and E, and the other led to C and F. The expansion of Y-chromosome lines D and E must be more recent than the YAP Y-chromosome mutation that defines them, most likely by 50,000 years ago. Lines D and E were possibly simply separated by D’s departure from Africa, or E’s return. D moved across Central Asia, the middle of the human star, and reached the East Asian point. E expanded around Africa in force. The evidence suggests that there E replaced many A and B Y-chromosome lines.


Meanwhile, at the Asian end of the cline, it seems that, like the mtDNA lines, the Himalayas had separated the Y-chromosome lines, C and F. Line C seems to have originated on the Iranian plateau, or further north. As it expanded a series of mutations again gave rise to regional varieties. Men with Y-chromosome C moved through Central and Eastern Asia, as well as into Northern India. The line is the most common Australian Aboriginal Y-chromosome.


Y-chromosome line F may also have originated around the Iranian Plateau, or in India. Some people believe a volcanic eruption had earlier emptied this subpoint of the human star. F may have moved into India with mtDNA line M. It seems that some of F’s descendants (G, I and J) replaced their own parent line (F) outside India. On the other hand they may have moved back north from there. Y-chromosome H probably evolved in India. Another of F’s descendants (K) seems to have moved east across India. This Y-chromosome K, along with mtDNA line M, made it to New Guinea from Southeast Asia. Y-chromosome K also made it to already occupied Australia.


There seems to have then been an expansion of Y-chromosome K and his descendants, perhaps along the coast and rivers, north to the East Asian point (N and O) and west back through the Indian subpoint, onto the Iranian plateau, and even as far as Africa (L, P and T). This expansion, along with many of the mtDNA ones, was probably associated with the improved boating technology necessary to reach the Australian point (“Into Australia” [Wallace’s Line]). The jury will see (“North to Alaska” [Genetic Evidence]) that P’s descendants were eventually able to expand north beyond 50°. They spread across the whole region from the American subpoint (Q) to the Northwest European point (R). Y-chromosome R also moved back south and spread through Africa, India and Australia.


Interpretation


There are a number of ways to interpret all this evidence. The jury saw that evidence for an African home for both mtEve and Y-chromosome Adam is provided by the fact that Africa has the greatest depth and diversity of lines. We find only more recent lines outside Africa. The main disagreement is between those who say that these modern humans replaced all other human types when they came out of Africa in a single migration, and those who believe modern human variations show a regional continuity going back more than 200,000 years. The evidence is perhaps capable of supporting either hypothesis (Smith et al 2005).


The fossil and genetic evidence is usually interpreted as showing that at some time more recently than 100,000 years ago (perhaps even 50,000) modern humans, presumably descendants of Y-chromosome Adam and mtEve, left Africa together. They are said to have then spread around the rest of the earth in a single wave of migration, rapidly displacing all earlier human populations. Supporters of this single origin theory claim no hybrids were formed between people from this expansion and other contemporary human inhabitants of the earth. However there appear to be many continuities of population difficult to explain away.


Richard Klein (1989) in “The Human Career” quotes a major objection to the single origin theory. Milford Wolpoff has pointed out that in most regions the very earliest modern humans look more like the present inhabitants of those regions rather than like any supposed common African ancestor. In other words regional variation exists right from the beginning of modern human development rather than there being a common ancestral type that later evolves into the regional variations we see today. Perhaps this is a result of “founder effect” (“Hybrid Vigour and Inbreeding” [Survival]). But in many cases the present inhabitants in each region also look a fair bit like the people who were there even before the modern humans. Of course this has been explained away as being parallel evolution (“Species” [Labels]) but is this likely to account for all the continuities? Chris Stringer and Clive Gamble (1993) even admit that, unlike the spread origin supporters, the single origin “camp has not so far produced a comparable theoretical dogma to account for evolutionary change”.


Another problem for the idea of a single sudden migration out of Africa is that although the mtDNA and Y-chromosome lines both come from Africa the expansions seem to be fairly independent of each other, they even originate as much as 100,000 years apart (Jobling et al 2004).


So how could various genes, including Y-chromosome and mtDNA lines, move independently through a population? The jury saw that the mutation that gave rise to Y-chromosome line C, for example, probably happened on the Iranian plateau. But the mutation must have occurred in a single individual. He must have had male children but they would have only half his genes, or nuclear DNA. His grandsons in turn would have on average just one quarter of his genes, his great-grandsons one eighth, and so on (“Pedigrees” [Ancestry]). If he had children with close relations the children would have shared many genes. But suppose members of this Y-chromosome line developed some new technology and moved a short distance away. He and his sons would then have had children with women who were more different. The original ancestor’s Y-chromosome would survive but his other genes would rapidly become diluted. We know Y-chromosome C’s descendants eventually reached Australia.

We can presume that neither the individual with the original Y-chromosome mutation, nor even his grandchildren, took part in the last stages of that movement though. It would have been a slow migration across Asia, picking up resident genes as it moved. Therefore, in effect, we can say the Y-chromosome itself moved independently through our species’ nuclear DNA or genes. Resident elements of these genes could easlily remain in spite of the expansion of Y-chromosome C. The same thing with mtDNA. In fact with any individual gene (“Hybrid Vigour and Inbreeding” [Wave theory of Evolution]). But mtDNA is replaced less readily than Y-chromosome; therefore, as you saw, mtDNA’s root is more ancient than the Y-chromosome root. Although the root of both is in Africa there is no reason at all why they must both go back to just a single population. And of course it is quite possible that either or both Y-chromosome Adam and mtEve descend from earlier migrations into Africa. There is no reason at all why they must descend from some branch of Homo erectus that had never left Africa.


If at times men carrying any particular Y-chromosome are able to move into an unoccupied region, or at least an environment that is not being exploited (“Species” [Ecology]), they must have some accompanying mtDNA lines. Otherwise there will be no descendants. However the jury saw in Part II (“Polynesian Origins” [Genes] and “Pacific Population” [Mixing]) that a Y-chromosome expansion is quite capable of picking up mtDNA lines not previously associated with it.


The reasons why so many people are so ready to accept a complete genetic replacement are many. The main reason is that it closely fits widespread cultural beliefs of how species, including the human species, originate. It is also easy to explain, as it is simple, much simpler than reality. And it gives us a defined point of origin, a beginning we can easily understand. It is possibly also influenced by some people’s attitude to the desirability or otherwise of maintaining the “purity” of their own racial or religious group.


These reasons are all based on assumptions, preconceptions and prejudices, Chinese drover’s clever dog syndrome. Unfortunately any simple single origin theory requires selectively ignoring much relevant evidence. The existence of a mtEve and a Y-chromosome Adam merely indicates that both these lines reduce to separate single male and female African lines dated between 50,000 and 200,000 years ago, more recently than the original Homo erectus expansion.


Tests on mtDNA in Neanderthal remains show that they too separated from modern human lines more recently than the original Homo erectus expansion, this time around 500,000 years ago (Wade 2001). According to the literal interpretation of mitochondrial DNA evidence, as used by single origin supporters, some Africans have been developing separately from Europeans for nearly half that long but these two groups have no trouble at all producing fertile hybrids. Culture has tended to separate them to some extent though. To a varying extent culture has also separated Europeans from the many different people they have encountered as they made their way around the world. Perhaps culture was all that separated Neanderthals from modern humans.


Mungo Man


Mitochondrial DNA evidence from prehistoric skeletons in Australia suggests that at least one of the earliest humans there was not a member of mtEve’s line. It parted from her line about 250,000 years ago but was totally modern looking, if not ultra-modern looking (Adcock et al 2001). The skeleton is dated to at least 40,000 years ago and possibly 60,000 years. We’ll assume this Australian skeleton represents the last survivor of modern human mtDNA lines contemporary with mtEve. If mtEve lived even just 150,000 years ago it took roughly 100,000 years, or 5000 generations, for the lines to be reduced to one. If mitochondrial lines are reduced at the rate of one per generation (“Pedigrees” [Populations]) this means there were at least 5000 females in the original mtEve population, i.e. well over 10,000 people. In fact by looking at chromosomes and DNA Dr. Sarah Tishkoff and her colleagues have suggested the original population involved was in fact at least twice this large and possibly consisted of up to half a million individuals (Wade 2001). It would have been a widespread population.


Some single origin supporters may believe there was indeed a mass exodus of more than 10,000 people from Africa some time more recently than 100,000 years ago. Led by a Moses, perhaps? I would bet the population was fairly well spread out and a great deal of gene flow occurred.


Most of us accept gene flow has occurred since mtEve lived (see for example Olson 2002) but some are sceptical when others suggest it was going on long before her time. The defence suggests that the distribution of the mtDNA and Y-chromosome lines even supports the conception modern humans are a product of gene flow backwards and forwards throughout the world. At times cultural and technological selection for various mtEve and Y-chromosome Adam lines has resulted in something between the single origin and spread origin scenarios. Groups often mix with locals as they spread out. In Part II the jury saw that this sort of mixing and mingling is common. In Part V “Conquest” the jury will see other examples that support the interpretation of the evidence the defence has offered (“North to Alaska” [Genetic Evidence] for example).


Both mtDNA and Y-chromosome research indicates that worldwide these each have relatively recent common origins. But this doesn’t eliminate the possibility nuclear DNA from earlier populations survives. Besides, scientists have discovered human nuclear DNA that shows a common origin much earlier in human history (Jobling et al 2004). And studies of X-chromosomes, which are mixed genetically in daughters but not in males as they have only one of them, give age estimates of between half a million and nearly two million years for our common origin (Ke et al 2001). This fact is confusing for single origin supporters but it makes sense to others of us. The defence mentioned in “Technology” [Progress] that each gene travels on its own wave, it has its own evolutionary history. Perhaps mtDNA and Y-chromosome should simply be regarded as just two more separate human genes that have moved through our species.


John Wilford (Wade 2001) has written, “Though the extent of mixing between the modern and Neanderthal populations is not known, Dr. Trinkaus said that the disappearance of Neanderthals could be attributed in part to interbreeding as well as competition”. Just like the grey duck in New Zealand (“Species” [Difference]). In other words perhaps mainly just their Y-chromosome and mtDNA lines have been replaced (“Technology” [Progress]).


See next :: "Out Of Africa"


Witnesses Called



Adcock et al (2001) Mitochondrial DNA Sequences in Ancient Australians. Proc. Natl. Acad. Sci. Vol. 98 pp. 537-542.

Behar et al (2008) The Dawn of Human Mitochondrial Diversity. Am. J. Hum. Genet. 82: 1-11

Jobling et al (2004) Human Evolutionary Genetics. Garland Science, New York.

Ke et al (2001) African Origin of Modern Humans in East Asia. Science Vol. 292 1151-1152

Klein, Richard G. (1989) The Human Career. University of Chicago Press, Chicago.

Olson, Steve (2002) Mapping Human History. Houghton Mifflin Company, New York.

Smith et al (2005) The Assimilation Model, Modern Human Origins in Europe, and the Extinction of the Neandertals. Quaternary Intern. 137, 7-19.

Stringer, Christopher and Gamble, Clive (1993) In Search of the Neanderthals. Thames

and Hudson, Great Britain.

Stringer, Christopher and McKie, Robin (1996) African Exodus. Random House, UK.

Wade, Nicholas ed. (2001) The New York Times Book of Fossils and Evolution. The Lyon Press, New York.


N.B. this post is a revised version of the first essay on this subject, which appeared on January 10th, 2008