ancestry, animals, biology, Education, evolution, Human, Mitochondrial DNA, Mitochondrial Eve, out of Africa, Popular science, science, Y-chromosomal Adam
In a paper published in 1987 in the leading science journal Nature, researchers claimed to have identified a female from whom all humans alive today are descended, dubbed “mitochondrial Eve”; nearly ten years later, another group of scientists published findings identifying her male counterpart, “Y-chromosomal Adam”. The choice of names, though evocative, was unfortunate and uninformative, since the use of “Adam” and “Eve” has led to several major misunderstandings about these two ancient humans. In this post, I’ll try to clear up some of these misunderstandings by explaining what these terms actually mean and how scientists have gone about trying to identify our most recent common male and female ancestors.
Although everyone alive today is descended from this pair of humans, they were not the only male and female alive at the time, nor were they the only ones to reproduce. In fact, unlike their Biblical counterparts, Eve and Adam lived tens of thousands of years apart and so could never have met, let alone mated. While we may look back and see them at the head of ancient lines of descent, they were unlikely to have been exceptional people during their lifetime. What is it, then, that makes them special in retrospect? How did scientists learn when and where they lived? To answer these questions, we have to first understand what is meant by the terms “mitochondrial Eve” and “Y-chromosomal Adam”. They were not the first pair of humans, nor were they only ancestors of everyone alive today; they were, in scientific terms, our most recent common matrilineal and patrilineal ancestor, respectively. So what do those terms actually mean and what can they tell us about human history?
The most recent common ancestor (MRCA) is the first person (or persons) encountered in common when looking back along the genealogy of a group of people. For example, the MRCA of siblings would be their parents, while a group of cousins would find that their MRCA is their shared grandparents. Ancestry can also be considered in a more limited sense; the matriline is the line of descent through the mother’s mother’s mother’s, etc…mother, while the patriline is the corresponding line through the father’s father’s father’s, etc…father. Confining ancestry to either the matriline or the patriline creates a much more limited scope for consideration. For example, the children of two sisters would have their maternal grandmother as their matrilineal MRCA, while their maternal grandfather, though an ancestor, would not be a matrilineal ancestor. Conversely, the children of two brothers would have a patrilineal MRCA in their paternal grandfather, their father’s father, while their father’s mother would not be a patrilineal ancestor. In fact, maternal grandfathers and paternal grandmothers, while certainly ancestors, are never included as matrilineal and patrilineal ancestors, because the line of descent from them is not confined to a single sex. The ancestors of any group of people therefore comprise more than the sum of their matrilineal and patrilineal lines of descent, which exclude ancestors with a line of descent through both sexes.
Scientists focus on studying matrilineal or patrilineal ancestry because these are easier to reconstruct since certain parts of our genome are inherited solely through the female or male line. In addition, these portions of the genome do not undergo sexual recombination, which means that they are inherited intact from one generation to the next, accumulating changes only through mutation. This makes them ideal tools with which to explore questions of ancestry. The Y-chromosome, which determines the sex of an individual, is usually found only in males and so is only inherited through the patriline. However, mitochondrial DNA (mtDNA), which is inherited solely through the matriline, is found in both males and females and plays no role in sex determination. In fact, the story behind mtDNA and its strictly matrilineal inheritance is far more intriguing.
Mitochondria: Our Alien Powerhouses
Cells are fascinating, complex miniature cities composed of “organelles” floating in a gel-like fluid called the cytoplasm. The nucleus houses the genetic information, which is used to guide the synthesis of proteins, tiny molecular machines that do nearly all of the work in the cell, from breaking down nutrients and building new cellular components to making up the filaments that support the cell’s structure and enable it to move. These machines need a constant supply of power to operate; most of this power is provided by mitochondria, tiny power stations found in every cell which generate energy through the process of cellular respiration.
Remarkably, mitochondria are thought to have once been free-living bacteria that survived being engulfed by another cell and took up residence within it. This means that mitochondria have their own separate genetic material, a circular DNA molecule 16,000 bases long (compared to about 3 billion bases in the DNA in the nucleus). Mitochondria reproduce within a cell by copying their own DNA and dividing; in mammals, this happens in response to the energy needs of the cell, which means it may not be linked to the cell’s own division cycle. During cell division, the mitochondria are randomly distributed between the two daughter cells.
An egg cell is one of the largest human cells and has a volume about 85,000 times larger than that of a sperm; within this volume are roughly 100,000 mitochondria compared to only 100 in the sperm. Since mitochondria are located in the cytoplasm, this large difference in size means that they are only inherited through the matriline. In addition, any paternal mitochondria that might manage to enter from the sperm are usually marked by the egg cell for later destruction in the embryo. Mitochondrial DNA (mtDNA) can therefore be used to reconstruct maternal ancestry – any mitochondria in an individual will have come from their mother, who got it from their grandmother, etc. Conversely, male children are a mitochondrial dead-end – a male will not pass on his mtDNA (inherited from his mother) to his children, who will inherit their mtDNA from their mother.
A small chromosome with a big story
The Y-chromosome is part of the DNA in the nucleus, which is arranged into 23 pairs of chromosomes. It is one of the two chromosomes in the pair which determines an individual’s sex. Under normal conditions, if both chromosomes in this pair are ‘X’, the individual develops as a female (XX); if one chromosome is X and the other Y, the individual develops as a male (XY). (It is possible for other factors to overcome the XY sex-determination system, resulting in an incongruity between the genetic and apparent sex; such conditions are termed “intersex”.) During fertilisation, the sperm adds either an X or a Y chromosome to the egg, which always contains an X chromosome. For the purposes of ancestry, the DNA on the Y-chromosome therefore serves as the male counterpart of mtDNA – the Y-chromosome is only passed on from father to son and can be used to trace an exclusively male lineage, analogous to the exclusively female lineage revealed by the mtDNA.
By studying mtDNA and Y-chromosomes, scientists can infer an individual’s matrilineal and patrilineal descent. Traditionally, this has involved cutting DNA into fragments and using the size of the fragments to reconstruct relationships. The advent of modern sequencing technology has greatly increased the resolution with which scientists can conduct such studies, since it is now possible to consider changes of just a single base in the DNA sequence. By applying these techniques to mtDNA and Y-chromosome samples from a wide range of humans living today, scientists have been able to reconstruct matrilineal and patrilineal trees leading back to mitochondrial Eve and Y-chromosomal Adam.
Adam and Eve
It may now be more clear why these names are as misleading as they are poetic. Mitochondrial Eve was not the only woman alive at the time, nor was she the only woman to reproduce. In fact, there is no reason to expect that she would have been exceptional or even have had particularly many children. Her sole distinction is to be found in retrospect: there is an unbroken line of ancestry from every human alive today back to her through the mtDNA inherited from their mother, grandmother, great-grandmother, and so on. What this means can be more clearly seen by considering several generations in a single family. While the children are descendants of their grandmothers on both sides, their mtDNA lineage can only be traced through their mother to her mother; if only this small group were considered, the maternal grandmother would be “mitochondrial Eve”. Similarly, although Genghis Khan may have been one of the most prolific men in history, his mother’s mtDNA will have reaped no benefit from his fecundity, since it could not pass on to his descendants. Many of the women contemporary with Eve are likely to have been ancestors of humans alive today, but somewhere along the way a male ancestor broke the line of matrilineal descent recorded in the mtDNA. A similar tale can be told of Y-chromosomal Adam; there may have been nothing exceptional about him during his life and living humans are descended both from him and from his male contemporaries, but only the line of descent from him is uninterrupted by a female.
Another common misconception engendered by the popular names is that Adam & Eve would have known one another and perhaps even mated. In fact, while mitochondrial Eve is generally thought to have lived in East Africa about 200,000 years ago, Y-chromosomal Adam was originally estimated to have lived in the same region roughly 50,000 years ago, though a recent study has revised this estimate back to 150,000 years ago. Nevertheless, Eve still predates Adam by several tens of thousands of years. Furthermore, the same study casts doubt on where Adam may have lived, relocating him to Central-Northwest Africa. This highlights an important point – because the titles are retrospectively bestowed based on studying DNA from a large number of people, our theories regarding when and where Adam and Eve lived may change as scientists gather and analyse data from more people.
As Eve’s mtDNA and Adam’s Y-chromosome passed through their descendants to us, random changes accumulated through mutation. It is possible to identify subgroups based on specific, shared variations in the mtDNA or Y-chromosome; these subgroups are called “haplogroups”. In principle, this is similar to dividing a family into branches, such as the children of each daughter; in fact, the matriarchs of the haplogroups have been called the “daughters of Eve”. Of course, the daughters of Eve were not sisters or even contemporaries – while some mitochondrial haplogroups originated over 70,000 years ago, others are as young as 10,000 years. Like Eve, these women may not have been exceptional in their lifetimes. Their claim to fame is retrospective – each daughter can claim to be the most recent common matrilineal ancestor of a subset of humans alive today, all of whom carry a copy of her mtDNA, which is a variant of the mtDNA she inherited from Eve.
These trees of descent provide a fascinating glimpse into the course migrations during human history. Although Adam and Eve were both in Africa, they lived in very different places and at different times. The record of migrations preserved in the mtDNA and the Y-chromosome are distinct and tell a tale of several waves of migrations and the subsequent branching and merging as humans spread from Africa across the surface of the planet. The earliest migration from mitochondrial Eve seems to have been towards southern Africa; several subsequent migrations spread her descendants throughout Africa until, sometime between 80,000 and 100,000 years ago, the L3 mitochondrial haplogroup crossed the Red Sea into Arabia and gave rise to all of the non-African mtDNA in humans alive today.
Finns: Immigrant or European?
The same methods can be used to answer interesting questions on a smaller scale, such as the history of a particular population. The most common Y-chromosomal haplogroup in Finland, N, is thought to have entered Europe from Asia sometime between 12,000 and 14,000 years ago. This haplogroup is found extensively throughout northern Eurasian populations; it is uncommon in Europe and is found at its highest frequency among the Finnic and Baltic people and in parts of Siberia and China. The specific sub-haplogroup found in Finland, N1c1, is thought to be about 8,000 years old, which provides a rough time estimate for this migration.
The story is not quite so simple, however, since the second most common Y-chromosomal haplogroup in Finland, I, is found throughout Europe and the Caucasus region and into Iran; it is present in above average proportions in Bosnia and Herzegovina, Croatia, Serbia, Norway, Sweden and parts of Germany, as well as throughout the Balkans. In Finland, this haplogroup is found in roughly 30% of the population, although the proportion is higher in Western than in Eastern Finland.
Finnish mtDNA tells a somewhat different story. Haplogroup U5, one of the oldest European haplogroups, is an exclusively European haplogroup which is found in a higher proportion in Finland than in the rest of Europe. Interestingly, it is found in a higher proportion in Northern than in Southern Finland, suggesting that the higher incidence among Finns is a result of their interaction with the Sami, who may have preserved this ancient European haplogroup. When the mtDNA and Y-chromosomal stories are combined, a tale emerges of several migrations into Finland between 7,000 and 10,000 years ago. An oversimplified version of this history might tell of carriers of haplogroup N coming to Finland from Siberia and encountering a population of Sami people who had migrated to Finland, having colonized Europe from the Volga-Ural region.
The distinct lineages formed by the Y-chromosome and the mtDNA can even provide insights about the history of language. A study published in the journal Science in September of last year found a correlation between Y-chromosome lineages and the language spoken by a population. For example, while the mtDNA in Iceland is primarily of British origin, the language, like the Y-chromosomal lineage, is Scandinavian. Indeed, a similar tale might be told of Finland, where the mtDNA is essentially European, while the most common Y-chromosomal haplogroup and language are both of non-European origin. While the reason for such a correlation remains unclear, some researchers have speculated that it may reflect historical differences in gender roles, such as the tendency for women to move after marriage.
We may be through with the past…
Knowing someone’s haplogroup can do more than simply tell an interesting story about their ancestral history; it can also provide information which may be of practical use. Some studies have shown a correlation between certain Y-haplogroups and infertility, while others have linked a specific mitochondrial haplogroup to protection from sepsis. As we study the human genome and learn more about different haplogroups, we continue to discover more such linkages and may become better able to explain and understand them. This sort of information could be of immense practical value; for example, it may be possible to tailor medical treatments specifically to an individual in order to minimize unwanted side effects and unnecessary risks.
Through the efforts of countless scientists building on each other’s work, we now have an astounding (though incomplete) picture of the story of humanity. While research continues to generate practical gains, we may have already reaped the greatest reward: a change in our understanding of ourselves. Like all good stories, humanity’s tale is rich and vibrant, full of false starts, near misses and great hopes. Each of us carries a record of that story within us, a record which tells us that there are no pure lines or true unmixed stocks. Every human alive today carries within them a chronicle of migration and intermingling leading back across the globe to Africa, where Adam and Eve lived but never met.
This post is also appearing as an article in the inaugural issue of the Arkadia Gazette, which launches today.
I read this article in the Arkadia gazette, and it got me thinking. So, as often happens when I take such liberties, I have a probably stupid question.
I read in the news at some point that some modern-day people in in Africa (around Sudan and Ethiopia) are the closest living descendants to these very early humans on earth. So my question is this: How does that make sense? If we anyway all descend from the same ancestor? Wouldn’t any measure of closest living descendant just be based on who is alive today through the fewest generations, rather then geographic location? Am I completely missing the point here?
I’m glad the article got you thinking!
It’s not a stupid question at all; the issue is exactly what is meant by “closest living descendant”. One way to measure this is simply by counting generations back to a common ancestor, which matches colloquial usage and seems to be what you had in mind. This is a pretty coarse measurement, though, so biologists usually think about how much genetic change there has accumulated over those generations — the less genetic change there is from an ancestor, the “closer” a given line of descent is considered to be.
The picture of Africa at the top of the post is a depiction of mitochondrial haplogroup lineages; L0, the oldest mitochondrial haplogroup, is located in eastern Africa (though it looks like it’s a bit further south than Ethiopia/Sudan). L0 is directly descended from Mitochondrial Eve, while the rest of the world is populated by descendants of L3, which had already diverged somewhat over about 50,000 years. On top of that, the L3 migrants met and mingled with other kinds of humans outside of Africa (eg, Ǹeanderthals and Denisovans).
So basically: if you measure closeness of descent by number of generations, Africans are no closer to Mitochondrial Eve than the rest of the world; if you measure it by genetic distance, Africans end up being closer because (1) L0, the haplogroup directly descended from Eve, is found in Africa (and only there); (2) the L3 population that left Africa continued to diversify genetically and also mated with other human populations.
I just want to stress that this doesn’t mean modern Africans are in any way more “primitive” than modern non-Africans. Not only would that represent a misunderstanding of what genetic change and evolution are, but it’s also a simplification to measure genetic distance based on just one thing (mitochondrial DNA). The only thing that we can really conclude from these observations is that modern humans are likely to have originated in a particular part of Africa at a particular time in the past.
Sorry for the long response; I hope it was clear!
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