Origin of man: In search of the original Y chromosomal Adam

Image Courtesy of the Wessex Reg. Genetics Centre (Wellcome Collection)

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Now we’ve met Eve, it’s time to meet Adam. He’s essentially the same as Mitochondrial Eve, but with one obvious difference. No, not that one - I mean his Y chromosome. 

Humans have 23 pairs of chromosomes- long strings of DNA in all our cells. There’s 22 matching pairs of autosomes, and then there’s the sex chromosomes, X and Y. People who are genetically female have two X chromosomes - one coming from mum, and one from dad, while people who are genetically male have an X and a Y, with the X coming from mum and the Y from dad. Because it’s only biological males that have a Y chromosome, they are the only ones who can pass it on, father to son down the generations. 

However unlike mitochondrial DNA, the inheritance of the Y chromosome isn’t as clean and clear-cut. Certain parts of the Y chromosome can pair up with the X chromosome and get swapped around when sperm are made - a process known as recombination - muddying the genetic waters. But there’s certainly enough DNA on the Y chromosome that doesn’t get messed about, providing a handy way of tracing male ancestry. And just as with mitochondrial DNA, geneticists are able to define a number of different Y-chromosome DNA haplogroups - people who all share highly similar Y chromosomal DNA - to look at patterns of evolution, migration and relatedness in populations all over the world

In a similar way to the analysis that led to the discovery of Mitochondrial Eve, researchers have been able to follow these genetic threads all the way back to our oldest common paternal ancestor: Y chromosomal Adam. Just like mitochondrial Eve, this man is believed to have lived in Africa, providing further support to the idea that modern humans evolved in Africa and then spread out into the world. 

Unlike the biblical story, where Adam came first, this genetic Adam seems to have appeared on the scene later than mitochondrial Eve - around 60 to 140 thousand years ago for Adam compared with the 140 to 240 proposed for Eve. There are a number of evolutionary reasons put forward to explain this, including different migration patterns of early human males and females, an imbalance in the proportion of female versus male babies born, and plain ol’ natural selection. 

However, this idea was shaken up by a paper published in 2013 describing the analysis of a DNA sample from an African American man submitted to a commercial genetic testing lab, which pushed back the birthday of Y Chromosomal Adam to more than 300,000 years ago. If you’ve been paying attention, you’ll notice that’s some way before modern humans are believed to have evolved in the first place. 

As might be expected, this announcement raised some questions and was challenged in a 2014 paper suggesting that this estimate was not correct because of poor DNA analysis. After conducting their own analysis, the researchers and came to the conclusion that Y Chromosome Adam actually existed between 163 and 260 thousand years ago, putting him back in the picture with mitochondrial Eve - although only in terms of time. We have no idea if they lived in the same place, and there’s still several tens of thousands of years each way, which is a heck of a long time to be waiting for a date…

This estimate was shifted again a year later by a paper from another consortium of researchers who placed Adam’s origin at around 245 thousand years ago, with a buffer of about 50 thousand years each way. Interestingly, this team also found evidence in the distribution of different versions of the Y chromosome to support not only the idea that a relatively small group of humans made if out of Africa to colonise Europe and Asia around 50 to 100 thousand years ago, but also that there was another significant genetic bottleneck around 10,000 years ago. Exactly what caused this is unknown, but it could be down to a ‘boom and bust’ population explosion and collapse associated with the introduction of farming around that time, or maybe it was to do with patterns of male migration and mating during various wars and conquests. 

Tracing the Y chromosome not only helps us find our earliest ancestors, it also provides information about the migration of these people. Researchers believe humans migrated down the coast of Africa and to Australia, India and Asia, while some others headed for Europe. This is backed by a recent study looking at Japanese males who were found to belong to a haplogroup of ancestors who originated in Africa. 

Speaking of wars and conquests, another notable figure who contributed to his own unique ‘genetic bottleneck’ is Genghis Khan, the famed leader of the Mongol Empire across much of Asia in the 12th century. Y chromosome analysis has shown that not only was Khan a fighter, he was a lover too - nearly 8 percent of the men living in the region of his former empire carry near-identical Y chromosomes, presumably Khan’s, adding up to around 0.5 per cent of the current male population in the entire world, or a staggering 16 million descendants.

Y chromosome analysis has shed light on a number of other intriguing histories and mysteries from the past.

For example, there’s the Jefferson-Hemings controversy, a long-running debate about whether the 19th century US President Thomas Jefferson had fathered children with Sally Hemings, an enslaved woman owned by him. Historians had argued for decades whether this could be the case, and many thought it wasn’t true. But in 1998, researchers analysing the Y chromosomes of male Jefferson and Hemings descendants showed that the President was indeed highly likely to have fathered Sally’s youngest son and by inference, her five other children too.

Moving from State to church, or rather, temple, scientists have found that modern-day Jewish men belonging to the Cohanim caste, often with the surname Cohen, are genetically linked and likely to be descendants of the Biblical high priest, Aaron, the elder brother of Moses.

Y chromosome analysis has also supported the long-standing folk history narrative of the Lemba people from South Africa who claim to have been led out of Judea by a man named Buba, and maintain a number of jewish practices. DNA analysis has suggested that they too possess the same Y chromosome haplotype as the Cohanim jews descended from Aaron, and represent a population that must have migrated back to Africa from the Middle East.

Y chromosome DNA has also helped to clarify the controversial origins of Brahmins, one of the largest ethnic groups in the Indian subcontinent. While some people have claimed that the Brahmins originated from a specific group known as Aryans, DNA analysis suggests that they are actually drawn from at least 12 different locations across the world, primarily Asia, India and the Middle East. 

And possibly most famously, Y chromosome and mitochondrial DNA analysis have been used to show that the skeleton dug up in a Leicester car park was most likely to be King Richard III, the last English King to die in battle during the 15th century War of the Roses.

There’s also the connection between Y chromosomes and surnames, which are another useful tool for tracing ancestry, cultural groups and even geographical origins. In Britain and many other patrilinear cultures, surnames are passed down the male line from father to son, along with the Y chromosome. This provides a link between people’s genes and their names, which is particularly strong for people with rarer surnames. So there’s a lot of interest in combining genealogy with genetics, to see what we can learn about families and their history. 

For example, because the UK is a group of islands it’s seen many waves of migration by different people throughout history, such as early Palaeolithic settlers, Romans, Anglo-saxons, and Vikings.  But how much have these people influenced our genetics today? 

To find out, researchers led by Mark Jobling from the University of Leicester have used their genealogical-genetic toolkit to help them track the migration of Vikings in the north of England, combining genetic data with archaeological, surname and geographical information. The researchers were able to draw a geographical map of where different Y-chromosomes could be found, revealing the contribution of the Vikings who came a thousand years ago to modern day people living in the north of England, suggesting a particularly heavy Scandinavian presence in the North West back in the day. 

Jobling’s colleague Turi King then took this idea in a different direction, carrying out research to evaluate how surnames and Y chromosome analysis could potentially be used in forensic investigations to solve crimes, by predicting a male suspect’s possible surname from DNA evidence left at the scene. She found there was a 24% chance of people sharing the same surname also sharing the same ancestry, concluding that “a large surname-based forensic database might contribute to the intelligence-led investigation of up to 70 rapes and murders per year in the UK.” Similar studies around the world have followed, to see how well this idea can be applied in other populations to help police home in on potential suspects.

Going all the way back to our original genetic Adam and Eve, what can we say for sure about them?

While they were individual human beings, we have no way of knowing who they were, or exactly when and where they lived. The closest we can get is being confident that they walked the Earth some time around 200,000 years ago, give or take several tens of thousands of years, and that they both lived in Africa. 

We can also be confident they weren’t the very first two humans that got it on and started our species. There were many others that came before them and lived at the same time, but due to luck, small populations and the vagaries of birth, breeding, migration and death, they’ve each ended up being the only male and female progenitor of all humans currently on earth, and it’s highly unlikely they ever met or mated. 

So, although the names Adam and Eve may be cute - and also rich in cultural resonance for biblical believers - maybe it’s time to retire these terms and think of something better. Send us your ideas on Twitter @geneticunzip and we’ll share the best ones.