What Does DNA Evidence Tell Us About The Past?

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What Does DNA Tell Us About the Past?

In the last couple of decades the importance of DNA evidence and its impact upon how we understand (big) history has been steadily growing.  From revelations regarding ancestors common to all living humans to precious insights into how modern humans came to dominate the earth, DNA evidence has been rearing its head in an ever-more prominent way for a number of years now.  But for all of the important insights delivered by this new source, the world of DNA and what it means for our understanding of the past remains a relatively obscure branch of knowledge.  In some respects I think this is linked to the nature of the evidence.  When I had my DNA tested I remember being incredibly excited by the possible implications contained in a tiny sample of my saliva – when the results were returned, however, that excitement was tempered by the realisation that a lot of work lay ahead of me if I was turn the string of numbers which I received into anything useful.  DNA history, in spite of the protestations of the various labs competing for your custom, is not a particularly accessible subject.  For that reason I decided to write this blog entry as a type of basic introduction to the world of DNA history; in fact, I might turn this into a series (if there is enough interest) exploring in more depth the issues surrounding this topic.

The first thing I think we need to understand is that evidence recovered from our DNA does not suddenly provide vast reams of information about our prehistoric world.  It provides insights, glimpses and, occasionally, moments of significant illumination but rarely does it help us to understand much about the day-to-day, or even year-to-year, lives of our ancestors.  DNA history is all about big history – great migrations and the spread of humans over thousands, or tens of thousands of years.  The evidence available has shown, for example, that all living humans share at least two common ancestors, known by scientists as Mitochondrial Eve and Y-Chromosomal Adam.  The first, Mitochondrial Eve lived around 200,000 years ago though there is no physical evidence to support her existence.  Instead, the mitochondrial DNA in modern humans converges, so to speak, on a single point about 200,000 years in our past; that convergence is a common ancestor from whom all modern mitochondrial DNA has descended.  Similarly, Y-chromosomal Adam lived anywhere from 60-190,000 years and, like his female counterpart, his existence is inferred from the evidence in DNA which shows a convergence around a single ancestor.  We know nothing about either of these individuals, only that they existed and that they appear, when one looks far enough back, in each of our family trees.

On the one hand, then, DNA evidence tells us very little.  We know that two individuals, separated by thousands of years, are the common ancestors for all living humans today but we don’t know anything about these individuals.  We know nothing of their lives, short of some very broad generalisations about how we imagine life in this period, and we know little about how their DNA came to dominate the human family tree.  On the other hand, the revelations provided by these developments are significant in their own right.  We now know that the human race is incredibly closely related and that we – all of us, regardless of race, gender, ethnicity, religion, or political beliefs – share common ancestors.  Pushing that a little bit further we can use that information, in my opinion, to draw one of the most important conclusions about the history of our species: every one of our wars has been, at its most fundamental level, a family feud.  Moreover, we so often write our history as the story of division – how we are different and why we fight about those differences – but this new evidence tells us that, when looked at from the broadest perspective, human history is instead the story of how we are all the same.  Isn’t that the richest kind of food for the best kind of thought?

We might not know anything about who Mitochondrial Eve was, but we know she existed and that is enough for us to provide a framework for how we think about the history of our species.  In a video I created several months back entitled The Scale of Human History I tried to make the point that we, as individuals, have a difficult time understanding the scale of our own past.  New information from DNA evidence has helped us, however, to create an image in our mind of what it means to be human (from a deep historic perspective) and to place the larger journey of our species into some type of context.  DNA evidence has, for instance, shown that all non-African people alive today are descended from just a few hundred individuals who struck out from Africa (the home of Mitochondrial Eve and Y-Chromosomal Adam) around 60,000 years ago.  Archaeological evidence has shown that modern humans left Africa before then but it was at this point in the past that our direct ancestors – those whose migration was successful – set off on the journey that would ultimately lead to modern humans appearing in Europe, Asia, Australia, the Americas, etc.  In that sense DNA evidence has helped us to develop a much clearer (though still muddied) picture of the human journey from Africa and their dominance of the planet.

It has also thrown up some interesting surprises, such as the Genghis Khan effect.  In a nutshell, approximately 8% of the men living in a large part of central Asia can trace their lineage back to a common ancestor who lived around the time of Genghis Khan.  Such a large proportion of the population plays nicely into the idea that Khan fathered many children to many different women on his famous campaign across Asia.  Indeed, the idea that such a large number of men can trace their lineage back to an individual – or closely related group of individuals: this effect may talk not just of Genghis but his brothers and male progenitors also – suggests that whoever this ancestor was, they were in a position to spread their DNA widely and that, in turn, their descendents were also able to do so.  Or, to put it another way, Khan’s DNA was spread far and wide during his lifetime (he apparently impregnated women in every place he captured) creating a large number of powerful descendents whose position, prestige, and level of power allowed them to continue the spread of his DNA.  Put simply, the Khans were a powerful family living during a period when they could claim multiple wives.  Their DNA was thus in the perfect position to prosper and spread as they had the means to support large families and the power to attract a wide range of sexual partners.

In some respects the Genghis Khan effect shows up the power and weaknesses of DNA evidence.  On the one hand, the lack of direct physical evidence is a problem which threatens to undermine many of the conclusions drawn.  On the other, it nevertheless serves to illustrate how interconnected and closely related our species is.  Studies of my own DNA were wrought with the same issues.  The evidence suggests one thing but a lack of physical verification means that it is all theoretical.  Take my study of my Y-Chromosomal DNA as an example.  This is the DNA my father passed to me, which was passed to him by his father, who received it from his father, and so on.  I chose to study that particular family strand as my grandfather never knew his father and, consequently, our family tree was left conspicuously incomplete.  Although DNA testing tells me nothing about this missing person, it does give me at least some insight into his family history and by comparing my DNA with that of many, many other people’s I was able to surmise that my Y-Chromosomal line dwelled not in my native Scotland but in Ireland.  But whether my missing great-grandfather ever set foot in that country or whether his family had left that area long before his birth remains totally unknown to me.  Would he have even realised that he was likely descended from Irish stock? Even after all the testing I know nothing new about my grandfather’s father – but I do at least have a sense of his, and my own, family’s deep history.

DNA evidence does not contain any easy answers.  In fact, it appears to raise many new questions in its wake but for all of those problems it has shed an important light on the big, broad processes that led our species to its current position.  It has helped us to paint (in broad strokes) a bigger picture of our history, the challenges we faced, and the challenges we overcame.  It suggests deep rooted links which bind our species whilst making a mockery of many of the divisions we seem so happy to draw and reinforce.  Deep ancestral history might not provide the type of everyday relevance that more modern history provides but it has demonstrated that there is a bigger picture when it comes to how we frame our understanding of the past and that, more than anything, is a precious thing.