Human Origins: A tale of tangled roots.

I wrote this a while back. Rather than letting it live out its days quietly on my hard drive, here it is: the first article that’s exclusive to my blog!

We have always used stories to explain our origins. Countless colourful explanations have flourished around the world. In the absence of any real evidence, discussion and doubt could only have limited value. Then, in the 19th century, two new concepts came together, allowing us to ask the right questions. Firstly, the idea took hold that modern species are the result of evolution from older life-forms, and secondly, ancient bones, similar but not identical to our own, began to be documented around the world. Neanderthals were unearthed across Europe, and Homo erectus, in Java.

“Le Moustier Neanderthals” by Charles R. Knight (1920). The discovery of our cousins, helped to add context to our own story.

From these discoveries, a new story developed: one of a world populated by many types of “cave-man”, and eventually of the appearance and dominance of Homo sapiens, ourselves. Questions have come and gone as the bones of this story have been fleshed out.  For decades, the paramount question was whether each of today’s populations evolved from those find in the local fossils, or whether modern humans appeared once, before spreading around the world, displacing their relatives.

It took modern genetic techniques to settle this argument. When the DNA of people from around the world is examined, more differences are found amongst the peoples of Africa than across the rest of the world. This strongly suggests that from an initial population of modern humans in Africa, a relatively small group leaving the continent to populate the world. But Africa was big, with a rich diversity of archaic humans. Where within that continent did these modern humans originate?

Firstly, let’s think about what we mean by “modern humans”.  Evolution is a gradual process, and to the untrained eye, the more-recent archaic species might not look out of place today.  Archaeological finds show the anatomical and cultural characteristics that define humanity slowly coming together, in millions of years of ancestral species. Somewhere around 200,000 years ago people appeared who are indistinguishable from today’s. These are known as Anatomically Modern Humans (or AMH) and they constitute the vast majority of our ancestry.  The oldest AMH bones known are from southern and eastern Africa. However, this is not proof that these were the first AMHs. Much of Africa has acidic soil, meaning that any bones would not be preserved.

However, our ancestors did leave other traces. In fact, everyone alive today carries the DNA of these early AMHs.  We inherit two copies of most of our DNA: one from our mother, and one from our father. These two copies are shuffled, and a single mixed copy is passed to our children. Over many generations, patterns become difficult to trace.

Luckily for geneticists, not all of our DNA gets shuffled. Some, called mitochondrial DNA (or mtDNA), is passed down only by mothers, and changes slowly only by mutation. These mutations can be used to construct a family tree of mothers and their mothers, going back through time. With every generation, some branches die out when a woman has no children, while the mtDNA on other branches become more common. Inevitably, if you go back far enough, all modern mtDNA must descend from one woman, dubbed the “Mitochondrial Eve”. This doesn’t mean that other women from her time didn’t contribute their other DNA to the modern population. It is just that, largely by chance, her mtDNA has been the only mtDNA to survive. The greatest diversity of branches is found across southern and eastern Africa, suggesting that this is where “Eve” lived.

mtDNA is not the only DNA that avoids shuffling. The Y-chromosome is passed down from father to son, and can be used to draw a similar tree. Interestingly, the greatest diversity of Y-chromosomal branches falls in Central Africa, suggesting a very different homeland for the “Y-Chromosomal Adam”.

Studying the rest of the DNA is more complicated, but not impossible, and these “shuffled” chromosomes suggest south-west African origin. So it seems that the genetic evidence is conflicting. How can the south, the east, the centre and the south-west all be the origin of AMH? In short, nobody knows, but we can think about the possibilities.

It is important to remember that modern populations do not represent the full diversity of the first AMHs. AMH may have appeared and spread throughout Africa before the times of our “Adam” and our “Eve”. Their contemporaries would have been every bit as anatomically modern, even though their Y-chromosomes and mtDNA have not made it to the modern day.  Similarly, its possible that “Adam” and “Eve” could have been members of different archaic populations before the dawn of AMHs. These groups could have met and inter-bred in later generations, producing us, the people that would (by a combination of natural selection and luck) out-compete its cousins and conquer the world.