Where do we come from? It’s a fundamental question in our understanding of what it means to be human. Yet, there are many gaps in our history, including the location of where modern humans - Homo sapiens - originated thousands and thousands of years ago.
Along with the Natural History Museum and the Max Plank Institute for the Science of Human History, researchers in the Francis Crick Institute’s Ancient Genomics lab have reviewed what we know about this early part of human evolution, and what remains a mystery. We spoke with group leader, Pontus Skoglund, and postdoc, Anders Bergström, to find out more.
What was the aim of this review?
Pontus: Most people, in the public and in academia, will have some knowledge of the origins of humans. But while there’s evidence that we all share ancestors who lived in Africa a few hundred thousand years ago, there are many remaining mysteries and the picture is not as clear as people may think.
We wanted to pull together the evidence, using the fossil record and DNA, to build a synthesis, and give our view of what we do know and we don't.
What does the review show?
Anders: We looked at three key phases in human history: the worldwide expansion of humans around 40,000 to 60,000 years ago; the emergence of present-day diversity in Africa between 60,000 to 300,000 years ago; and finally, even further back to around 300,000 to 1 million years ago, when the ancestors of modern humans separated from archaic human groups, like Neanderthals and Denisovans.
One key argument we make is that there isn’t yet sufficient evidence to say where modern humans originated. A popular idea is that of ‘Mitochondrial Eve’, the most recent maternal common ancestor of all people today, typically conceived to have lived in a single, small population, perhaps in southern or eastern Africa, and that this population then expanded to give rise to everyone alive today. But this idea can be somewhat misleading, because in the rest of our genomes we have many more ancestors who did not necessarily live at the same time or place as Mitochondrial Eve. Furthermore, it’s not known in which part, or parts, of Africa these various early ancestors actually lived.
Pontus: Another important point is that there is only strong evidence of four occasions when modern humans mixed with Neanderthals and Denisovans in the period 40,000 to 60,000 years ago. And only three of these seem to have contributed to present-day ancestry. This is despite us knowing from fossils that these groups lived closely, and we could indeed expect more of these events, but so far there isn’t strong genetic evidence for other episodes of contact.
What challenges are there in this area of study?
Anders: We’re talking about such a large timescale – we’re wanting to find out what happened up to 500,000 years ago. There actually aren’t very many human fossils from these early periods, and there is even less ancient DNA. A lot of the remains aren’t preserved well enough, especially if they’ve been in a hot climate. In Africa, the oldest ancient DNA recovered so far is from only 15,000 years ago. Much of what we know about early human evolution therefore relies on extrapolation into the deep past from recent or present-day patterns of genetic diversity, but those patterns contain very little information about the geographical whereabouts of our ancestors.
Pontus: There’s also the basic question of defining the ‘origin of modern humans’. We’ve been talking about this from a genetic perspective, so looking for a period in time when most of the genetic ancestry of present-day people was found in a specific geographical area. By 'most ancestry', we are referring to the multiple paths through every person's giant family tree that different parts of our genomes take when we follow them back in time.
But there are different perspectives other disciplines would take, for example looking instead at the evolution of traits, that is, when our ancestors became sufficiently similar to present-day humans in terms of anatomy, behaviour, physiology or cognition. The full history of our genetic ancestry and the evolution of particular traits will not necessarily align in a simple way, but these perspectives are often conflated when talking about human origins.
Will we ever be able to locate where humans originated?
Pontus: There are lots of interesting avenues of study to explore and new resources will become available to help us carry out this work. For example, more fossils will be unearthed which could shed new light. And advances in research tools, like in the field of ancient proteomics which is currently in its infancy, could help us collect genetic information from older or less well-preserved samples. There is also more to be done with statistical models, to better analyse the data we do have.
Anders: We’ll be able to find out much more about our past, but it’s hard to say how far this will go and whether we will ever be able to locate with certainty where modern humans originated. But as a question which goes to the fundamentals of who we are, and provides perspective on our history, how our ancestors lived and adapted, its one which is we’ll continue to explore.