Everyone who ever lived

It’s been roughly 200,000 years since the first anatomically modern humans, our species Homo sapiens, arose in Africa, and since then we’ve pretty much got everywhere. Our amazing story is written into our genes, mixed up with genes of the other early humans - such as Neanderthals - that we met and mated with along the way. This genetic journey is the subject of a new book - A Brief History of Everyone Who Ever Lived - by writer, broadcaster and geneticist Adam Rutherford. Kat Arney caught up with him to find out about the story behind his story of our story.

Adam - It’s got a certainly ambitious title. It is a brief history of everyone who ever lived. It is literally true in a sense that what it does is trying fuse older academic fields which is history and archaeology with a very much newer one which is genomics. So using DNA as a historical source to compare tests, verify, debunk what we think we know about history using the genome.

Kat - We hear a lot about all these things like 23andMe, you can get your genome done, people seem to be extracting DNA out of everything they can dig out of the ground. How has the science of DNA changed what we understand about our origins and how recent are we talking that these changes have come about?

Adam - It’s not an exaggeration to say that genomics has revolutionised palaeoanthropology, so the study of the evolution of humans. It’s all really happened in the last 10 years – mostly in the last 2 years. So, the first non-Homo sapiens human genome was only sequenced in 2009 and 2010 which was Neanderthal. From Neanderthal 1 – one of the first three Neanderthals found – this one being the one from the Neander Valley in Germany. We immediately began to make comparisons between us and them, and discovered a whole bunch of stuff which included the stuff that people now already know which is that we successfully interbred with Neanderthals. Now I argue in the book that that means that they were not a different species from us which I can defend…

Kat - Controversial, but yeah…

Adam - It is a little bit and I think the problem really is less to do with what the data says and more to do with our species definition. There isn’t one single species definition but the most widely used is that two organisms who are incapable of producing fertile offspring must be a different species. The fact that we can have our DNA tested – you and me as white skinned Europeans or largely white-skinned Europeans in my case – that we both have Neanderthal DNA in our genomes. We know that because we compared it with the Neanderthal DNA that was taken out of Neanderthals. The fact that we carry their DNA to this day means they had fertile offspring.

Kat - So they must've bonked each other basically.

Adam - Absolutely. I mean categorically, we can demonstrate that’s true. We know of – initially, it was only five but now, that number has gone up - gene flow events is what we refer to them as.

Kat - Euphemistically.

Adam - Euphemistically that. It does mean sex. It does mean sex in a sort of grand population scale rather than just two people getting it on. Many times that Homo sapiens met Homo neanderthalensis, we see gene flow events. We see it from males to females and females to males. We see it from Homo sapiens into Homo neanderthalensis and we see it from Homo neanderthalensis into Homo sapiens. So there isn’t a version of this which involves the production of infertile offspring.

Kat - A lot of us will have seen the image of the March of Progress, that diagram of evolution that starts with a knuckle-dragging monkey goes through various kind of heavy-browed knuckle-dragging ancestors, and the arrives at modern humans. So that very neat idea of a tree or a progression, that’s not right then, is it?

Adam - No. I think we’re ready to abandon that. I think we’ve been ready to abandon that for a few years now. That was an idea drawn in the 1930s and I think the fewer samples we had and the few data that we had, the more confident we were about – as you say, the March of Progress is what it’s known as. There are two things I argue are wrong with that. The first is the name and the direction of travel. So it suggests that ape-like ancestors on all fours were somehow simpler than us and that we have progressed into being bigger-brained and we used tools and that the final one is a sort of bearded man with spear. So, he is now Homo sapiens, an intelligent version of what had come before. That’s wrong. There is no direction to evolution. We are evolved, all organisms are evolved to exploit their environment at that time. So, we are no more or less evolved than any other organism on Earth. So there is no direction to the travel. There is no inevitability about tool use, and there's no inevitability about having bigger brains, and the intelligence that we enjoy. That’s the first thing. The second thing that’s wrong with it is with another path.

Kat - It seems to be more like not even a family tree but this enormous family bush, strangling thing all over the place.

Adam - Yeah, that’s right. I think maybe also, we need to think quite harder about abandoning the tree as a metaphor. I think it works on a broad scale, but of all the human species that we’ve discovered so far, it’s very difficult to put them on any sort of tree-like structure. We can't really draw the lines between any individual species as they’ve been traditionally known. We now can say because of genomics that Neanderthals did successfully mate with Homo sapiens. We know another new species, the Denisovans which was discovered in the last 5 years, very few specimens, a couple of teeth, and a finger bone. But we got the DNA out of that, got the genome out of that, and know that it’s not the same as Neanderthals and it’s not the same as us. And so, it’s another human species. We can position them on a geographical map and we can position them on a chronological map, and we can also look at the proportions of people living today, extant people and see how much DNA they carry of Denisovans. What we see is that the further east you go, the more Denisovan DNA there is, and the less Neanderthal DNA there is. If you go back into Africa, you see no Denisovan DNA and you see almost no Neanderthal DNA. What we do see is from modern Europeans migrating back into Africa. So we effectively have to redraw the map of how early humans migrated and spread all over the world over the past 250,000 years.

Kat - So that’s going way, way back in tens of thousands, hundreds of thousands of years back. What can our DNA tell us now about where someone like, you and I have come from? There's lots of services now. You can spit in a tube, you can get back this reading that says, “You're part German, part from here, part from there.” What can we know about where our populations today have come from?

Adam - It’s a great question. It’s a tricky one to answer because DNA will tell you such powerful things about immediate family, and more and more where people are discovering siblings and uncles, and aunts, and parents if they were adopted. Particularly in this era when there were a lot of war babies. We can use DNA with great accuracy to identify immediate family. So it’s very powerful in that regard. As in exactly the same way it is for identifying people, for being used in forensics. But the further back you go, the dimmer the past becomes and that applies to every bit of evidence that we use, and that includes DNA. The way I think about this is that the 2 parents, and 4 grandparents, and 8 great grandparents, and 16 great great grandparents, and so on, on a fully outbred family tree, the number of ancestors you have doubles every generation. So if you go back to 1066 which is almost a thousand years ago, then an outbred family tree has something like 100 trillion people.

Kat - Okay, that is more people that have ever lived on Earth. The math doesn’t work in this equation.

Adam - It doesn’t work at all. It’s a thousand times more than the number of people who’ve ever lived on Earth and we know that. That’s not controversial to say. So we figured that our family trees become pollarded and they overlap very soon a few generations up. And then if that’s true which has to be because of the maths, there's the concept called the isopoint where all family trees cross. The isopoint for Europeans is around about the 15th century. So there was hypothetically and actually – although we can't say who it was – a person who existed through which every single person’s family tree flows. Every single European person has a common ancestor of about 500 years ago, which I find just absolutely bonkers.

Kat - I found this mind bending when I was reading the book. There's so many amazing things in there. You're digging into the story of Richard III, you dig into the recent story of Jack the Ripper, you're looking at where are we going, what can our DNA tell us. If there was kind of one thing in a sentence that you could sum up about, what you want people to take away from this book, your take away points, what do you think that would be?

Adam - I think it would be that humans are much more complex than we thought they were. And that I think is something worthy of celebration. The second thing is that if people are trying to give you simple answers to complex problems, then they're probably selling something. It is the same answer. It’s the same answer. I'm not sure why we thought there would be simple answers to the questions of, what makes us us, how we came to be what we are when we are so beautifully complex. For a long time, we thought there were going to be simple answers because I don’t think we’d thought about it hard enough. And now, what we find is that we’re even more complex in terms of our genome than we are in terms of our behaviour and that’s exciting!

Kat - Science writer Adam Rutherford, and his book A brief history of everyone who ever lived is available in all good bookshops and online retailers. And if you're in need of another idea for a Christmas present for the genetics buff in your life, my own book, Herding Hemingway's Cats - Understanding How our Genes Work, is out now too.