Who We Are and How We Got Here Ancient DNA and the New Science of the Human Past

You may have missed it, but archaeology is undergoing a silent revolution. The story of our deep history used to be based on skeletal remains, linguistics, and the analysis of objects and tools our ancestors left behind, but since about three years archaeologists have a new tool in their arsenal. The analysis of DNA from old bones, or ancient DNA. David Reich has been at the forefront of developing this technique and argues that it is rewriting most of what we thought we knew about the last 350,000 years or so of human history.

Why did we have to wait so long for the ancient DNA revolution? This is perhaps the only part where I feel the book misses a beat. Reich does not really go into this, but DNA is notoriously unstable and quickly degrades after an organism dies. What little DNA you can retrieve is typically fragmented and mixed with DNA of microbes that feasted on the corpse when it died. With effort, care, the right techniques, and extreme measures to prevent contamination with human DNA from for example the researchers themselves, it is possible to isolate, amplify and puzzle together the genome of humans that died many millennia ago. Reich details how this technique works and what you can do with the data. I struggled a bit to get a clear understanding of the finer details. Even so, you will come away from this with at least the broad picture. We *can* isolate DNA from archaeological samples and we *can* use this to reconstruct how human populations are related.

Ancient DNA pioneer Svante Pääbo showed that Neanderthals, one of our evolutionary cousins, and humans mated with each other before Neanderthals went extinct (see his book Neanderthal Man: In Search of Lost Genomes). Today, non-Africans have some 2% Neanderthal in their DNA. The pace of discovery has accelerated tremendously since. Reich is at pains to stress that this book is not a definitive overview. New discoveries are constantly being made, refining or negating previous theories. Even Reich has had to backtrack on some of his published findings as more data have accumulated. Nevertheless, he wants to give an overview of what we know at the moment, acknowledging that this field is rapidly progressing. And this is where things get interesting.

The story of human evolution has long run along these lines: we evolved in Africa and then we left and spread around the world (the out-of-Africa hypothesis might sound familiar), our evolution resembling a tree that keeps dividing into smaller branches. So, all groups alive today can be neatly traced back to common ancestors. Ancient DNA reveals a picture that is much more complicated. Populations have mixed, split up, various split-off groups breeding with others again etc. The whole resembles a complex network more than anything else. Reich and others have even inferred so-called ghost populations, groups of people whose remains have not yet been found, but who have left distinctive traces in ancient DNA. Excitingly, for some of these we have recently found remains, confirming these predictions. Reich takes us on a world tour, showing how ancient DNA is challenging virtually all our previous ideas.

The history of Europe is one of continuous mixture and splitting of groups. South Asia has seen the incursion of migrants from the north, the Indo-Aryans (yes, the very same Aryans the Nazis got all obsessed with – ironic, as ancient DNA is now showing that this group wasn't the pure-bred human lineage they imagined). The idea that the Clovis people were the first humans to move into the Americas at the end of the last ice age? Sorry, nope, there were already people there who since disappeared. East Asia? Polynesia? Far from the simple stories of a single migration following one path, here too we can now reconstruct ghost populations and multiple waves of migrations and displacements. And what about Africa? Yes, we went out of Africa. But other groups went back in afterwards, and complex waves of migration are the rule here too.

Basically, if Reich's work is to be believed, whatever archaeology book you read next, you'll need to take it with a grain of salt and wonder how ancient DNA would change the story. Reich makes no secret of the fact he thinks this method yields superior data and insights – things we just couldn't deduce from the archaeological record so far. I think he has the data on his side so far, though there will no doubt be dissenters.

Human race has become a taboo topic. Anthropology has been so horrifically abused in the past to justify eugenics that no sane career scientist is willing to go there anymore. As a result, anthropologists and biologists have reached a politically correct consensus that whatever differences we can discern are so small as to be negligible, and, anyway, variation within groups is far great than variation between groups. By collectively avoiding this topic, Reich says, we are leaving the stage to crooks and ideologues with potty, pseudo-scientific ideas. Just because the orthodoxy of no meaningful differences can no longer be maintained, doesn't mean the heretics are right. Reich bravely aims for a new middle ground. Ancient DNA is showing that the old stereotypes of human race are just plain wrong. I have to applaud his bravery for being willing to table this topic.

All we can say for now is that the present-day structure of the human population is not the same that existed only a few thousand years ago. If anything, we are complex mosaics; mixtures of groups that themselves differed from each other and were results of previous mixtures etc. Many of these details are still being worked out, and the prospect of establishing an ancient DNA atlas of humanity, detailing all the mixtures and migrations over the millennia is an exciting one. In places it will corroborate archaeological finds, written records and legends and myths, in many other places it will present a far richer story than what we have been telling ourselves so far.

One last thing worth mentioning: the illustrations. Here, finally, is a book that makes good use of timelines and maps. From the endpapers giving a grand overview of the book, the timelines at the start of chapters putting the details in context, to the custom-drawn maps and figures: they are legible, use clear fonts, use contrasting symbols that work in greyscale, and avoid clutter. Editors at publishing houses should pay close attention.

Who We Are and How We Got Here is a fairly technical read, but stick with it. Once the story gets going, it is an incredibly exciting overview of a revolution in the making.