Who We Are and How We Got Here

David Reich is a professor in the Department of Genetics at Harvard Medical School, and an innovator in the field of ancient human DNA analysis. In Who We Are and How We Got Here (2018) he explains his work, referring to the “ancient DNA revolution” as the ability to extract DNA from ancient bones and sequence ancient genomes. It has revealed wide-ranging information about the interactions and migrations of different ancient humans throughout the world.

I appreciated that at the beginning of each chapter there is a diagram of the migrations covered in the chapter. This was a useful overview of the information. Chapter 1 begins with a very clear explanation of what DNA is and how it is extracted from ancient bone. The information and terminology presented in this chapter is referred to throughout the book, so the it is helpful for the reader to pay attention. That being said, with this background information, the further explanations are very clear to follow.

The research that Reich and his team have carried out reveals even more than what they initially intended. For example, it’s very interesting that now we can learn about ancient humans without having a trace of their skeletons, teeth, artifacts – or even their own DNA. Instead, “statistical reconstruction” can be done, based on information in other DNA samples. In referring to a group of ancient humans for which no DNA has yet been found, he writes that “we know important facts about them based on the genomic fragments they have left behind in samples for which we do have data.”

An example of a migration pattern that his work has focused on is in the chapter Ancient DNA Opens the Floodgates. He presents a theory different from “Out of Africa” in which there is a clear, straightforward path from Africa into Europe and Asia in one great migration. Instead, Reich’s research indicates that the ancestral population of modern humans, Neanderthals and Denisovans lived in Eurasia, descended from the original Homo Erectus that came out of Africa, and that there were further migrations back to and out of Africa again. This chapter has a very useful diagram entitled, “A Plausible Scenario in Which Modern Humans Ancestors Were Not Always in Africa,” which illustrates his argument very well.

Reich presents an analogy that replaces the one of a “tree” of human evolution:

            “The case of the Ancient North Eurasians showed that while a tree is a good analogy for the relationships among species – because species rarely interbreed and so like real tree limbs are not expected to grow back together after they branch – it is a dangerous analogy for human populations. The genome revolution has taught us that great mixtures of highly divergent populations have occurred repeatedly. Instead of a tree, a better metaphor may be a trellis branching and remixing far back into the past.”

Even where complete ancient DNA is not yet known, it can still shed light on the movements of ancient humans, as Reich speculates:

            “We do not yet have ancient DNA from the period before fourteen thousand years ago from southeastern Europe and the Near East. We can therefore only surmise population movements around this time. The people who had waited out the ice age in southern Europe became dominant across the entire European continent following the melting of the Alpine glacial wall. Perhaps these same people also expanded east into Anatolia, and their descendants spread farther to the Near East, bringing together the genetic heritages of Europe and the Near East more than five thousand years before farmers spread Near Eastern ancestry back into Europe by migrating in the opposite direction.”

Some of Reich’s speculations do not seem to have a clear basis, in my view. In the chapter, Encounters with Neanderthals, he mentions that DNA studies indicate that there was not a lot of interbreeding between Sapiens and Neanderthals. He rejects the argument that Neanderthal and modern human offspring were not as fertile as the mating between members of the same (species) and instead believes there wasn’t much interbreeding for social reasons. He writes, “Even today, many groups of modern humans keep largely to themselves because of cultural, religious, or caste barriers. Why should it have been any different for modern humans and Neanderthals when they encountered one another?”

I feel that this is not a valid argument since we cannot assume that what is true today was true for humans in prehistory. In fact, many archaeologists and anthropologists have jumped to false conclusions based on their cultural biases.

But the main focus of the book is how the DNA revolution is helping us understand how humans moved and got to where they are today. Reich explains,

“It is in the area of shedding light on human migrations - rather than explaining human biology - that the genome revolution has been a runaway success. In the last few years, the genome revolution - turbocharged by ancient DNA - has revealed that human populations are related to each other in ways that no one expected.”

The detailed descriptions of these migrations, and how they change our view of history, race, and culture, is the focus of this book. Reich refers to what his work has revealed as “a story about how our interconnected human family was formed, in myriad ways never imagined.”

In the chapter, The Future of Ancient DNA, is a paragraph that summed up for me the impact of this new technology:

            “The measure of a revolutionary technology is the rate at which it reveals surprises, and in this sense, ancient DNA is more revolutionary than any previous scientific technology for studying the past, including radiocarbon dating. A more apt analogy is the seventeenth-century invention of the light microscope, which made it possible to visualize the world of microbes and cells that no one before had even imagined. When a new instrument opens up vistas onto a world that has not previously been explored, everything it shows is new, and everything is a surprise. This is what is happening now with ancient DNA. It is providing definitive answers to questions about whether changes in the archaeological record reflect movements of people or cultural communication. Again and again, it is revealing findings that almost no one expected.”

Further findings include those that disprove beliefs that have been used to justify racism and cultural superiority. These are detailed in the chapters, The Genomics of Inequality and in The Genomics of Race and Identity.

All in all, I found this book to be both readable and fascinating, and it truly helped me understand who we are and how we got here.