# [Book review] Who we are and how we got here

David Reich is a leading researcher in the field of ancient human genetics. In his book **"Who we are and how we got here"** (2018) he describes the latest discoveries in a field that is quickly rewriting the (pre)history books. The field is moving so fast that today, only three years later, there are some chapters that start to be outdated.

The first thing to say about this book is that it is not especially long (~300 pages pocket edition) but is **dense.** I often felt overwhelmed by the amount of relevant information packed into every paragraph. At times, it feels more like a long scientific paper than a book for the general public. I personally appreciate Reich’s efforts to explain his field in as much detail as possible, but some other people might differ. 

If I had to summarize the most relevant ideas of this book in just three points, they would be:

*   The ancient DNA technology is providing a more detailed and rich picture of the prehistory of humans than any archaeologist could have dreamed of. It is sort of the ultimate technology to clarify the events that took place in the human past.
*   Two main processes are fundamental to understanding human diversity: migration and admixture, i.e. genetic exchange between different groups. All human populations have experienced both in the past, extensively. It is meaningless to talk about pure races; there is no such thing. In virtually every case, the current location of different peoples does not reflect that of their distant ancestors.
*   Human genetics show, with no doubt, important differences between different groups of people and between sexes. There has been a tendency in the last decades, coming from within the academy, to deny or reject these differences for political reasons. This path does not lead anywhere good: the differences are there. What we choose to do with them is a different matter

I will now go through some of the most relevant ideas in this wonderful book. Note: the titles and structure of this summary do not correspond to the structure of the book.

What is ancient DNA?
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The study of the distant human past is an enterprise that has been traditionally in the hands of archaeologists, historians, palaeontologists and lingüists. Since the 60s, some geneticists have tried to infer the past by sequencing the genomes of living people and comparing those sequences. The resolution that can be obtained by using these sequences is limited: if you want to know how those sequences were thousands of years ago, you rely on statistical inference subject to many different problems.

In recent years, the advent of certain new technologies and methodological innovations provides us with a source of information that is as good as it gets: the DNA sequences of people living thousands of years ago. Now you don't rely on inferring what happened in the past by looking at the present time and making statistical extrapolations: now you can look at the past straight into it. Some of the technological breakthroughs making all this possible are:

1.  Cheaper and more powerful sequencing
2.  Different techniques that filter non-human DNA in the samples. Most of the human remains are covered with other life forms such as bacteria and fungi, which results in just a tiny fraction of the DNA present in the samples corresponding to human sequences.
3.  The development of laboratories akin to the clean rooms used to create microchips, where scientists avoid the contamination of external modern DNA fragments using astronauts-like costumes, filtered air, sterilization with UV light, etc.
4.  The discovery that the petrous bone in the skull preserves more high-quality DNA than any other bone in the body.

The first ancestral genomes were published in 2010: A few Neanderthals, a Denisovan and a 4000 years old (4kyo) individual from Greenland. From there, every year new genomes have been sequenced from all over the world. In 2017, Reich's lab alone had produced genome-wide data for more than 3000 samples.

A recent origin for all human beings
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Around 7 million years ago, a population of primates split in two. One branch gave rise to chimpanzees. The other would eventually become modern humans. We will call that branch of the tree of life the hominids. At some point in the past 2-3 million years, a population of hominids left Africa and spread all over the Eurasian continent. We call this hominid *Homo erectus*. Then, *Homo erectus* diversified in different regions into different species.

For a long time, there was a big debate in the scientific community about where the different humans came from. There were two contenders: 

*   **The multiregional hypothesis:** That states that the different human populations come from the *Homo erectus* that inhabited the African-Eurasian continent, by parallel evolution
*   **The Out of Africa Hypothesis:** That states that the extant human diversity is much more recent and the product of a migration of humans that left Africa about 200 000 years ago (200 kya) and colonized the rest of the planet

This question was mostly answered by sequences of living human beings. By comparing the mitochondrial sequences of many people around the world and estimating the time of divergence, it was clear that human diversity could be largely grouped into two categories: Africans, and the rest. In a certain way, we are all Africans. Out of Africa is now widely accepted as the correct hypothesis. 

Prehistoric interspecies sex
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Although now there is a single hominid occupying the planet (us), there was a different situation 40kya. We know from the study of fossils that Neanderthals, a different branch from *Homo sapiens* within the tree of hominids, inhabited the caves of Europe and the Middle East. The remains of some Neanderthals were analyzed by Svante Pääbo, one of the pioneers (if not the founder) of ancient DNA, who was able to produce sequences covering the full genome and published them in 2010. The Neanderthal genome hid a big surprise: by comparing it with the genomes of different extant human populations, it was revealed that some segments of our own genome were clearly more similar than what we could expect by random chance to the Neanderthal genome. The only possible explanation was that at some point after the Out of Africa event (because the similarities do not occur with those human populations that remained in Africa), humans and Neanderthals had sex and we descent from these hybrid individuals.

Another big discovery came after sequencing a pinky finger bone found in the Denisova cave, in Siberia. From this bone alone it was possible to extract a high-quality full genome of a new hominid that wasn't known by other human bones (except a handful of teeth that were huge and suggested a herbivore diet). The genome of this new hominid (named Denisovan after the name of the cave where it was found) revealed two important surprises:

1.  Denisovans were more different from humans and Neanderthals than these two groups among them, indicating an early diverging population
2.  A comparison with the genomes of human populations revealed that New Guineans have inherited a significant amount (5-8%) of their DNA from Denisovans, which reveals the occurrence of a different hybridization event.

From the size of the fragments acquired by hybridization (the technical term is introgression), it is possible to determine that the mating occurred more recently than the hybridization with Neanderthals (54-49 ky Neanderthals, 49-44kts Denisovans). 

A geographical puzzle
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If you think about these two hybridization events while looking at a map, you might be a bit surprised. First, humans left Africa and occupied the rest of the world: some people arrived in Asia, some others took a turn to Europe. Those going to Asia eventually arrived at the Bering strait and walked into America. But the populations leaving Africa and taking the "Asian road" never made it to Europe, where the sequenced Neanderthals were living. How is it that all humans on Earth (but some Africans) have inherited Neanderthal DNA? Or in the case of New Guineans, if the pinky bone was taken far north in Siberia, did humans take a huge detour before arriving in New Guinea? 

If you are confused about this, think that any fossil that we sequence (or by extension any fossil that we find), is very unlikely to be the direct ascendant of ourselves, they are just "other branches" in the tree. The Asian populations with Neanderthal segments descend from humans that probably mated with Neanderthals in the Middle East. The ancestral population of New Guineans mated not with the Denisovans that were sequenced but with a different population (potentially distantly related) that occupied a different geographical region: Reich calls them Australo-Denisovan. We are only beginning to understand the ancient diversity of humans.

Use metaphors carefully
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The Tree of Life is one of the most successful metaphors in modern science but it is one to be used carefully. At the scale of human evolution, we see that populations do not only diverge creating a branching pattern but they also intermix, which suggests that (at least at this level), a trellis could be a better metaphor for evolution. 

Although we accept Out of Africa as correct, it is true that our current view of human evolution is (up to some point) a synthesis between this hypothesis and the multiregional hypothesis. Ancient hominids interbred with our ancestors and left significant traces of their genomes in ours. In a certain way, Neanderthals or Denisovans didn't become extinct; we are their descendants.

How we got where we are today
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Reich discusses different large geographical areas (Europe, India, South East Asia, America and Africa) and the things we know about them. The full picture of human migrations and admixtures is very complex and our knowledge varies drastically from region to region. For example, Europe is the best-known place, due to a long history of academic research in the field. Other places lag behind for different reasons: in America for instance, the study of ancient DNA is very often hindered by the Native American populations, who control the use of human remains belonging to their tribes. In China, it is forbidden to transport human remains outside the country for research, which prevents access to first-class ancient DNA laboratories (though I guess this situation might have changed by now). 

In spite of these differences, it is possible to detect some general patterns across the world: we see very successful peoples (who often master new technology, such as agriculture, the wheel or a new type of boat) expanding over large areas of land and displacing the previous populations. These populations carry with them their culture and their language, which become prevalent over large areas. For instance, the Austronesian languages (Indonesian, Polynesian, Malagasy) can be found across a large geographical area (from Madagascar to Eastern Island) due to the success of a population of expert sailors, which expanded and conquered a vast area over thousands of years. These episodes of population expansion and spread obviously entail the dissemination of culture over vast areas of territory. However, we should be careful interpreting any widespread culture as the result of migration: For instance, the Bell-Beaker culture, characterized by a certain type of pottery, spread in Europe for one thousand years, starting in 2800 BC. Although archaeologists were tempted to explain this expansion by a large wave of migration, the sequencing of ancient DNA has shown that in this specific case the spread of the Bell-Beaker culture was due to the movement of ideas and not to migration.

How Europeans got where they are today
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In this summary, I will focus exclusively on Europe, the region that scientists understand best. Reich does explain other regions in his book, but I felt that covering all would make this summary a bit too long. 

Simplifying a lot, we can divide the history of Europe into several phases

1.  A group of humans from the OoA event arrive in Europe. They are hunter-gatherers. They spread all over, possibly displacing the Neanderthals living at the time: the arrival dates of these humans pretty much overlap with the last Neanderthals living there. The hunter-gatherer population are not static: they evolve, migrate and mix over thousands of years. The mutation that confers blue eyes to some people appears in this phase. The skin color of these hunter-gatherers was very dark.
2.  Agriculture develops in the Fertile Crescent. Farmers coming from there arrive in Europe and spread, sometimes mixing with the local population. These farmers had light skin and brown eyes.
3.  The Yamnaya spread into Europe. The Yamnaya were most likely responsible for Indo European languages (Russian, Spanish, French, English, Persian, Hindi, all these languages have a common origin) being spoken in so many different places today. They inhabited originally in the Steppes of Central Asia, where they learnt how to domestic horses, took the wheel and putting them together created the first war chariots. They also carried the mutation for blonde hair.

Most European populations nowadays are a mix of those three groups. A very interesting result is that most people in Europe inherited the Y chromosome from the Yamnaya. Since this is a chromosome that is inherited only by the paternal line (see also the third part here), this is telling us that the Yamnaya took women from the then-local population.

Some final takes
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Reich talks not only about ancient DNA but also about the implications of genetic studies. Genetics has been traditionally a very sensitive field, due to how certain individuals are willing to quickly misuse its results to justify their prejudices, which can go in one direction or the opposite. Reich explains that although the traditional definition of race is nonsense, there are important genetic differences corresponding to different human populations. Denying this will only pave the way for those criticizing the Academy as politically biased and uninterested in pursuing the truth. Reich also discusses in his book that genetics studies in ancient DNA are here to stay and they are fundamental to understanding human evolution. Ancient DNA will be as fundamental to archaeology and history as radiocarbon dating, if not more.

It is not easy to find a book such as “Who we are and how we got here”. As I said in the introduction, Reich does not shy away from explaining the complex-nuanced version of his field instead of a simplified version that could potentially be more accessible to a larger number of readers. Even if some of the chapters will become outdated in a few years (some of them already are), I know for certain that I will go back in the future to this book to refresh some of the most important ideas. In a few days, I am starting a few courses on Anthropology, and this book has been one of the major reasons why I decided to follow that path.

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Thanks a lot to Miranda for the corrections and the general feedback.

All errors are my own.