Who We Are And How We Got Here - Part One - Introduction
Genetic research promises to unlock the secrets of the past
Img. Source: CC BY-SA 3.0, https://commons.wikimedia.org/w/index.php?curid=227326
The vast majority of human history took place long before the written word was invented. This period encompasses the time from humans left first the African continent some 50,000 years ago and spread across the globe to the when the first civilizations began roughly 5,000 years ago. What happened during this vast period of time went unrecorded and is mostly lost to us, except for perhaps vaguely preserved stories and sagas passed down through the generations.
Archaeologists began excavating various sites around the world in order to fill in some of the blanks about this vast stretch of prehistory. However, due to the lack of any kind of writing, most of what they had to go on consisted of random bits of material culture. Various prehistoric cultures were described by archaeologists on the basis of where they were first documented or distinctive features of their artifacts.
The most common type of artifact was pottery, because fired clay pottery does not decay unlike organic materials like wood, cloth and leather. Pottery also tends to feature distinctive designs and motifs which can help differentiate various cultures. Other material indicators include styles of weapons and tools, metalworking and other technological innovations, and distinctive burial practices. Of course, how those cultures referred to themselves and to what degree they saw themselves as distinct from their neighbors will forever be unknown.
The sheer number of prehistoric cultures that have been subsequently identified by archaeologists all around the world is remarkable. For example, here is a list from Wikipedia of Neolithic cultures just in China alone: Neolithic Cultures of China. The right-hand bar on that page contains a list of Mesolithic and Neolithic cultures from other parts of the world including the Fertile Crescent, the Nile Valley, and Europe. Here is map of Neolithic cultures designated by archaeologists from 4000 to 3500 BCE in Europe:
For a long time debate raged in archaeological circles about whether the changes in material culture were caused by cultural diffusion, or whether they were caused by demographic replacement. Experts went back and forth about whether cultural changes were caused by the movement of peoples or by the spread of ideas. This debate was often summed up by the adage, "pots or peoples?"
This debate also fed into the study of various language families. It had been known for a long time that certain languages are related to one another and likely descended from a single common ancestor. For example, as far back as 1786 Sir William Jones demonstrated a relationship between ancient Greek and Sanskrit. This led to the subsequent discovery that most of the major languages of Europe were related to one degree or another and even had a distant relationship to languages spoken as far away as Northern India and the Iranian plateau.
This language family was subsequently dubbed Indo-European, and the existence of this language family implied a diffusion of peoples from an ancestral homeland at some unknown point in the distant past. Languages—like genes—accumulate minor variations over time. The rules for these changes are somewhat consistent and can be worked out by linguists. From these analyses, language “family trees” can be constructed. The "source" language of the Indo-European languages, for example, has been reconstructed using these rules and is known as proto-Indo-European.
Other major language families were later identified such as the Bantu languages of sub-Saharan Africa and the Austronesian languages which are spoken as far east as Madagascar and as far west as Hawaii. The spread of these language families implied a similar movement of peoples and cultures as well. But the question still remained as to what degree this process reflected the movements of peoples and to what extent it could be explained by other factors such as conquest, intermarriage and cultural assimilation, and how exactly it unfolded.
Recently a powerful new tool came on the scene offering the tantalizing possibility of definitively answering a lot of these persistent questions. That tool is genetic analysis based on ancient and modern DNA samples. Our newfound ability to "read" people's genomes offers the possibility of finally resolving many of these questions for the first time, promising to rewrite many chapters of the human story in the process.
These remarkable discoveries are the subject of David Reich's book, Who We Are and How We Got Here. The book does a terrific job of summarizing many of the major discoveries that have been made by Reich's lab at Harvard University and other laboratories around the world over the past decade or so. Other notable researchers in this field include Svante Pääbo of the Max Planck Institute for Evolutionary Anthropology, and Eske Willerslev of the University of Copenhagen and Cambridge University.
How ancient DNA is transforming our view of the past (BBC)
Improved techniques for the extraction and enrichment of ancient DNA along with increasingly powerful computational tools have made it possible for the first time to reconstruct the movements of peoples using biological material recovered from archaeological sites. Because each individual is unique and the differences between any two people in a given group can be larger than those between different groups, two types of DNA are especially valuable for this type of research.
The first of these is Mitochondrial DNA. Mitochondria are present in all human cells. Due to their origin as a separate species of bacteria, mitochondria have their own unique DNA signature which is not recombined, unlike our own individual DNA which is inherited from both parents and “reshuffled” every generation like a deck of cards. Mitochondrial DNA, by contrast, is inherited exclusively from mothers and passed directly to offspring allowing the construction of family trees based on shared mitochondrial DNA. These family trees have since been given letter and number designations.
The oldest mitochondrial lineage—known as L0—arose roughly 160,000 years ago and is found today in the southern part of the African continent. This discovery helped confirm the Out of Africa hypothesis and invalidated the competing Multiregional Hypothesis, which was the idea that modern humans are the result of interbreeding between isolated populations which evolved independently all over the world (although there is evidence of admixture with groups of archaic humans).
"Mitochondrial Eve" was the nickname given to the first female lineage to bear this signature from which all subsequent lineages branched off at some point. It should be noted that Mitochondrial Eve was not a specific individual person, but rather the term refers to the mitochondrial DNA signature possessed by the most recent common female ancestor of all humans alive today.
The other important portion of DNA for researchers is the Y-chromosome which is inherited exclusively from fathers. Females inherit two X chromosomes during embryo fertilization but males inherit an X chromosome from their mother and a Y chromosome from their father. By looking at the non-recombinant portion of the Y-chromosome (Y-chromosomal short tandem repeat sequences), family trees can also be constructed for paternal lineages as well.
This led to the notion of "Y-chromosomal Adam" to compliment “Mitochondrial Eve.” Again, it should be noted that—unlike the Biblical story—these are not specific individuals, nor were they a couple living together at the same point in the past. Y-chromosomal Adam is currently estimated to have lived between 200,000 and 300,000 years ago, roughly consistent with the emergence of anatomically modern humans in Africa.
Minor mutations in mitochondrial and Y-chromosomal DNA have allowed humans to be grouped into various haplogroups based on their shared maternal and paternal ancestry (as the image at the top of this post shows). Information about the various haplogroups can be found on the internet for the curious. Mitochondrial and Y-chromosomal DNA has allowed the journey of humankind to be reconstructed to some degree since humans’ departure from Africa between 70 and 50,000 years ago. We can also use genetic diversity to understand this process, as various “founder” populations break off from the “trunk” of the gene pool the farther you go from the African cradle.
Chemical reactions that take place between the environment and fossils break down genetic material over time, so it's not always possible to extract DNA even from fairly recent discoveries. Cooler and drier conditions (such as those found in caves) are the best; warm and wet conditions are the worst. A major breakthrough was the discovery that the petrous bone—a thickened region of the skull behind the ear—preserves a higher density of DNA than other parts of the skeleton allowing DNA to be retrieved even under suboptimal conditions. Another promising method is the extraction of ancient proteins from tooth enamel. This process allowed DNA to be extracted from the 800,000 year old Homo antecessor fossil discovered in Spain—the oldest DNA recovered so far.
Much of this research was made possible by the development of technology which allowed the enrichment of DNA samples and the conversion of of millions of bits of DNA into sequenceable parts that could be efficiently analyzed. These techniques focused on parts of the genome called Single Nucleaotide Polymorphisms (SNPs), which are nucleotide changes (mutations) that occur at specific positions ("loci") in the genome.
SNPs ('snips') are the most common type of genetic variation between individuals, and slight mutations in SNPs can be used to group people together and assess the degree of relatedness between different populations. Using these tools, researchers developed statistical methods to assess the degree of relatedness between genetic samples from different groups—methods such as the Three Population Test and the Four Population Test, which measure correlations in allele frequencies among sets of two, three, or four populations. Reich describes one such technique:
A SNP microarray contains hundreds of thousands of microscopic pixels, each of which is covered by artificially synthesized stretches of DNA from the places in the genome that scientists have chosen to analyze. When a DNA sample is washed over the microarray, the fragments that overlap the artificial DNA sequences bind tightly, and the fragments that do not are washed away.
Based on the relative intensity of binding to these bait sequences, a camera that detects fluorescent light can determine which possible genetic types a person carries in his or her genome. The SNP microarray that we analyzed was able to study many hundred of thousands of positions in the genome that harbor a mutation carried by some people but not others. By studying these positions, it is possible to determine which people are most closely related to which others.
The technique is much less expensive than sequencing a whole human genome since it zeroes in on points of interest—those that tend to differ among people and thus provide the greater density of information about population history. (pp. 131-132)
Of course, it should be noted that genetic evidence is one additional source of information—it is not a replacement for other complimentary methods of investigation such as linguistics or archaeology. The genetic evidence still needs to be evaluated against the historical, linguistic, and material evidence when reconstructing human migrations and cultural/technological diffusion. Each type of evidence provides an important piece of the puzzle.
What the studies ultimately revealed was that most current human populations are the results of profound mixing that occurred in the distant past due to extensive migration of peoples over many thousands of years. As a result, what they've also demonstrated is that ancient human populations were far more mobile than we previously believed. One fascinating result of genetic analysis is the discovery of various ghost populations: genetically distinct groups of people who existed at some point the past but no longer exist in any coherent form today due to migration and admixture.
Adam Rutherford provides a good summary of the results of this research in A Brief History of Everyone Who Ever Lived (pp. 74-75; emphasis mine).
There was a time, not that long ago, when we assumed that the history of humankind was simply a series of what evolutionary biologists call "founder events"—small tribes moving further from the motherland of Africa, setting up camp, growing, and then budding off to form new small tribes who would repeat the process in new uncharted territory.
Early genetic studies (meaning "in the last decade," such is the ridiculous pace of this science) indicated that this model may well have been accurate, as small samples of chunks of DNA seemed to cluster together. You would also expect to see a decline in genetic variation the further we moved from Africa, which is broadly what we see.
The latest analyses incorporate the fact that that current residents of geographical area are not necessarily very good representatives of the residents of the deep past. This is obvious if we look at areas that have been subject to migration by modern Europeans. The majority of peoples of Australia or North America today are from Europe in the last 500 years, and so their genomes are not representative of the indigenous people who were there first. But this impermanence is also not necessarily true for much older populations of humans.
The assumption, for example, that the Siberian farmers are going to be most similar to the first settlers of the American continent because of their proximity to the Bering Land bridge 15,000 years ago (now the Bering Straits) is not correct. In fact, digging up old bones and wheedling out their DNA shows that today's Siberians are more like East Asians, but ancient Siberians were more like Native Americans, mixed in with some northern Eurasian.
All this means is that we made assumptions about patterns of migration that were much more linear and spread like ripples, rather than the picture that has emerged in the last couple of years, which says that we moved in all directions all the time, and laid our hats and flowed our genes in a matted crisscross, instead of a nice clean radiation. Oceans and mountains are good barriers to gene flow, but on big open continents the horizon is the limit...
I see several overarching patterns that can be detected from this research.
One is that the spread of languages and material culture often does roughly correlate with the spread of peoples. That is, demographic replacement does indeed explain quite a lot of the changes seen in the archaeological record, as well as the extent of many language families today. For example, the spread of farming does appear to have been caused by the expansion of farming peoples rather than the widespread adoption of farming by hunter-gatherers. That’s a significant finding.
This is not true in every case, however, and there are instances where material culture appears to have spread via diffusion rather than assimilation and replacement. For example, in the far north of Europe, certain farming techniques do appear to have been at least partially adopted by hunter-gatherer groups there. Some of the spread of the Bell Beaker culture in Europe was also due to cultural diffusion, for example.
The other thing to note is the expansion of food producing peoples. The populations which expanded the most over the millennia were apparently those who adopted highly effective food production techniques—whether those were rice cultivation, cereal cultivation, mixed farming techniques, or herding and animal husbandry. The adoption of these food production techniques allowed these populations to grow and expand much more rapidly than their neighbors according to both the linguistic and genetic evidence. The descendants of these food-producing groups demographically swamped and assimilated the surrounding groups which did not adopt these techniques, explaining patterns we see today. This helps explain why farming and horticulture displaced all other modes of life such as hunting and gathering around the world in all but the most remote and marginal locations despite its disadvantages.
I'm going to deliberately skip over Part 1 of the book, which describes the discovery that prehistoric humans interbred with a number of different species of archaic humans such as Neanderthals and Denisovans. I think this fact is pretty widely known by now, and has been extensively documented in the popular media.
It is notable, however, that some of this interbreeding took place remarkably recently, including possibly in the last 15,000 years among the people of New Guinea. More recently, additional ghost populations have been discovered using these techniques for whom there is yet no clear fossil evidence. For example, it was recently discovered that West Africans interbred with an unknown population of archaic humans who lived around half a million years ago (a discovery too recent to be included in the book):
Scientists have found evidence for a mysterious “ghost population” of ancient humans that lived in Africa about half a million years ago and whose genes live on in people today. Traces of the unknown ancestor emerged when researchers analysed genomes from west African populations and found that up to a fifth of their DNA appeared to have come from the missing relatives. Geneticists suspect that the ancestors of modern west Africans interbred with the yet-to-be-discovered archaic humans tens of thousands of years ago, much as ancient Europeans once mated with Neanderthals.
Scientists find evidence of 'ghost population' of ancient humans (The Guardian)
I’m going to jump right into part two, which describes the movements of various ancient peoples in the creation of modern human groups on multiple continents. This field is so recent and so fast-paced that many new and groundbreaking studies have come out in recent years since the book was published in 2018. Research studies take time, effort, and funding, and new developments are continually emerging.
CHAPTERS:
'The Dawn of Everything' by David Graeber & David Wengrow, essential reading on the subject.