After I read and reviewed Who We Are and How We Got Here: Ancient DNA and the New Science of the Human Past, I thought I knew about the changes to the story of human evolution based on studies of DNA. And given that Ancestors in Our Genome was published a few years before that book, I was curious what it could add to what I had been reading so far. As it turns out, a lot. As with my previous review, I should preface this one with the same warning that things are about to get complicated…
Author Eugene E. Harris used to be an anthropologist of the old school, those that thought that the study of the anatomy of bones and fossils was the best instrument to understand our evolutionary past. With technology to study DNA still being relatively primitive, its power to reveal details about human history was somewhat limited until well into the 1990s. But Harris has been at the forefront of technological progress and has experienced first-hand what the study of genetic material can nowadays reveal.
Ancestors in Our Genome fairly quickly becomes technical as it tackles the first big topic in the book: which great ape is our closest relative, the gorilla or the chimpanzee (and their close cousin the bonobo)? The reason the book becomes technical is that it has to explain the difference between species trees and gene trees. The former is the kind of family tree you will be familiar with: a branching diagram that shows how species are related. The latter is similar, but its resolution is far more fine-grained. Rather than showing relatedness based on evolving populations, it shows relatedness based on genes. And as the genome (i.e. the whole of an organism’s genetic material) consists of many genes (some 21,000 in humans), you can make many such gene trees for a single species. And these gene trees do not necessarily agree with each other, something that initially had researchers flummoxed as they started using this tool.
If all this sounds complicated, well, that is because it is. Luckily, Harris includes many helpful illustrations to make his point, but this section will require your attention. It also sets the tone for the rest of the book. Genomics (the study of genomes) is a powerful tool, but it does away with simplistic explanations.
“Genomics […] is a powerful tool, but it does away with simplistic explanations”
Harris proceeds to describe how genomics has revealed traces of a population crash in our distant past (without invoking the controversial topic of catastrophic volcanic eruptions such as discussed in my review of When Humans Nearly Vanished: The Catastrophic Explosion of the Toba Volcano), talking about effective population sizes and the relative contributions of random genetic drift and positive and negative selection. Another technical topic that benefits from helpful illustrations.
A good example of genomics revealing a complicated story is the question of what genetic changes are responsible for such human traits as increased brain size, bipedalism, hand dexterity, language, etc. If you were still holding out hopes for researchers turning up a gene for, for example, increased brain size, I have to disappoint you. We share the same genes with many animals, and for a trait such as brain size and complexity it is not the absence or presence of genes that makes the difference, but rather how these genes are regulated by other genes. You might know that in humans the 21,000 genes that make us supposedly us make up only 1.5% of the approximately 3 billion letters of our DNA. The rest was initially branded as genetic dark matter or junk DNA (see Junk DNA: A Journey Through the Dark Matter of the Genome). It turns out that at least part of this junk DNA contains regulatory DNA regions that, much like the dimmer-switch on a lamp, can up- or down-regulate genes (remembering that genes code for proteins, which are the actual biological workhorses in a cell). And that is part of what is responsible for above-mentioned human traits.
“The time for simplistic just-so stories is well and truly over.”
And, as also highlighted in my last review of Blueprint: How DNA Makes Us Who We Are, many traits are not caused by one or a few genes of large effect, but are polygenic, i.e. influenced by many genes each having a small cumulative effect. Another complication is that the use of genomics to study human origins was initially done by analysing the Y-chromosome (inherited exclusively via the father) or mitochondrial DNA (exclusively inherited via the mother). The reason was that both these are small stretches of DNA with few genes, so they were manageable and inexpensive at a time when analysing genetic material was expensive and labour-intensive. But those days are gone and we can now routinely and cheaply analyse all genetic material of an individual. Harris describes several projects that are collecting thousands of genomes of representative groups of the human species. And analysing this material, which is arguably more comprehensive, is frequently telling a different story than previous analyses based on only mitochondrial or Y-chromosomal DNA.
Finally, there is the topic of ancient DNA, the analysis of DNA recovered from archaeological remains. Harris discusses what it has revealed so far, including Pääbo’s work that showed that human DNA contains small amounts of Neanderthal DNA, suggesting we mated with each other (see also Neanderthal Man: In Search of Lost Genomes). But far more, he shares the excitement of the many new questions that are thrown up by this technique.
In the end, Ancestors in Our Genome is a fairly technical work, but it has to be to do justice to the complexity of the topic. The time for simplistic just-so stories is well and truly over. Where Reich’s Who We Are and How We Got Here talked predominantly about ancient DNA revealing how our ancestors migrated and mixed with other groups, this book casts a wider net. What traces of past episodes of natural selection linger in our genes? How do we relate to our primate cousins? And especially, what new light can genomics shed on age-old questions that still keep anthropologists up at night? Harris does this with verve, answering some questions and throwing out many new ones. So, sure, pick up Reich’s work, but certainly read this book as well.
Disclosure: The publisher provided a review copy of this book. The opinion expressed here is my own, however.
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