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Leap Into Our Branches

Paleogenetics is evolutionary sleuthing. The way DNA sequencing throws light back into the foggy paths of prehistory is explained lucidly by a practitioner.

Leap Into Our Branches
Leap Into Our Branches
Who We Are And How We Got Here: Ancient DNA And The New Science Of The Human Past
By David Reich
OUP | Pages: 368 | Rs. 495

In 2010, a team of paleo geneticists at the Max Planck Institute for Evolutionary Anthropology, led by biologist Svante Pääbo, published an ast­onishing result—they had mana­ged to sequence the Neanderthal genome and found that there had been interbreeding between Neanderthals and West Eurasian humans. In a sense, the field of paleogenetics had come of age. Paleogenetics, or the science of using genetics to study ancient hum­ans and other populations, relied heavily on advances in technology to extr­act and sequence genomes since the Human Genome project. This new book by David Reich, a member of this team, is a popular exposition of the revolutionary potential of paleogenetics to understand humanity’s origins and ‘histories’.

The workhorse of paleogenetics is the DNA molecule—a helix-shaped molecule which is the blueprint for life. It consists of arrangements of four bases denoted by the letters, A,T, G, and C whose ordering determines the coding of amino acids and hence the production of different proteins in a cell. During replication, sometimes there is an error in the copying of the bases and a wrong base is added to the new DNA strand. These errors or mutations are what form the basis on which natural selection operates. Int­erestingly, the rate at which mutations accumulate in a genome is constant over generations, allowing us to determine how long back two segments shared a common ancestor. What we have is a kind of biological stopwatch.

The DNA in a cell is of two kinds—nuc­lear DNA or nDNA which is what we usually refer to as our genetic code, and a second type of DNA found only in structures known as mitochondria, which are outside the cell nucleus. The mitochondrial DNA or mDNA  is much smaller than nDNA and much simpler to sequence. Also, mDNA is passed down from the mother and refl­ects a purely matrilineal heritage.

With advanced gene sequencing mac­hines, sequencing DNA from living peo­­ple can now be done rapidly, in a relatively straightforward manner. But doing the same from samples of ancient populations is extremely challenging. The DNA in ancient remains degrades with time, the sample gets contaminated by micro­bes and fungi and finally there is a chance of contamination from human handling.

Reich’s book makes it clear that though we all share our ancestry with other populations, there are real genetic differences on the average between members of different groups.

This book offers an overview of how the study of ancient DNA has radically transformed our view of prehistory. Around two million years ago, an archaic species of humans emerged in Africa—the ancestor of us Homo sapiens, and of at least two other populations: Neanderthals and Denisovans. Around 700,000 years ago, Neanderthals and modern humans separated while the Denisovans separated from Neanderthals later. The Denisovans inhabited the eastern part of the Eurasian continental mass while the Neanderthals were concentrated in Western Eurasia. Modern humans migra­ted out of Africa around 50,000 years ago.

Reich traces the origins of modern hum­ans, including the peoples of modern Europe, the Indian subcontinent, East Asia and native Americans. The determination of the lineages of these major population groups reads like a detective story, where scientists follow clues from anthropology, archaeology and linguistics to establish our origins with genetic evidence. Though the book is written in an easy style and technical aspects are explained lucidly, the case studies can be confusing. The terms for various populations and their chronologies are easy to mix up for the lay reader.

There are salient points from the case studies here. Firstly, there are no ‘pure’ populations. All of us share our ancestry with other populations. The multitude of interconnected populations has contributed to each of our genomes. This does not mean that repeated adm­ixtures have made population differences meaningless. In fact, populations are different; both in the average and in their variation. Scientific analysis shows real genetic differences on the average between members of different groups. Yet, accepting this truth does not mean that one needs to accept racism or sexism. As the author says, we accept larger differences among individuals within a population and aspire to treat them with equal respect. The differences across populations are on an average much smaller. There is no reason to treat them differently.

Genetics has revolutionised the study of our past. The future will throw up many more surprises. This book is ess­ential reading for anyone who wants to understand how we know what we know about our ancestors. And, as the subtitle suggests, in doing this, it sheds some light on who we are. In these days of increasing identity politics, it serves as a reminder of our shared heritage at the most fundamental level.

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