Why do we walk on two legs? Why are antibiotics becoming increasingly ineffective? Why do we need vitamin B in our diet? Why do people with serious disabilities continue to be born? Why are people in the tropics darker-skinned than those in temperate zones? Why do bats fly only at night? Why are flowers brightly coloured and patterned? Why do people get covid even after they are vaccinated? The detailed answers to those questions vary from case to case, but there is also a common answer: because of evolution. Evolution is the central organising principle behind living matter. That means all of us – viruses, microbes, fungi, plants and animals.

It is rightly said that nothing in biology makes sense except in the light of evolution. Just as physical laws account for fire, rain, earthquakes, motor cars, aeroplanes, artificial satellites and mobile phones, evolution accounts for the makeup and functioning of living organisms. Evolution explains how the huge variety of living creatures came into being. It is a history of life on earth, and not merely a descriptive history. It explains why particular features are characteristic of different forms of life. The explanations fall into three broad groups: those that depend on accidents of chance alone, those that depend on chance combined with reproductive advantage, and those that depend on chance combined with mutual advantage. Let me give examples of each. They are chosen deliberately to illustrate how an understanding of evolution clarifies aspects of human life.

Among the roughly 300 year-old Amish sect in the US, a significantly higher proportion have more fingers or toes than the usual five. Why? It so happened that very early, some Amish had a genetic condition which leads to extra digits. The number of Amish has always been small, and they largely married within themselves. Therefore, purely on account of chance (to which the small population size also contributed), extra digits occur in the Amish more frequently than in others. Next, consider the evolution of resistance to an antibiotic. It originates from a chance event, a mutation, in a single bacterium. In someone who is taking the antibiotic (and also in environmental waste that contains the antibiotic), resistance gives that bacterium and its descendants an advantage. They survive and reproduce more effectively than bacteria that are sensitive to the antibiotic. Eventually, resistant bacteria become widespread. In this case evolution has taken place because of chance and reproductive advantage. Finally, consider vitamin B. It is an essential nutrient, but our body cannot produce it. We depend on the bacteria which live in our gut to make vitamin B for us. In return, our metabolism provides the bacteria with the food that they need. The original association between the two species must have occurred by chance. Once it did, mutual benefit ensured its persistence.

As these examples show, evolution can take place in several ways. Among them, the second in our list has become famous. In it, evolution is the result of cumulative changes, each spreading within a population following a chance event that results in a slightly improved ability to reproduce. That way of evolving is known as natural selection. It was proposed simultaneously by Charles Darwin and Alfred Wallace in 1858. Popularly, it goes by the name ‘survival of the fittest’ – an expression that has given rise to so much confusion that biologists hardly ever use it. Sometimes natural selection is also called Darwinian evolution (most unfairly to Wallace). It remains the dominant explanation for evolution, especially when the outcome appears to be an adaptation. Adaptation is an evolutionary outcome which makes the members of a species respond optimally to the challenges posed by their surroundings, which include other living creatures too. Optimality is judged as an engineer would, and roughly means ‘as efficiently as possible’. Adaptive evolution leads to better swimmers, fliers, runners, or an improved ability to digest food, see, hear or smell, resist infections, and so on. Molecular biology illustrates the first evolutionary mechanism in our list – chance alone – dramatically. Chance is behind a large number of evolutionary changes in DNA and protein sequences that persist even though they are neither beneficial nor harmful. The third mechanism, in which chance and cooperation combine, makes us look at evolution in a new way. We now see that living creatures (including ourselves) are not isolated individuals. Rather, they are communities that include a huge number of microbial species that live on and within them in an arrangement that helps both sides – or one should say all sides. The emphasis on cooperation reflects a shift in perspective from simple-minded Darwinian evolution, which stresses competition.

Unexpected facets of evolution are emerging. The role of the external environment as a stimulus for evolutionary change is one. The role played by the mechanics of cells and tissues in guiding the development of form is another. Thanks to technical advances in molecular biology and computer analysis, the study of evolution is passing through an exciting phase today. We are learning with whom we share our roots and how long ago that was. The deep history of our own species, Homo sapiens, is acquiring shape. We know that our forefathers originated in Africa and, as hopeful migrants, went out from there to populate other continents. There are tantalising glimpses of the people whose descendants we present-day Indians are. They include the earliest humans to reach here from Africa as well as immigrants who arrived in diverse streams from the north-west, north-east, and south Asia. Two statements sum up what evolution tells us so far. First, life on earth is a spectacular example of unity and diversity, both of them based on common ancestry. Second, the diversity has come about through a combination of accidental events, the laws of physics, and natural selection. In the memorable words Darwin used at the end of The Origin of Species (a book that repays reading even today), there is grandeur in this view of life. The study of evolution is endlessly fascinating. Its goal is to explain the most striking aspects of living creatures, the sorts of things that attract a child’s wonder. At the same time, it throws light on the deepest doubts that have engaged humans. Who are we? Where did we come from?

However, there is a question that evolution seems ill-equipped to answer. Why on earth would anyone want to deny young Indians such richness, when it costs nothing? NCERT appears to have done just that by omitting the topic of evolution from its X standard text book. The reasons given for the removal that I have come across (it is too difficult, Darwin is a controversial figure) make no sense. It used to be thought that evolutionary biology was not utilitarian in the sense that mathematics, chemistry, computer science or engineering are. In this age of organ transplants across species and strategies to counter the impact of climate change, even that is not true. But there is a far more important point behind the teaching of evolution to children at several levels and in progressively increasing detail. An appreciation of evolution enriches the mind. To delete it from the syllabus is as foolish as not teaching a language, or poetry, or history (though I seem to recall an entrepreneur saying that teaching language was a waste of time). Being exposed to evolutionary thinking should be considered an essential element of human culture, on a par with being told about the planets, stars and galaxies and where they come from. One does not have to know about evolution to make a living. Also, cutting down on its teaching will not matter to those who study in private institutions that use their own curricula. They will find, or be directed to, sources of knowledge other than officially prescribed texts. On the other hand, the move will hit precisely those children whose needs should be addressed by a progressive educational policy, those who come from deprived backgrounds, are compelled to study in publicly funded schools, and retain the hope that education is liberating.

Vidyanand Nanjundiah used to be at the Indian Institute of Science and is now at the Centre for Human Genetics. He studies evolution, especially the evolution of social behaviour in microorganisms. He can be contacted at vidyan@alumni.iisc.ac.in. The views expressed in this piece are personal.