two events separated by one hundred years mark scientific revolutions that symbolise, more than anything else, the web of life. The first event was the publication of Charles Darwin's
The Origin of Species in 1859. In this book Darwin pointed out that life on earth was a product of evolution.
The book advances a theory for how evolution takes place. This theory is known as natural selection. According to it, some organisms leave behind more offspring than others because they have different heritable traits. Such traits eventually spread through a population.
Natural selection was immediately hailed as the correct explanation for evolution. But it had gaps. Darwin was unable to explain what causes variation, what makes one individual differ from another. He was unable to explain the mechanism of heredity -- what makes children resemble their parents.
It required another revolution for the gaps to be filled. That revolution came with the discovery of the double helical structure of
dna by James Watson and Francis Crick in 1953. There is a curious parallel between the achievement of Watson and Crick, and that of Darwin. Even before Watson and Crick made their discovery, there was evidence to show that
dna and the carriers of heredity -- genes -- were very likely the same. However, a model of the structure of
dna at once showed how, via the genetic code, hereditary traits could be embodied in
dna. Just as the hypothesis of natural selection convinced people that evolution must be true, the explicit model for
dna structure made people see that
dna had to be the hereditary substance.
What does
dna do, how is it linked to evolution and what makes it crucial to the web of life?
dna contains coded information for making proteins, which are the essence of what we are. Our tissues are made of proteins; we move, see, smell, hear and think with the help of proteins; we digest food because of proteins; and our tissues get nutrients thanks to proteins. We have about 50,000 different kinds of proteins. Each is a string of smaller molecular units called amino acids.
dna is also a string of molecules, known as bases that are combined with a sugar and a phosphate. Four kinds of bases occur in
dna and 20 kinds of amino acids are found in proteins. The genetic code enables a particular amino acid to be made for every three bases of
dna. In addition,
dna can copy itself via a chemical reaction; this is why children resemble parents.
Variation, the essential requirement of evolution, depends on differences in
dna.
dna bases keep changing because mistakes can occur in chemical reactions. As a result, the
dna molecules that encode the same protein may slightly vary in different individuals. Therefore, they may have different varieties of the same protein. Natural selection will result if some
dna molecules encode proteins that are better than the proteins encoded by other
dna molecules. An enormous variety of living forms is possible because a huge number of different answers are possible to the question of what it means to be 'better'.
Every creature that is alive today -- every human, chimpanzee, elephant, peacock, snake, frog, fish, fly, worm, bacterium,
neem tree, coconut and grass -- has two things in common. One, they all carry versions of the same
dnas. Two, the code that they use for converting the information in
dna into proteins is also the same. It is as if there were a large number of human groups, each with a language made up of words that sounded similar, with their meanings the same, and with the words being strung together into sentences using identical rules of grammar. One would guess that the languages are derived from a common source.
This is what our
dna tells us. We must all be descendants of the same ancestors. The living world is linked by 'relationships', with some of us being more closely related with each other. The closer the relationship, the more recent the common ancestor is. All humans have a common African ancestry dating back to approximately 150,000 years. Our last common ancestor with chimpanzees lived about five million years ago; with mice, 50 million years ago; and with earthworms, 500 million years ago. In more than one sense, we are all part of a big world.
Vidyanand Nanjundiah is a professor, department of molecular reproduction, development and genetics, Indian Institute of Sciences, Bangalore 
