At first sight, two humans may seem to differ enormously. But looks can be misleading. Among animals, humans make up an unusual group. All humans belong to the same species (
Homo sapiens, meaning 'wise human'). Technically, this implies we can exchange our genes with each another. Biologically, it means any two humans are essentially the same.
Our closest relatives -- chimpanzees, gorillas and orangutans -- are also unusual in the same way we are. There are no more than one or two kinds of each (because it is now accepted that black-faced, 'pygmy' chimpanzees are a separate species, they have been given a new name, 'bonobo'). Taken together, the four share a very recent common ancestry. Comparisons of
dna and proteins, as well as anatomical and biochemical studies, prove this.
Just how close is the relationship? There are two ways to answer the question, both depending on what we know about evolution. The first way involves a conversion of time into distance. If you have to go very far back in time to find a common ancestor of two species, the two must be separated by a large evolutionary distance. Conversely, if the common ancestor lived more recently, the distance is smaller. This calculation is based on the principle that all living forms have branched from a smaller set of ancestral types.
Everything alive today had a common origin in the distant past -- about 400 crore, or 4000 million, years ago. But closer to the present, some of us still had the same ancestors, while others (say, a peacock or a python) branched off. Our most recent common ancestor with the chimpanzee and the bonobo lived about 5 million years ago; with the gorilla, about 7 million years ago; with the orangutan, about 15 million years ago; with the mouse, about 50 million years ago; with the shark, about 400 million years ago.
We can use these numbers to calculate the relative distances between any two living species. For example, if we take humans and mice to be a single unit of distance, or 100 per cent apart, the distance between humans and chimpanzees becomes 5 million divided by 50 million, or 10 per cent. Again, if the unit is the distance between humans and sharks, humans and chimpanzees are separated by a short distance of only 1 per cent .
The second way is to examine the extent of overlap between blocks or segments of
dna. Parents shuffle the blocks randomly and each transmits half a combination to a child. Because there are two parents, the child receives a full set of
dna and tends to have a lot of their
dna. Cousins will have similar
dna sequences, but not as similar as the
dna of brothers and sisters. That is because even though cousins have different parents, they share at least one set of grandparents. By extending this reasoning, we see that
dna similarity between any two individuals shows how recently they had a common ancestor, and therefore, how closely related they are.
As a species, humans share 98.8 per cent of
dna sequences with chimpanzees. This means one of two things. The first conclusion is favoured by most scientists. It says that the 1.2 per cent of different
dna is vital: it must be responsible for most of the important differences between the two species, for example the differences in the ability to use tools or language.
The other conclusion follows if you assume that for all practical purposes, 98.8 per cent and 99.9 per cent are the same. If this hypothesis is correct, the differences between us and chimpanzees may be not be genetic, but epigenetic (less to do with the
dna itself than with the way in which the
dna is organized and the way it functions). In a deep sense, we are really a variety of African ape.
The same
dna block-based comparison shows how closely individuals resemble each other. On the average, two human beings share 99.9 per cent of their
dna sequences. What about the remaining 0.1 per cent? Close examination shows this miniscule figure comes from differences among Africans and the rest. Equally, the same level of variation is found within Africans themselves. This fits in with the view that in a fundamental sense,
Homo sapiens consists of recent migrants from Africa. We are also all Africans.
Vidyanand Nanjundiah is a professor, department of molecular reproduction, development and genetics, Indian Institute of Sciences, Bangalore 
