I love zoos. There is one less than ten miles from where I live, and I always swear that I will go to it far more often than I do. At the same time, I must admit that I find a visit to the monkey house is a sobering experience. We stare in through the bars. Looking right back at us with wise, knowing eyes is someone roughly our shape and size, standing like us on two feet. Maybe he is peeling a banana with hands just like our own, or maybe he points out of the cage and is apparently laughing at us. He bears an uncanny resemblance to old Uncle Fred. At times like this, we need to look twice to make sure who is on the right side of the bars.

It is easy to believe that of all the creatures in the animal kingdom, the chimps, gorillas, and orangutans are nearer to humans than any other. The question is, how close?

Half a century ago, we could offer only limited answers. Humans, chimps, and gorillas are different species, because fertile inter-breeding is always stated to be impossible. (Which makes me wonder. Was it tried, and if so by whom? I don't even like to ask.) But fifty years ago, any statements of similarities and differences had to be based on the comparison of muscle and bone structure and general anatomy.

Now we have new tools for the comparison of species. The genetic code that defines a gorilla, chimp, or any other animal is contained in the DNA, that gigantic long molecule normally organized into a number of strands called chromosomes. Given a single cell from a chimp and a cell from a human, we can take the DNA strands and do a point-by-point comparison: the structure is the same here, different there. The extent to which the two DNA samples are the same is a good measure of the closeness of the two species.

This analysis has been performed, and the results are breathtaking. Humans and chimps share more than 98 percent of their DNA. Each of us is, in an explicit and meaningful way, less than two percent away from being a chimp.

The same exercise, carried out with DNA from orangutans, shows that humans are rather less closely related to them. I would argue that this agrees with our intuitive ideas. The orangutan looks a little bit farther from us than the chimp, who is uncannily like us in both appearance and behavior.

But why stop there? How close are we to elephants, whales, cats, camels, ducks, snails, and wasps?

The same kind of DNA comparisons can be used to establish "measures of similarity" between different creatures. This in turn allows us to construct a modern version of the "tree of life" which you could find in old science textbooks. (Humans, rightly or wrongly, were always shown at the top.) The analysis confirms most of our intuitive ideas. The differences between our DNA and those of other creatures increases steadily as the species become more obviously "different" from us in form and function. We are more like every other mammal than we are like any bird, and we are more like every bird than we are like any mollusk or insect.

Sometimes, however, we (or at least I) don't have much intuition to go on. Is a camel, say, more "different" from a deer than it is from a pig? And what about a blue whale, which as a sea-going mammal can grow to a hundred feet long and close to two hundred tons in weight. You might feel that such a creature is so different from any land animal, it must have diverged from a common ancestor in the very remote past.

DNA analysis answers these questions and the results are, to me at any rate, surprising. Whales and dolphins are close relatives of each other, as you would expect from their form and lifestyle. They are also, however, rather close to the hippopotamus, apparently finding hippo legs more of a hindrance than a help when they took to the seas. Cows and deer are closely related to each other, but less close to whales and hippos. Whales and hippos are, so to speak, first cousins, while cows and deer are second cousins to both, and pigs perhaps more like third cousins. Camels, whose ancestors diverged longer ago from all these other animals, are outsiders who should not be invited to family get-togethers at all.

In addition to the DNA within the cell nucleus, tiny within-cell structures known as mitochondria have their own DNA and reproduce separately. The mitochondria are believed to be bacteria that long ago formed a symbiosis with animals, and now we could not survive for a moment without them. Combining information from nuclear and mitochondrial DNA, we can estimate how long ago different species diverged from each other. For example, we can date the time when humans and chimps had a common ancestor. That time is about five million years ago. Humans and gorillas diverged at much the same time, as did chimps and gorillas from each other. We and the orangutans parted ancestral company farther in the past, about twelve to fifteen million years ago.

Five million years may sound like a long time, but there has been life on Earth for more than three and a half billion years. We and the great apes separated very recently on the biological time scale.

We really are close cousins, more so than the whales and hippos are to each other. No wonder that we and our ape relatives feel an odd sense of family recognition when we meet.

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