‘Who knows? Maybe over there in the dark alleys of Islamic extremism, maybe some day that same energy will be turned towards environmentalism.”

This is Edward O Wilson, biologist, Harvard professor, grand 77-year-old Southern gentleman, world expert on ants, magnificent writer and — one doesn’t often get the chance to say this — one of the greatest men alive.

Wilson’s greatness lies in the intensity of his love. He loves life, all of it. No plant or animal species is, to him, expendable, not just because it might be good for us, but because it is good in itself.

“The creature at your feet,” he has written, “dismissed as a bug or a weed, is a creation in and of itself. It has a name, a million-year history, and a place in the world. Its genome adapts it to a special niche in an ecosystem. The ethical value substantiated by close examination of its biology is that the life forms around us are too old, too complex, and potentially too useful to be carelessly discarded.”

Or, as he puts it to me: “It is a spiritual matter.”

He has even given his love a quasi-scientific name, biophilia. But it is not just his love. He believes it is in all of us. We have and will only ever have only one home: Earth in all its living diversity. Evolution has embedded in us a longing to be a species surrounded and, indeed, infested by millions of others. We find peace amid nature. In every human culture, he points out, our gardens are more or less the same — miniature models of the trees, grasslands and water of the African savanna, the landscape which, for most of our evolutionary history, was all that humans knew.

Yet this life-lover finds himself living in the sixth great extinction of planetary history, comparable to the catastrophe that wiped out the dinosaurs 65m years ago. Human activity — through global warming, elimination of the rainforests, agriculture, pollution — is destroying the biodiversity of our world. Humanity is the one species capable of behaving unnaturally, of destroying its home. Future generations, he warns, will find themselves living in an era he calls the Eremozoic, the Age of Loneliness.

Wilson is, crucially, a bottom-up environmentalist. His eyesight was damaged in childhood by a fishing accident. As a result he finds it easier to study small rather than large organisms. Ants are his supreme speciality. Luckily, ants turn out to be amazingly intricate, globally successful creatures. The total weight of ants in the world, he points out, is greater than the total weight of humans.

It was from ants that he developed his primary scientific discipline — sociobiology, the study of the genetic determinants of behaviour. When he first published on this subject in 1975, he was excoriated by the left as the prophet of a new genetic determinism that would send women back to the kitchen and enslave supposedly inferior races. This was nonsense and, indeed, over time, the left relented. Now, in the form of “evolutionary psychology”, sociobiology has been generally accepted as one of the key scientific categories of our time. For that contribution alone, Wilson would be one of the giants of our era.

Wilson’s insight was simply that the complex society of ants must be somehow programmed in their genes since there are no textbooks of ant life. Since we are also evolved creatures, the same must be true of us, though our behaviour is complicated by the fact that we are reasoning, self-aware creatures that can decide not to do what our genes tell us.

Evolutionary psychology, the offspring of Wilson’s idea, is simply a way of finding the genetic framework of our behaviour beneath the cultural additions. We are, for example, plainly tribal creatures — probably because that was the best survival strategy on the African savanna.

But it is environmentalism that really puts him on the mountain top. His bottom-up emphasis distinguishes him from top-down environmentalists like his great British friend, James Lovelock, creator of the Gaia hypothesis that sees the Earth as a single organism. Lovelock studies planet-scale systems. But they come to the same terrifying conclusion — our time of plenty, of global dominance, perhaps even of our existence, is running out.

“Jim works down,” says Wilson, “and gets down to the level of the species. My approach tends to be bottom-up. My background is in systematics and biology at the species level. But when I talk about species, I’m talking about the eco-system as well. Gaia can be thought of as a system of ecosystems. We’re really talking about the same thing in the end.”