Sitting on a log, in a clearing by the banks of the River Matamata close to where it flows into the Amazon, Sara Bennett was encircled by her audience. This audience, too, sat on logs. We were composed of men and monkeys. The human contingent were my six travelling companions and I. The monkeys - well, they too had names, but I could no more name them than name the half-dozen different monkey species they came from.

The reddish-furred monkey in Dr Bennett's arms was a female howler monkey: this I did recognise - by the fearsome, echoing roar she made as she clung like an anxious child to her human matriarch. But as for the simian miscellany that sat solemly on logs, pretending to understand as Sara talked about her monkey rescue work, or clowned around, running up tree trunks, swinging from shrubs or playing with rocks and sticks, I cannot begin to identify them.

They ranged from something with a broken tail, the size of a squirrel, to a woolly monkey the size of a labrador, and two dark-coated creatures as big as children, their fur so long and shaggy that it fell over their eyes, while long-tailed monkeys, pale or dark, hardly bigger than cats, put on a spirited display of gymnastics. “Now stop showing off and settle down, Chimboshi,” said Sara to the most extravagant performer.

Dr Bennett runs a sanctuary for orphaned monkeys in the Amacayacu National Park in Colombia. The park is enormous - nearly 300,000 hectares - reaching from the banks of the Amazon deep into primary forest; but the modest Amazon port of Leticia is only a couple of hours downriver and Leticia (though unconnected by road) is a substantial little town; so the interface between Man and monkey brings its crop of casualties on the simian side.

“People in Leticia know I look after young monkeys that have been orphaned or wounded,” said Sara, “so they bring them here.” Her sanctuary is not caged - just a patch of forest around her small cabin - and the animals are free to come or go. A scientist whose work is now more in conservation than pure science, people admire and respect her.

Who could fail to? But awkward questions can be the most interesting, so I asked: “Obviously you're helping the monkeys in your care, but has your work any significance for the rest of the Amazon's monkey populations?” She was honest enough, and a sufficiently good scientist, not to pretend to any easy confidence in the answer.

Perhaps I should not try to impute motives, but it was my strong impression that Sara Bennett does not measure the good she is doing in strictly scientific terms; nor is any wider contribution she may make to South American monkey populations what mainly impels her to take in and care for these creatures. I think she just loves monkeys, and in particular her monkeys. She loves them as individuals; and they fascinate her - as, indeed, she fascinates them. This struck me as a wholly and self-evidently good thing, and in need of no further justification, even if that could be provided.

At which point I can imagine a sniffy response from some of the people I have met and talked to along the borderline between science, conservation and ecological campaigning. To a way of thinking common among their mindset, Sara's continuing involvement in the lives of her orphans would be seen as a problem. According to this view, Man should so far as possible stand away from “Nature”. Nature starts where the human domain ends, and the aim of serious environmental campaigners should be to withdraw so far as possible the hand of Man, and erase so far as possible the mark of Man, his stamp on the world. Man distorts. Man is bad; Nature is good; the distinction is clear, and the best among us should be on the side of Nature.

But what, then, of the isolated indigenous tribes in the Amazon part of the wild? Are they part of Nature? Environmental campaigners like to insist that such people will establish a “balance” with their environment, but what if they don't? No natural law says that an indigenous tribe may never multiply, beating the forest back. But we outside the forest almost seem to be defining indigenous tribes as part of Nature, not Man - an insult, properly considered.

The truth is that some environmentalists form the fundamentalist outriders for what, even among millions of the less zealous, amounts to a kind of religion, not a science, for it invests data with moral qualities unknown to science. To many, love of “Nature” is the flipside of distaste for Man, or an embarrassment - even shame - about being human.

At the heart of this religion sits a weird variation on the old, old story: the story of the Garden of Eden. In Genesis, God expels Man from the garden. But in the 21st-century version, Man is urged to expel himself; then declare the garden a national park.

Well, I'm not against national parks. Almost 10 per cent of Colombia is a national park. We should have more of them. And there may be places where we do wish to stop and freeze invasive environmental change; and species we do want to ring-fence and preserve from extinction, even self-inflicted. The precautionary principle, meanwhile - that we should be careful about changes that may get out of hand - is simply a matter of prudence, requiring no doctrine for its justification.

But make no mistake: this is not withdrawing from Nature. The very act of selectively extracting ourselves from chosen places, is an act - perhaps the ultimate act - of control. The Earth is our garden, our Eden. We can make new breeds, new plants. We can make lakes and level mountains. We can help to shape and tend our planet as no other species has, and the bits we choose to leave “wild” - like the classic English country garden - are part of the plan. Our plan. The plan we choose to implement.

Stewardship - control - is not an idea we can honestly duck. We must stop retreating into the metaphysical mists of a theory of division between Man and Nature, and cheerfully accept that we ourselves are “Nature”, and we're in charge: the top species. We can design this garden for succeeding generations, according to our human taste, because we love our own species.

We love monkeys too, and therefore we will have monkeys, lots of them, of every kind. And we will run orphanages for them. It gives us pleasure. So hats off to Dr Bennett and her audience in that forest clearing, all of us - including the humans: so much a part of Nature that one of the bigger monkeys sat down beside our companion Karl and, in a spirit of scientific curiosity, looked into his eyes and stroked his beard.

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