In 1968, six years after Rachel Carson published Silent Spring - the book regarded as marking the beginning of modern environmental consciousness - a young American entomology professor at Stanford University, California, published The Population Bomb. The tenor of Paul Ehrlich's book echoed the revolutionary sensibility and pervasive anxiety of the time. In it, Ehrlich and his wife, Anne, presented a neo-Malthusian scenario of imminent population explosion and ensuing disaster. "The battle to feed all of humanity is over," the Ehrlichs warned. "In the 1970s and 1980s, hundreds of millions of people will starve to death in spite of any crash programmes embarked upon now. At this late date, nothing can prevent a substantial increase in the world death rate..."

Not surprisingly, the second part of his message - that society must find ways to limit population growth - drew howls of protest. The left saw it as immoral, and feared that the right would use the idea of overpopulation to promote only the right kind of social or ethnic bloodlines. The right worried that population control might limit the rights of individuals. And virtually every one objected to the discussion of human reproduction as a condition of food and habitat as if discussing, say, a population of fruit flies.

Forty years on, the message from Ehrlich, now 76 and the Bing professor of population studies in the department of biological sciences at Stanford, has barely mellowed. He and his wife have just published a new book, The Dominant Animal, the central theme of which is how one species, Homo sapiens, has become so powerful that it can significantly undermine the Earth's ability to support much of life.

It is undeniably timely as we lurch from one grim realisation to another: a climate crisis, then an energy crisis, now a food crisis. And underlying them all is the issue of population. When Ehrlich wrote The Population Bomb, there were 3.5 billion people on Earth; there are now 6.7 billion. "The connections are so obvious it's appalling that they're not made," he says. "Each person we add now disproportionately impacts on the environment and life-support systems of the planet."

There is a growing sense in the environmental movement that population will again emerge as a central component of the debate on global warming. But it's a discussion that's open to distortion on one side by fringe groups who use the issue as cover for positions on race and immigration, and on another by superstitious thinking that technology will arrive to support and improve living standards for ever greater numbers of people, or that some kind of natural phenomenon - such as a disease, perhaps with a moral or spiritual component - will take the problem out of our hands entirely.

Ehrlich says: "There was a period when people saw the connections - connections that are often quite complex. Obviously, if the US still had the 140 million people we had at the end of the second world war we wouldn't be dependent on foreign oil, and we'd be emitting far less carbon dioxide in the atmosphere."

He believes that studying our cultural evolution - "where we came from, how we came to dominate the planet, and what that dominance means for our future" - is key to opening a more realistic discussion. "It's absolutely incomprehensible to me that we're in a [US] presidential campaign and no one is discussing any of the issues in The Dominant Animal, issues that are the concern of the majority of the scientific community."

For Ehrlich, though, the critical scalding he received for The Population Bomb and the problematic timeline of its central prediction forces him frequently to revisit and defend his ideas. Except for some developing countries, the globe was not racked by food shortages through the 1970s because advances in farming and technology were able to sustain larger populations.

In 1968, environmental studies was still fringe science. The intervening years have seen not only a boom in the field, but also in the variety and breadth of the issues at hand. "When I wrote The Population Bomb, I knew there would be a problem with climate, but I thought I'd be dead by the time we started to worry," Ehrlich says. "One thing after another has come up - the biodiversity crisis, ozone depletion ... all of which means I have to know more about more things."

Yet the issue of overpopulation and its equally thorny partner, overconsumption, remain near the centre of Ehrlich's study. Reiterating what environmental scientist James Lovelock stated recently, Ehrlich says: "We have grown in number to the point where our presence is perceptibly disabling the planet like a disease." No longer is it clear that technology, so often cited as means of maintaining growth, isn't ecologically counterproductive and fostering a population bubble that must sooner or later burst.

The charm and the curse of the population debate is that one must inevitably return to the subject of fruit flies. When a female finds a pile of rotting bananas, she lays her eggs and the population explodes. When the bananas are used up, the population crashes. Some females find another pile of fruit, and the process starts over. "Our problem is we only have one pile of bananas," Ehrlich says.

Entrenched

The issue of consumption, Ehrlich believes, may be more thorny even than population. So entrenched is the culture of consumption, that debate in the US tends reflexively to skip over the question of curbing domestic energy use and carbon emissions to the question of how to curb growing Indian and Chinese pollution. "Over 50 or 60 years, we turned the US from a country for people to a country for cars," Ehrlich argues. "We should be spending the next 50 years reversing that."

Part of an effective effort, Ehrlich holds, would be to add what economists call the "externalities" to the cost of energy. With the price of petrol reaching over $4 (£2) a gallon and Americans rapidly rethinking their love of huge cars, the price still does not reflect its true cost. For example, Ehrlich says, "we don't pay our share of the US military budget that goes to keeping the flow going, and we don't pay for the treatment for cancers caused by the particulates from burning fossil fuels. We don't pay the full costs of our behaviours."

There is cause for guarded optimism. He says: "If you look at it historically, the rise of environmental consciousness has been extremely rapid. We're only 40 years into it. The trouble is, the environment has been going downhill far faster."

· The Dominant Animal: Human Evolution and the Environment, by Paul and Anne Ehrlich, is published by Island Press.