Thirty years ago this month, the students were tearing up the cobblestones of Paris and hurling them at the riot police in the name of workers' freedom and free love.

Twenty years ago this summer, the youth of Britain was rallying against the Nazis. This decade opened with the fireworks of the poll tax riot. And with that final outburst, the political fire of young Britain was spent. Passion was replaced by ecstasy; causes by special effects.

With one exception. The injustices suffered by other people might no longer awake moral indignation, but the exploitation of animals did. That this spark of righteousness persists is due in large part to one man, Peter Singer. The Australian philosopher's 1975 book Animal Liberation is the Das Kapital of the movement which took its name. For a certain sort of activist, animals are the ultimate proletariat, since they will never object to actions taken on their behalf, buy their council houses or cross picket lines, although some of them are willing to work for peanuts. At a more general level, animal sympathisers are open to the charge that they want to believe that animals are people.

Peter Singer has in fact argued that great apes should be considered "persons", with legal and moral rights equal to those of humans. He is anything but sentimental, though. His philosophy is a variety of utilitarianism, the doctrine which judges actions by their results, and identifies morality as the pursuit of the greatest good for the greatest number. Its appeal, to borrow the title of one of Singer's books, is as a system of 'practical ethics'. It stands opposed to systems of ethics which take their cue from conscience or from God. Darwin, on the other hand, has been seen as a useful partner of utilitarianism by a number of thinkers, including Herbert Spencer, the founder of Social Darwinism. In some quarters, Singer is regarded as a descendant of Spencer or worse. A few years ago, protesters in Germany attacked Singer - physically, on one occasion, smashing his glasses - because he favours euthanasia in certain circumstances, such as those of very severely handicapped babies. They were apparently unaware of his writings about animals, and the fact that three of his grandparents perished in the Nazi camps with which they associated him.

Although it's facile to judge a person's thought by the range of people it annoys, the German episode does illustrate that Singer is an independent thinker. Despite his advocacy of animal rights, he does not mistake animals for people. He recognises that people are animals. Darwinism therefore applies to us as well.

Nor is he a single-issue animal. Over the past fortnight, he has been in this country to speak at two events organised by the Darwin Centre at the London School of Economics. One was a seminar on 'Ethics and Nature'; the other a lecture entitled 'A Darwinian Left?'. This talk was a brief version of an essay which will be published next year by Weidenfeld & Nicolson, in its forthcoming Darwinism Today series.

Even if you think the Left is history, and you're feeling Darwinned out by newspaper comment-section articles, his rhetorical skills are seductive. Although 'philosopher' is his job title (his base is the Centre for Human Bioethics at Monash University in Victoria), Singer's success arises in large part from his skill as a publicist, in the Continental sense of a journalist who writes as an advocate of a cause. Developing themes aired in earlier works, particularly How Are We To Live? Ethics in an Age of Self-Interest (Oxford University Press), he coolly sets out a case implying that we need to make proper use of the 'D'-word, instead of regarding it merely as a fashionable name to drop, and take up the 'L'-word once again. If the Left has disintegrated, then it is necessary to reinvent it.

When Singer talks about the Left, he is not referring to what trade unionists meant when they invoked 'this great movement of ours', or Thigmoo for short. The essence of the left, in his vision, lies in the urge to reduce suffering in the world, and to side with the ridden instead of the riders. "If we shrug our shoulders at the avoidable suffering of the weak and the poor, of those who are getting exploited and ripped off, or who simply do not have enough to sustain life at a decent level, we are not of the left," he writes. "If we say that that is just the way the world is, and always will be, and there is nothing we can do about it, we are not part of the left."

As well as the rump that still sees itself as the Left with a capital L, he is talking about the left of centre, which of course is lot further to the right than it used to be, about liberal opinion, about the editorial stance of newspapers such as this one; and "a spectrum of ideas about achieving a better society". He is addressing people who may not see themselves as especially political, but whose worldviews are based on the assumption that culture explains humans, and nature explains all the other species on the planet.   

That he is preaching to this very broad church may take a while to become apparent, since 'A Darwinian Left?' talks a great deal about Marxism, and may therefore seem to be flogging a dead horse. "The Left has really been a Marxist left in many ways, even for those who did not think of themselves as being Marxist," he observes. "I wanted to get people to see that there were things in a Marxist view - even a small-m marxist view - that were incompatible with Darwinian thinking, and it was time for the Left to face up to that a bit more explicitly." In seeing human history as a function of social relations, he argues, Marx's vision implied that there is no fixed human nature.

This is probably something which has escaped many of those who are or have ever been part of the Left. A lot of his intended audience might say, 'Well, we were only ever social democrats, we never had any utopian illusions about the perfectibility of man’.

"They will still very likely have accepted the view that has been so prevalent in the social sciences, that human nature is malleable," Singer replies; "that it’s culture that determines things far more than biology, and that therefore we can change things through changing the culture." This view, he believes "does owe a lot to Marxist thinking". Until the convulsions of the 1980s, conventional wisdom in the social sciences had acknowledged this influence in the observation that 'we are all Marxists now'. The disintegration of the Left as a coherent body of thought has obscured the fact that in our assumptions about society and the individual, many of us are Marxists still.

To a degree, Singer considers, this is as it should be. "It is time for the Left to take seriously the fact that we have evolved from other animals," he declares in 'A Darwinian Left?', "and that we bear the evidence of this inheritance, not only in our anatomy and our DNA, but in what we want and how we are likely to try to get it." This means absorbing the insights of mathematically oriented theorists like Robert Axelrod and John Maynard Smith (a man whose own roots are firmly in the Left) about game theory, which have helped illuminate how co-operative behaviour can become established between self-interested individuals. It means considering the ideas of evolutionary psychologists like Leda Cosmides, who has argued that evolution has endowed the human mind with faculties designed to detect cheating.

A Darwinian Left would naturally adopt Singer's earlier project of animal liberation, seeing continuities rather than sharp distinctions between ourselves and other species. Singer points out that Darwin recognised the roots of morality to lie in the social instincts possessed by many species. A few years ago he edited Ethics, an Oxford Reader anthology. He notes in his essay that he was pleasantly surprised by the reaction to a section entitled 'Common Themes in Primate Ethics'. The dog of outrage did not bark - which was surprising, since the section juxtaposed Jane Goodall on chimpanzees with Jesus on turning the other cheek, and Robert Axelrod on why that doesn't work.

Singer began to incorporate evolutionary themes into his writing in 1979, when he wrote a book called The Expanding Circle. This was intended to refute Edward O. Wilson's arguments in his groundbreaking - and vilified - book Sociobiology: The New Synthesis. Singer believes that Wilson was guilty of confusing what is with what ought to be. But in the process of constructing his counter-argument, he became impressed with the power of the Darwinian paradigm. "It’s nonsense to think that you can deduce ethical premises from biology," he says. "On the other hand, philosophers have made a mistake in thinking that you can understand ethics without even looking at biological explanations for why we make some of the ethical judgements that we do; why we’re inclined to give such a strong place to kinship relations or obligations of reciprocity."

Despite his track record of nettle-grasping, Singer shows an inclination to defer some of the tough questions surrounding evolutionary politics. While he is ready to dally with evolutionary psychology, he ignores the presence of its evil twin, behaviour genetics. "The reasons why people are unemployed for long periods, or on welfare, are very varied," he writes, "and we know little about whether it has any kind of genetic basis." That may be true, but there are plenty of behaviour geneticists who say otherwise, and they have got the bit between their teeth.

His vision of a Darwinian Left is of a consciousness that accepts an unchanging core within human nature, which is happy with free markets but not global capitalism, which recognises that people are both naturally competitive and naturally ready to co-operate, given the right conditions. It remains true to the ideal of solidarity with the downtrodden, and, well, does what it can to create the conditions for co-operation. And that's probably about as far as it can go, until somebody works out how to integrate evolutionary models, based on individuals, with a theory of what the sociologist Emile Durkheim called 'social facts': customs, institutions, nations and other entities that are more than the sum of their individual parts.

Singer realises that his conclusion is "a bit depressing", so he ends his discussion with a note of hope for the distant future. Maybe one day, he suggested to the LSE audience, "those more utopian dreams of the Left may be in our power", if science becomes capable of "genetically engineering a better society". And with that brave new bombshell, Peter Singer was his old self again.