Books: Where to park your sabre-toothed Jaguar

Blood Rites: Origins and History of the Passions of War by Barbara Ehrenreich, Virago pounds 18.99

By The Time I finished reading Blood Rites, the book and I were in quite different states. The margins of my copy were left crawling with scribbled snorts and rhetorical question marks, while I was left with that warm glow that comes from feeling that the time one has spent with a book was not only pleasant, but intellectually profitable. It's not uncommon to enjoy disagreeing with an author whose world view offends one's own; much rarer to disagree thoroughly without irritation. Ehrenreich engaged my sympathy and sustained it, even though I think she is fundamentally wrong.

The question she poses is a very good one. She asks not why humans kill, but why they appear compelled to ritualise killing and make it sacred. From the obsidian knives and pulsing hearts of Aztec sacrifice to the intercessionary pleas made by living Japanese people to those who died in war, and thereby were deified, homicide is a profoundly spiritual matter.

Ehrenreich's survey extends to profane matters too, such as the antagonism between knights and foot-soldiers, and the idea that a standing army demands a modern state to maintain it, rather than the other way around. Her range of historical reference guarantees that all except the most erudite will learn something new and surprising here. But her ambitions reach much further. Admirably, she wants to integrate history with prehistory; recorded history with evolutionary history. Earlier this year, Ehrenreich co-wrote a trenchant article for The Nation which pointed out that in denying that biology can offer any valid perspectives on human behaviour, scholarly critics of science are guilty of exactly the same charge of essentialism that they bring against their targets. It is just as essentialist to say that we are radically non-biological as it is to say that deep down we are animals.

Such a statement shouldn't be worthy of note. It should be taken for granted. But in the current climate, it requires an effort - and sometimes a degree of daring - to rediscover the 19th-century vision of a single stream in which all the human sciences flow together. Unless that vision is generally embraced, we may look forward to a indefinite trench war in which "biological determinists" and "postmodernists" aim loud but ineffectual barrages against each other's straw men.

Moving towards a single but inclusive human science requires more than overcoming disciplinary rivalry or prejudice against rationalism. It needs models that demonstrate the usefulness of an integrated approach, and which are persuasive. Evolutionary accounts are routinely dismissed as "just so" stories. The tellers of these tales, it's said, simply cook up a notion that suits their view of the world, and none of the stories is more valid than any other.

Nowadays, though, they are more than stories. They are models too, and it is harder to stop a structurally unsound model from collapsing than to shore up a flawed narrative. Above all, modern Darwinism offers a foundation in which costs have been weighed against benefits. Some currents of evolutionary argument have thus come to read like rational-choice economics, and this may be one reason why Ehrenreich has ignored evolutionary calculation. She wishes to emphasise that war is often not a matter of rational choice. Evolutionary thinking does recognise the role of irrational choice, however. In his perfectly titled book Passions Within Reason, Robert Frank has explored how apparent irrationality can pay off. It may be irrational to threaten to break your neighbour's legs for stealing your parking space, since the costs include prison, but the man with a reputation for such outbursts will never have to leave his car in the next street.

Ehrenreich's central idea is that blood rites are a legacy of practices which our ancient ancestors developed in the face of predators. The thrill that accompanies the prospect of blood is an echo of the fear induced by lion, jaguar or sabre-toothed cat. Rituals may have their roots in collective tactics used against carnivores. As she notes, the Gond people of northern India have in recent times been observed to steal tiger kills by shouting and waving sticks at the beasts. The impact can be greater when such gestures are made in step. Parade-ground drill and church chants may have a common root.

As societies developed, Ehrenreich observes, the threat from carnivores declined. So, as the fossil record shows, did stocks of large game animals. Hunting skills thus diminished in value. Perhaps, she suggests, war developed to give men something to do, and a way of gaining prestige. The idea is sold short by the offhand way in which she puts it. She does not examine what prestige is, or why it should be so valuable compared to other things a man may desire. Referring to the "status of a specific group", the combatant males, she overlooks the possible rewards to individual males that might make fighting worth the risk. The answer is expressed euphemistically in the motto "none but the brave deserve the fair". Sociobiologists call it "reproductive success".

Ehrenreich sees human evolution as a struggle to the top of the food chain. She doesn't really mean the food chain in its proper ecological sense, though, but the Great Chain of Being in which Man dominates the animals. The fossil record, and behavioural ecology, suggest that the Homo genus has probably maintained a similar dietary range from its inception. They also suggest that Homo may have conserved an even older tendency to form mutually hostile male groups. Carnivores have certainly always been among the many threats that Homo species have faced, but without evidence such as analyses of damage to bones, Ehrenreich fails to make a case that they were the most dangerous animals our ancestors faced. Big cats had a choice of prey; other Homo were competing for the same resources.

Ironically, Ehrenreich has based her argument on the very division she criticised in The Nation, in assuming that for our ancestors, us and them meant humans, or nearly-humans, and animals. But the evidence suggests that the enemy is us, and always has been.