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DISTASTEFUL as it may seem to men, it鈥檚 women who are responsible for the intelligence of the human race. According to a controversial theory, our female ancestors were smart enough to pick brains over brawn when choosing a mate. They spurned the local Hercules or Adonis, and plumped instead for the man with a sparkle in his eye that hinted at wit and intelligence. Humanity should be grateful, men especially. For if these female ancestors had preferred bright red rumps, men might still be sporting these and keeping company with gorillas and chimps in Africa.

Where did this bold idea come from? It has its origins in IQ studies. The average IQ of men and women is pretty much equal. But look closely at intelligence and you find some strange differences between the sexes. For a start, men have a broader range鈥攖here are more mentally subnormal males, and more geniuses. And the pattern hasn鈥檛 changed as far back as the records go. What鈥檚 more, mothers are more likely to transmit their mental disabilities to their children than fathers. It was clues like these that, 30 years ago, led Robert Lehrke of Brainerd State Hospital in Brainerd, Minnesota, to suggest that many genes for intelligence are concentrated on the X chromosome.

Now a team of German geneticists has set out to prove him right. And their findings are likely to stir up a hornet鈥檚 nest. Not only do they argue that the X chromosome plays a central role in intelligence, but they also believe this proves that generations of females have been turned on by brainy men. And that by favouring intelligence over strength and beauty our female ancestors set in motion a process that eventually created our awesome brain, which today excels in mathematics and physics, and delights in art, music and literature.

The idea that mate choice, or sexual selection, influences evolution is as old as Darwin. He was convinced that only female choice during mating could explain oddball traits such as the peacock鈥檚 tail or the lion鈥檚 mane, which confer no obvious survival advantage. These days the idea is universally accepted and biologists have even begun to explain how such biological exuberance can mark out a good mate. But the notion of the human brain as a sexual ornament is more contentious.

Evolutionary biologist Geoffrey Miller of the University of New Mexico, Albuquerque, proposed the idea little more than a decade ago and argues the point in his recent book The Mating Mind. Sceptics abound, some of them highly critical and highly influential. Steven Rose from the Open University, for instance, describes the idea as 鈥渨holly unacceptable鈥� and 鈥渇rankly, pornographic鈥�. And the latest genetic developments are sure to reignite the debate.

Central to the new findings is the idea that the X chromosome plays a special role in sexual selection. We have 23 pairs of chromosomes in our bodies: 22 鈥渁utosomes鈥� that don鈥檛 determine sex and a twenty-third odd couple that does. The X chromosome and the much smaller Y are in fact not a pair at all. Men have one copy of each. Women have two X chromosomes. So, while we all have two copies of every gene found on the autosomes, men have only one copy of any gene on the X. And it鈥檚 because of this that sexual selection gets to work its magic.

Evolution occurs when a genetic mutation results in a trait that makes an individual more successful, and so more likely to pass on the mutated gene. But most mutations are 鈥渞ecessive鈥濃�攚hen paired with a normal version of the gene they have no effect. That鈥檚 why mutations on autosomal genes rarely show up. With no discernible trait to be selected, the mutation will eventually drift out of the gene pool. But a mutation on the X chromosome, dubbed X-linked, is a different matter. It is expressed in all males who have it, even if it鈥檚 recessive, because there is no corresponding gene on the Y to cancel it out. Males with the mutation will be different in some way, and if females notice this and happen to prefer them that way, the genes associated with the new trait will be passed on to their offspring.

Imagine, then, an X-linked mutation arising in a member of a tribe from our distant past. It may happen in an individual of either sex, but if the mutation is recessive it will only show up in a male. If this male, being a tad brighter than his dim-witted brethren, is extremely attractive to women then many of them will mate with him and pass on the mutated gene to their daughters. (Sons receive their father鈥檚 normal Y chromosome, so won鈥檛 get the X-linked gene.) Females with only one copy of the mutant gene don鈥檛 express it, but they do pass it on to some of their daughters and sons. As females continue to prefer bright males, the mutant gene soon becomes common within the population. Eventually it is so prevalent that increasing numbers of females will have two copies and express the trait. Any other X-linked mutation that confers increased intelligence will thrive in the same way. And, lest female readers become too smug, it should be noted that men are clever first.

If this is how our complex brains evolved, we would expect to find an excess of genes linked with intelligence on the X chromosome. And that鈥檚 exactly what Horst Hameister and Ulrich Zechner from the University of Ulm in Germany and their colleagues claim to have found (Trends in Genetics, vol 17, p 697). Of course, 鈥渋ntelligence genes鈥� are objects of much dispute, so the German team chose to look at genes that cause mental disability when mutated鈥攁nd assumed that in their normal state these contribute to intelligence. It鈥檚 a reasonable assumption, they argue, particularly for mutated genes that give rise to disorders where mental impairment is the only diagnostic factor.

They looked for entries related to mental impairment on a database called the Online Mendelian Inheritance in Man (OMIM)鈥攁 catalogue of human genes and genetic disorders. The team found a total of 958, of which 202 were mapped to the X chromosome. This means that the X contains a whopping 21 per cent of the genes for mental retardation, although it accounts for only about 4 per cent of the total human genes. Researchers, however, have always been wary of attributing an excess of any kind of genes to the X chromosome. This is because X-linked mutations, which show up so readily in males and have telltale patterns of inheritance, are much easier to identify than mutations on other chromosomes. Hameister鈥檚 team needed to show that their finding wasn鈥檛 simply an anomaly.

Their analysis is very elegant. Using OMIM they looked at other disorders such as cleft palate, growth retardation, and polydactyly鈥攁 condition in which a person is born with extra toes or fingers. There鈥檚 no reason to suspect that any of these is particularly associated with the X chromosome, yet the team found that geneticists had identified, on average, 2.35 times as many genes for these disorders on the X as on the autosomes. Even after accounting for this bias, there are still 3.1 times as many genes for mental impairment on the X chromosome as you would expect if they were evenly distributed among all the chromosomes. 鈥淭hey have demonstrated that the preponderance of mental retardation genes on the X chromosome is real,鈥� says Jennifer Marshall Graves, an expert on sex chromosomes at the Australian National University in Canberra.

But are these genes really associated with intelligence? There鈥檚 convincing evidence for a few that have firm links with brain structure. At least three are involved in a pathway that regulates the activity of a family of proteins called GTPases. Hollis Cline of Cold Spring Harbor Laboratory in New York and her colleagues have shown that GTPases influence the tree-like branches at the ends of nerve cells in the brain called dendrites (Nature Neuroscience, vol 3, p 217). The formation of connections at nerve endings is central to learning and memory. Many children with X-linked mental impairment have severely reduced networks of dendrites, clearly showing the link between brain structure and cognitive ability.

Gene genius

What鈥檚 more, these three genes are among the group that lead to mental impairment with no other diagnostic symptoms when they mutate (see Diagram). They are the genes that Hameister and his colleagues believe are most likely to contribute to intelligence in their normal form. Josef Gecz, an expert on X-linked mental impairment at the Women鈥檚 and Children鈥檚 Hospital in North Adelaide, Australia, agrees. He also points out that while 10 of these genes have already been found on the X chromosome, none has been found on the autosomes. So genes with the strongest links to intelligence do seem more prevalent on the X chromosome.

Even so, Gecz cautions that having extra copies of these genes is not going to make you a genius. 鈥淚f you have a car and you pull one plug, you may make the car stop. But by putting in two extra plugs, you won鈥檛 make it run faster,鈥� he says.

For some of the other genes fingered by Hameister and his team, the link with intelligence is less clear. One, recently identified by Italian researchers, influences the metabolism of fatty acids (Nature Genetics, vol 30, p 436). Others code for receptors for signalling molecules on the membranes of cells. 鈥淚f we thought originally that we would find a common theme, it鈥檚 not true,鈥� says Gecz. Nonetheless, most geneticists believe the German team has uncovered a real effect鈥攁n excess of X-linked genes responsible for intelligence, known in genetic parlance as the large X-chromosome effect. Many, including Graves, are also convinced that this is the result of female choice.

But where鈥檚 the evidence that women choose intelligent men to father their children? Some would say, just ask the exasperated single women who are still looking. Miller, of course, has grown accustomed to this line of questioning. His research highlights the amount of effort men put into activities that advertise their intelligence, and he argues that they would not waste so much time writing sonnets, composing music and creating works of art that have no obvious survival benefit, unless it made them more attractive to the opposite sex. What鈥檚 more, if women really do seek out clever, witty men to father their children then they should find them particularly attractive in the few days around ovulation. His current experiments with female undergraduates at the University of California in Los Angeles indicate that this is indeed the case: women who are not on the Pill favour intelligent, creative men when they are most likely to conceive and men with greater resources at other times of the month.

And you may not guess this from watching beer commercials, but it鈥檚 not just women who prefer brainy partners. Miller, for one, is convinced that it works the other way too, and that鈥檚 why both sexes are equally clever. While our male ancestors used their ever-increasing brains to show off to potential mates, the females also needed intelligence to recognise it and to beguile men, persuading them to stick around and help out in the onerous task of raising children. As evidence that both sexes prefer quick-witted sexual partners, he points to a study by David Buss from the University of Michigan in Ann Arbor. Buss asked people from 37 cultures what they found most attractive in the opposite sex and although he didn鈥檛 include 鈥渟ense of humour鈥� in his list, it was the number one choice written in.

Such arguments have never convinced Miller鈥檚 critics, and the new genetic evidence will do nothing to persuade those who believe that our big brains can be explained without resorting to theories of sexual selection. David Houle, an expert on sexual selection and mate choice at the Florida State University in Tallahassee has an alternative explanation for the excess of intelligence genes on the X chromosome. He says it could simply be the result of natural selection choosing intelligent males and females who were better at, say, finding berries, killing mastodons, or surviving winters. 鈥淭hey are sort of jumping the gun in favour of a particular model. It鈥檚 certainly consistent with Miller鈥檚 hypothesis, but it鈥檚 not a special confirmation of it,鈥� says Houle.

Hameister accepts that intelligence has survival advantages beyond those of attracting a mate. But, he believes, this strengthens his argument because evolution of the human brain doesn鈥檛 follow the same pattern you find in other sexual ornaments such as the peacock鈥檚 tail. While female preference drives peacocks to evolve ever larger tails, eventually the tails become a burden鈥攎ales are not agile enough to escape foxes and other predators. So sexual selection and natural selection work in opposition to keep the tail in check. But for intelligence, with both processes working in tandem, there is nothing stopping evolution. Together, natural selection and sexual selection have forced evolution鈥檚 hand, resulting in a threefold increase in our brain volume in a mere 2.5 million years.

Perhaps surprisingly, Miller鈥檚 response to the link between intelligence and the X chromosome is quite cautious. 鈥淚t鈥檚 great to see geneticists come along and find this connection,鈥� he says. 鈥淏ut I鈥檓 a bit reluctant to stick my neck out and say that it鈥檚 an obvious proof of my ideas. It is one bit of supporting evidence.鈥�

Premature speculation

Rose, by contrast, sees no evidence to back up what he describes as the 鈥渃ompletely pathetic argument鈥� that our big brains are the product of Palaeolithic women lusting after men who painted caves. He says the genetic findings don鈥檛 address the theory鈥檚 major shortcomings such as the failure to explain why women selected men with greater cognitive abilities and why there is no evidence for differences in the brains of men and women or in their average IQ. Then there鈥檚 the observation that in the modern world, at least, intelligent men don鈥檛 have more children: 鈥渢he reverse, if anything,鈥� he says. Rose does accepts that if genes connected with cognitive performance are concentrated on the X chromosome, that probably says something about evolution and is interesting and worth exploring. 鈥淏ut I鈥檓 constitutionally (probably genetically) opposed to premature speculation,鈥� he adds.

Besides, isn鈥檛 all this talk of genes controlling sexual behaviour and intelligence making a case for genetic determinism? 鈥淣o way,鈥� says Hameister. The environment has a significant effect on these complex traits in an individual, he says. Graves agrees, noting that it鈥檚 hardly surprising that there would be genes governing our sexual preferences or mating habits. 鈥淭here are genes that regulate everything else,鈥� she says. 鈥淚鈥檇 guarantee that there would be hundreds of genes that would affect sexual behaviour. But it鈥檚 obvious we can override our genetic inheritance. That鈥檚 one of the great things about being human.鈥�