Did human intelligence peak thousands of years ago?

Although he says that his ideas arise from anthropology and neurobiology, Crabtree's argument mostly comes down to genetics.

Here's the gist: We all know that genes are little DNA snippets that perform many vital functions in building this biological contraption known as "us," including passing down traits from our parents. Each gene type comes in several flavors called alleles. For example, a gene (one of many) related to height might code for either tallness or shortness. We inherit half of our genes from each parent, and some dominant alleles, such as brown eyes, tend to, well, dominate over recessive ones, like blue eyes.

Sometimes an allele undergoes an abrupt, random change called a mutation. Somatic mutations affect a single organism -- you, for example -- while germ-line mutations can be passed on to offspring. Although most mutations are harmful, in rare cases, one can increase a creature's fitness to live on and breed. Hence, mutation is the engine that drives evolution and natural selection.

Based on studies of intellectual deficiencies linked to genes on the X chromosome (or XLIDs such as fragile X syndrome), Crabtree estimates that at least 10 percent of human genes participate in intelligence and believes that every single one of them plays a vital role in intellectual and emotional function. Intelligence, for Crabtree, is a genetic house of cards, and a bad mutation in even one gene can bring the whole affair tumbling down [sources: Crabtree; Crabtree].

To make matters worse, intelligence is particularly susceptible to harmful mutations and is not readily passed down to children, says the geneticist, which means you need a strong selective force in play if you want to keep your bean keen from generation to generation. Between 50,000 and 500,000 years ago, he argues, the unrelenting struggle for survival put hunter-gatherers under pressure aplenty, but more stable agricultural societies selected for traits that better met the needs of their way of life, such as resistance to the diseases it gave rise to. As for modern society, its various safety nets, though desirable, enable people across the intelligence spectrum to breed. Crabtree offers no evidence to back up these assertions [sources: Crabtree; Crabtree; Mitchell].

In short, intellect and emotional maturity are as fragile as a dream, and we are in for a rude awakening.

Crabtree's hypothesis appears to embrace all flavors of intelligence and emotional development. For example, he includes the "intuitive ... non-verbal comprehension of things such as the aerodynamics and gyroscopic stabilization of a spear while hunting" [source: Crabtree]. This strategy makes his claim hard to prove or refute. After all, other factors involved in survival, such as coordination, training or social qualities, might also map to intelligence -- or fail to, depending on which definition better supports Crabtree's thesis.

This vagueness is part of the problem. Scientists, of course, need to carefully define their terms, specify their problems and outline the criteria by which further research might support or refute their hypotheses. On these grounds, and others, Crabtree's critics take him to task.

Critics fault Crabtree for his loose phrasing, sparse proof and reliance on "it stands to reason" arguments. They also disagree with his readings of certain genetic, neurobiological and anthropological studies [sources: Kalinka et al.; Mitchell]. In short, the head bone of contention extends beyond Crabtree's claim of brittle braininess to the very essence of his evidence.

For example, Crabtree dismisses language's role in expanding the frontal cortex and endocranial volume -- two changes vital to the evolution of abstract thought that he says occurred 50,000-500,000 years ago. But Alex Kalinka and his colleagues at the Max Planck Institute argue that language capacity stretches back much further, to when modern humans split off from Neanderthals, which could suggest an early, language-related ballooning of the brain [source: Kalinka et al.].

Mitchell, meanwhile, defends society's role in bolstering brainpower. What Crabtree views as a crèche that coddles the less clever, Mitchell sees as the very crucible from which acumen arises, pressured by the complexities intrinsic in social dealings [source: Mitchell]. Crabtree replies that societal impacts -- including environmental, nutritional, perinatal and educational improvements -- merely offset genetic decline [source: Crabtree].

On that topic, Kalinka and company dismiss Crabtree's XLID-based estimate of 2,000-5,000 intelligence deficiency genes. Here's why: The X chromosome boasts more than its share of brain-expressed genes and virtually bristles with links to neurological genetic disorders. In other words, any estimate based on the X chromosome should skew quite high and therefore offer a poor basis for estimation [source: Kalinka et al.].

Returning to the easily broken heart of the matter, Mitchell attacks Crabtree's idea of delicate intelligence on the ground that it would work only if mutations affected humans not as individuals, but as a species, which isn't the case. If it were, he argues, every trait touching on our fitness would degrade over time as we accumulated one nasty mutation after another (sidebar). Not only does this not occur, says Mitchell, but intellectual deficiency genes (ID) tie into a vast web of other genes whose mutations do not cause ID -- a far more robust system than Crabtree describes [source: Mitchell].

Finally, say critics, competing models of genetic development strongly argue for intellectual resilience. Intelligence might not derive from genes that specifically code for it, they say, but rather arise as a side effect of general fitness, a hitchhiker on the coattails of other selected-for traits (or vice versa). Such a model would help explain why intelligence often goes hand-in-hand with various physical and mental health characteristics, including cardiovascular and psychiatric diseases [sources: Deary; Kalinka et al.; Mitchell; Yeo et al.].

In the end, Crabtree himself views his fragile intellect idea as a useful hypothesis, one he admits is in need of greater study and substantiation [source: Crabtree]. Meanwhile, the debate itself underlines just how complex our genetic dance might be -- and how much work remains to be done.