Big Brain: The Origins and Future of Human Intelligence

By Gary Lynch and Richard Granger

Palgrave Macmillan, 259 pp., illustrated, $26.95



Your Inner Fish: A Journey Into the 3.5-Billion-Year History of the Human Body

By Neil Shubin

Pantheon, 229 pp., illustrated, $24



Twenty years ago my high school biology teacher drew a leafless tree of life on the blackboard. At the base of its trunk were single-celled protists, and as the tree extended upward, it forked into the plant and animal kingdoms. The limbs of each kingdom quickly subdivided into phyla: ferns and flatworms, conifers and echinoderms. Toward the top of the tree, a branch was labeled "chordates," and at the very top of that branch's mess of twigs, our teacher labeled the highest shoot "human beings."

This sort of diagram is called a phylogenetic tree and offers a succinct representation of the diversity of life. It beautifully illustrates the genetic connections between organisms as disparate as a cheetah and an eelworm. It says: We are all cousins. Darwin used the analogy in "The Origin of Species"; international biologists are currently assembling an amazing example at www.tolweb.org.

What I've been realizing lately, though, is that by placing humans at the very top of the tree, my high school teacher may have done his students a mild disservice. The implication was that big-brained Homo sapiens, perched at the top of the topmost twig, formed the pinnacle of evolution. This is a fallacy. Evolution proceeds by blind chance, not by design. We are adapted creatures, but we are not optimized creatures. As counterintuitive as it might seem, it's inaccurate to suggest that humans are more evolved than, say, horseshoe crabs, which have been living and dying for 400 million years.

Two esteemed brain scientists, Gary Lynch and Richard Granger, in their new book, "Big Brain," call the top-of-the-tree paradigm "species chauvinism." "We often," they write, "fall into a fallacy of thinking - an almost irresistible fallacy - imagining that a feature or characteristic that we possess must have been carefully built that way, just for us."

In "Big Brain," the authors reintroduce the Boskops: an extinct, controversial, 10,000-year-old African hominid that possessed a brain about 30 percent larger than ours is now. The Boskops had little frames and massive heads, and their mere presence in the fossil record throws the idea of unidirectional evolutionary progress into question.

Were the Boskops more intelligent than we are now? More capable? If they were, why did they die out? Lynch and Granger offer up several theories. Ultimately they conclude that the Boksops "would far exceed contemporary people in at least some mental capacities."

"Big Brain" is about more than the fossil remains of a few large-headed ancestral relatives. Sometimes dry, sometimes riveting, the book tracks the evolutionary development of the human brain and ultimately argues that pure chance, in concert with natural selection, has given us our currently unmatched cognitive powers. Along the way, the authors are exceedingly careful to point out that evolution never proceeds in a straight line but rather takes a path "of apparently aimless wandering interrupted by surprising leaps."