Beyond Stones & Bones

Besides making brain structure more complex, genetic change also advanced the brain's chemistry. In 2005, Matthew Rockman of Duke University and colleagues discovered that a gene called PDYN began accumulating changes 7 million years ago, soon after our oldest direct ancestor appeared. This gene regulates production of a molecule called prodynorphin, which is like the brain's soup stock: depending on what other ingredients are added, it can change into neurochemicals that underlie perception, behavior or memory. "Fossils can tell us a lot, but it is genomes that tell us what was involved in making language possible and in making brains the way they are today," says Rob DeSalle, co-curator of the American Museum's new hall.

It surely took more than prodynorphin's magic to modernize a brain and thus jump-start the creation of new species. To find what else made us human, scientists led by neurogeneticist Daniel Geschwind of UCLA are examining which combinations of genes are active in the cortex, the seat of higher thinking, of chimps and people. Among the genes turned to "high" in people, they reported last year, are those that influence how fast electrical signals jump from neuron to neuron and therefore how fast the brain can process information, those that enhance connections between the cells and thus learning and memory, and those that promote brain growth. This pattern of gene activity, it appears, began emerging when Australo-pithecus species did.

And it helps explain why Lucy's kind were the way they were. Afarensis women and men stood three to five feet tall and weighed 60 to 100 pounds. They had small teeth good for fruits and nuts, but not meat. (The available prey was enough to make one a confirmed vegetarian: hyenas the size of bears, saber-toothed cats and other mega-reptiles and raptors.) That suggests that early humans were more often prey than predators, says anthropologist Robert Sussman of Washington University, coauthor of the 2005 book "Man the Hunted." The evidence is as stark as the many fossil skulls containing holes made by big cats and talon marks from raptors.

The realization that early humans were the hunted and not hunters has upended traditional ideas about what it takes for a species to thrive. For decades the reigning view had been that hunting prowess and the ability to vanquish competitors was the key to our ancestors' evolutionary success (an idea fostered, critics now say, by the male domination of anthropology during most of the 20th century). But prey species do not owe their survival to anything of the sort, argues Sussman. Instead, they rely on their wits and, especially, social skills to survive. Being hunted brought evolutionary pressure on our ancestors to cooperate and live in cohesive groups. That, more than aggression and warfare, is our evolutionary legacy.

Both genetics and paleoneurology back that up. A hormone called oxytocin, best-known for inducing labor and lactation in women, also operates in the brain (of both sexes). There, it promotes trust during interactions with other people, and thus the cooperative behavior that lets groups of people live together for the common good. By comparing the chimp genome with the human, scientists infer that oxytocin existed in the ancestor of both. But it has undergone changes since then, perhaps in how strongly the brain responds to it and in how much is produced. The research is still underway, but one possibility is that the changes occurred around the time our ancestors settled into a system based on enduring bonds between men and women, about 1.7 million years ago.

That was a formula for success, and one that may have also left a mark on the brain. Besides revealing the size of a brain, paleoneurology examines impressions of surface features that the brain leaves on the inside of the skull. That yields clues to its organization. Comparing the shapes of the brains of two hominids that lived 2.5 million years ago, Australopithecus africanus and Paranthropus, scientists find major differences in the shape of the frontal lobe, which controls higher cognition. "Paranthropus has a teardrop shape, whereas africanus is more squared off, and africanus has a swooping down on the bottom where Paranthropus is sort of peaked," says Dean Falk of Florida State University. That configuration suggests that africanus had a better-developed region called area 10, which plays a key role in decision-making, taking initiative and advance planning. It may be why africanus evolved while Paranthropus came to a dead end.