Groping toward a fuller understanding of how we became human, some researchers are looking at how and when we came to act as women and men.
Hominids diverged from the chimp branch of the family tree roughly six million years ago, producing a series of evolutionary dead-ends before the ultimately successful genus Homo emerged. H. neanderthalensis and H. sapiens grew out of the same line at different times.
The Neanderthals' ancestors diverged first, and they left Africa a couple of hundred thousand years ahead of ours. Homo sapiens arrived in Europe about 50,000 years ago. The two species overlapped for perhaps 20,000 years before the Neanderthals went extinct. Scientists still aren't sure why.
A lack of all but the most primitive weapons found at Neanderthal sites suggests they may have had to band together to kill large animals, says Steve Kuhn, an anthropologist at the University of Arizona. And numerous fractures found in skeletons of Neanderthal females, just like the males, indicate that both sexes may have participated in dangerous work.
Kuhn and his wife, anthropologist Mary Stiner, are the main proponents of a theory that says Neanderthals engaged in coed hunting - and did so, in part, because they lacked other options. Archaic humans, by contrast, came out of Africa with the ability to fashion early traps, nets and clothes, and to grind seeds into edible form.
In present-day hunter-gatherer societies, the women collect and process most of the plants, trap small game, and sew most of the clothes, says James Adovasio, director of the Mercyhurst Archaeological Institute in Erie, Pa., and one of three authors of the new The Invisible Sex: Uncovering the True Roles of Women in Prehistory. (Men apparently make most of the shoes.)
Adovasio didn't set out to enter the battle of the sexes. He's best known for finding artifacts in Western Pennsylvania that push back estimated dates for the peopling of the Americas. Through chance events, he also came to specialize in relatively perishable artifacts - baskets, blankets, fishing nets, shoes and textiles.
What little is preserved is surprisingly well-made, he says. Some Stone Age impressions left in clay show fabrics woven as fine as Brooks Brothers shirts.
Most of these items, crucial for survival in harsh times, would have been created by women, he says. Yet virtually all texts and exhibits emphasize men hunting cave bears or mammoths or some other ridiculously oversize prey.
"If you pick up even some of the most recent prehistory books you'll rarely see any images of females doing anything," Adovasio says. (Also left out of the limelight were the men who made the shoes.)
A majority of researchers believe the roots of separate sex roles date far back in human evolution, probably to the time when the size ratio of male to female reached something close to our current differential.
In other animals, this ratio can reveal something about the relationship between the sexes. In species with a big difference, such as gorillas, the males often fight to control harems.
In species with males and females closer to the same size, the sexes are more likely to work in pairs, cooperate, and share the burden of protecting their young.
Modern men and women are similar in body size compared with our closest relatives. So determining how long ago we reached our current ratio should point to when our ancestors stopped organizing themselves like apes and started acting more like people, says Stanford University anthropologist Richard Klein.
Some researchers see evidence placing this date at least 3.2 million years ago, the age of the diminutive australopithicine known as Lucy. Others favor a mere 1.7 million or 1.8 million years.
At any rate, by half a million years ago, our evolutionary limb had separated from the Neanderthals'. Their ancestors migrated from Africa to the Middle East and Europe by 250,000 years ago, while ours stayed put. When so-called modern Homo sapiens did reach Europe and Asia, others were already settled in.
Klein is skeptical of the claim that Neanderthals had no division of labor.
But the husband-and-wife team from Arizona, in a paper published in December's issue of Current Anthropology, contend that Neanderthals may have had no other way to survive. They had grown to depend on big game for food: deer, horses, cattle, bison and, very occasionally, a mammoth.
Our ancestors, on the other hand, may not have gone after much big game, says Adovasio. In fact, the fossil record hints that not all meat was hunted. At some hominid sites, animal bones show signs of having been gnawed first by other, fiercer creatures. In other words, Adovasio says, our ancestors were scavengers at least some of the time.
He believes that a more appealing picture - Paleolithic man (not woman) as macho hunter - is partly a product of 20th-century culture.
"Nobody wants to be based on road kill," Adovasio says. Plus, exhibit curators may have figured there was little excitement in the gathering of fruits and roots: "Who wants to see a diorama of people hunting a wild pumpkin?"
Hunting animals did become more important for both Neanderthals and modern humans as they moved from the plant-rich tropics to more northern climes. Weapons and protection provided an edge.
"What women do then is go heavily into technology," says Kuhn, the Arizona anthropologist. "They make water- and wind-proof clothes," he says, as well as devices for trapping small animals.
Olga Soffer, an anthropologist at the University of Illinois and an author, with Adovasio and Jake Page, of The Invisible Sex, says it was our ancestors' understanding of gender and gender roles that made us fully, cognitively human as recently as 40,000 to 50,000 years ago - the time that cave art and jewelry start to appear.
Before that time, she says, mothers cared for their children but males and females largely fended for themselves, finding food however they could.
Seeing themselves as men and women, she says, served as a catalyst for the emergence of other forms of symbolic thinking. It was sort of a Helen Keller moment for the species: As the word "water" written into Keller's hand opened up the concept of symbolic language to a deaf and blind girl, so the idea of gender introduced the world of conceptual thinking to humanity.
Most researchers agree that something dramatic changed around this time. But what?
Stanford's Klein sees no connection with sex roles. Gender distinctions, in his more mainstream view, go back more than a million years. He believes a genetic change spurred artistic and cultural advances.
A mutation in the DNA, he says, might have reorganized the brain without changing its size. It may have helped humans create more complex communication - thus offering an advantage that would spread through the population.
A mutation like he envisions having hit modern humans would have contributed to our survival and the Neanderthals' demise. Klein is hoping the actual mutation will surface as scientists continue to decode bits of DNA scraped from Neanderthals' bones, and compare them with our own.
Such a mutation, he says, would have spread not just because it made men better, more specialized hunters. It would have helped both men and women do almost everything better.
Contact staff writer Faye Flam at 215-854-4977 or email@example.com.
* 5 million to 7 million years ago: Divergence of chimps and hominids.
* 1.7 million to 3.2 million years ago: Hominid males and females evolve to current size differential. Genus Homo emerges.
* 250,000 years ago: Homo neanderthalensis established in Europe.
* 100,000 years ago: Homo sapiens evolves in Africa.
* 50,000 years ago: Homo sapiens spreads to Europe and compete with Neanderthals.
* 40,000 to 50,000 years ago: An explosion of intellect and creativity gives humans a compitive edge.
* 30,000 years ago: Neanderthals, unable to adapt, die out.
* 15,000 to 20,000 years ago: Humans travel to the Americas.