Meeting Report

Highlights

• The study of human evolution is an ever-changing field that draws on many disciplines, including archeology, paleontology, biology, and the social sciences.
• Factors that distinguish modern humans from their ancestors include physical traits such as large brains, small teeth, and flat faces, in addition to social and cultural behaviors such as food sharing.
• The recent discovery of Homo floresiensis has challenged long-held assumptions about the evolution of modern humans.
• Many question marks remain on charts that delineate the path of human evolution.
• Radical changes in environment forced dietary changes and other physical adaptations, especially in the last three million years.
• Larger brains in modern humans are offset by smaller digestive systems, an adaptation made possible by a higher-quality diet and the development of cooking.
• Along with physical adaptations came cultural adaptations, such as giving food to infants even after they were off the breast.
• Biological and cultural adaptations led humans to move out of Africa about two million years ago.
• It is the interaction of biology and culture that makes us human.

A passion for evolution

In the September 26, 2005, Anthropology Section Inaugural Lecture at the New York Academy of Sciences, Leslie Aiello of the Wenner-Gren Foundation for Anthropological Research discussed her 30-year passion for the study of the evolution of adaptation in modern humans. Drawing on research on climate change and the evolution of human walking, diet, cooperation, language, physiology, and thermoregulation, she attempted to unravel the complex course of our seven-million-year evolutionary history and show how biology and culture interacted to make us human.

What makes us human, anyway?

Introductory courses in human evolution have long taught that three factors distinguish us as humans. One includes body size, body shape, and two-footed walking. "We're at least 50% again the size of the chimpanzee, and we're 50% again the size of some of our early evolutionary ancestors. We have long legs and narrow bodies and we've lost a lot of the musculature in our arms, and of course we walk on two legs," Aiello said. The second defining factor includes having small jaws and teeth as well as flat faces, all indications of a radical dietary change. The third is our large absolute and relative brain size, and associated language and cognitive abilities.

To this traditional list, modern anthropologists have added two more items. One is life history, including slow development, longevity, and for females, a long period of post-reproductive life. The fifth factor is social behavior, cultural activities such as food sharing and economic division of labor.

A fantastic discovery

A recent finding challenges long-held ideas about what it means to be human. About two years ago, a new species called Homo floresiensis was discovered on the island of Flores in Indonesia. Standing only about three feet tall, floresiensis had tiny brains yet were evolved enough to have very advanced tools, including fire.

The finding also challenged the belief that we modern humans had the earth to ourselves for tens of thousands of years, because floresiensis lived as recently as 12,000 years ago. Showing a chart that places human species on a time scale going back 1.6 million years, Aiello said Homo floresiensis seems to be a relic of an ancient population.

Homo floresiensis, which lived as recently as 12,000 years ago, seems to be a relic of an ancient population.

"This is very controversial," said Aiello. "Many people think this is just a dwarfed modern human with a microcephalic brain size, but it's not. If you know your post-cranial anatomy (from the neck down), the skeleton looks like a skeleton that would've been on one of our ancestors about three million years ago. This again raises issues about what humanity is, because many of the things that have been found with floresiensis would correlate with later phases of human evolution."

A long and twisted road

The idea that our ancestors may not have been alone until quite recently isn't unusual when you look at our entire ancestral road map with its multiple species, said Aiello, showing a slide of 22 species placed on a time chart stemming back seven million years. Referring to the chart, she noted the difficulty of finding the actual route between species. "There are more question marks than solid lines linking them," she said.

That led Aiello to the first of her two major initiatives. One was to find the thread that connects us through this "labyrinth to the past." The other was to explain changing hominid adaptations. "We can draw on a variety of fields for these, particularly paleontology and archaeology, to get the scaffold in for the evolutionary framework," she explained. "Biology and the social sciences provide us with hypotheses to explain what may have been going on."

A changing environment

Recent examinations of environmental change have yielded surprises in terms of just how radical that change was. To illustrate those changes, Aiello first showed pictures of Australopithecus afarensis, the species that includes the famous Lucy, found in Ethiopia in 1974. This species had mixed human and chimpanzee characteristics, and lived almost four million years ago. "If we saw this species on the street, we would think it was a chimpanzee standing up," she said. "They still were very heavily muscled in the shoulders, although they had a definite adaptation in the hips and legs to walk habitually on two legs." They also had a very small brain case and quite a large face, and foraged in a mixed-type environment of forests, woodland, and lakeshores.

Australopithecines had longer legs, the first signal that the species was broadening its diet and moving more efficiently.

Then, about 2.5 million years ago, a radical change began and the climate became cooler and much more seasonally variable. Several new species appeared, including Australopithecus garhi. In this species, the skull is australopithecine-looking, with a relatively small brain and large teeth, but what's new is that the material found in a nearby site of the same age included animal bones with cut marks. "This is what you find when hominids are using stone tools to strip meat off of associated fauna," Aiello said. Also, these australopithecines had longer legs, the first signal that the species was beginning to broaden its diet and perhaps move in a more efficient way, particularly in covering longer distances.

Also appearing on the scene around this time was a suite of species called the robust australopithecines, including the one sometimes called Nutcracker Man. "The teeth on some of these are as large as the end of your finger," Aiello said. "They're absolutely amazing in the force that they would generate between their jaws, and they seemed to have been eating large quantities of food taking a lot of masticatory preparation."

The genus Homo also appeared in this period, beginning around 2.5 million years ago. By the appearance of Homo ergaster, about 1.8 million years ago, these early ancestors looked much like us. "If you saw one of these individuals on the street, particularly if wearing a hat, you probably wouldn't notice that they were that much different than ourselves," said Aiello. Unlike the australopithecines, Homo controlled animal-based food in that they would actually beat scavengers to it, as evidenced by stone tool marks that were imprinted on bones before the scavenger's teeth marks were. It was Homo that would make the jump to leave Africa.

Out of Africa

Similar discoveries in the former Soviet republic of Georgia and the Far East indicate that early humans expanded out of Africa almost two million years ago. Adaptations inferred from those early Homo skeletons have helped piece together what would have made it possible for them to leave. One adaptation was an expanded brain size, only two-thirds the size of our own, but virtually double the brain size of the australopithecines. Other adaptations were a much larger body size, very extended lower limbs, and a much narrower form.

Said Aiello, "Through the years of my studying this, I've come to appreciate that this change between the australopithecines and Homo was probably more fundamental than the division seven million years ago between the line leading to modern apes and the line leading to the australopithecines. These early hominids lived for five million years as basically bipedal apes, and it's here at this stage where you make this jump to these early members of our own genus."

The increase in brain size of Homo sapiens was accompanied by an almost perfectly balanced decrease in digestive system size.

Trade-off for a better brain

To answer the question of how our expanded brains evolved, Aiello, together with colleague Peter Wheeler in England, established the "expensive tissue hypothesis." The hypothesis was based on the simple fact that whereas we modern humans have the basal metabolic rate one would expect for a mammal of our body size, we have a brain of tremendously large size relative to body size. "The problem is your brain takes up a very large amount of energy, and if we were just a normal animal with a huge brain, you would expect our basal metabolism to be much higher."

So where did that missing energy go? "It turns out that it disappears into your gut," Aiello said, explaining that we have a decreased digestive system size that almost perfectly balances the increase in brain size. "If you look at the energetics of it, the cost of your gut in metabolic terms is about the same as the cost of your brain. Now how can you get a small gut? You eat a high-quality diet."

Researchers disagree about what food source was most responsible for increasing brain size, with some emphasizing meat sources and others crediting tubers. But no matter what Homo was eating, the species may also have cooked its food at this stage, because it is hard to believe that they could have chewed enough raw food to get necessary nutrients to support their body size. "I used to ask my students to sit down with a bag of carrots and eat until their jaws wear out," Aiello said. "You can't get too far into that bag of carrots before you get really tired."

Getting socialized

Because infants can neither dig tubers nor hunt meat, such a diet signals another very significant adaptation, one that is cultural rather than physical.

"One of the major differences between ourselves and nonhuman primates is that we share food, particularly with infants," said Aiello, explaining that once nonhuman primates are off the breast, they must forage for themselves. Furthermore, once the leap is made to food sharing, there are more types of habitats that can be lived in. "Some researchers believe that expanding out of Africa, crossing ecological zones, and adapting to different food resources wouldn't have been possible without having some sort of social system in place."

With changes in diet also came changes in the lower face, including smaller teeth, a deeper jaw, and a flattened profile. "This actually set up the geometry of the mouth that allowed muscles to move the tongue," said Aiello. "[This] was the preadaptation to producing the sounds that we use in modern human language."

In conclusion, she again posed the question of whether it was biology or culture that made us human. "My own feeling is that it's the interaction between the two," she said. "Once you get environmental changes forcing dietary changes, there's a tremendous number of [consequences] from it."

Open Questions

What made us human, and what were the varying roles of biological and cultural change in this transition?

What were the major biological and cultural milestones in our evolutionary past?

What is the role of the paleontological and archaeological sciences vis-à-vis the biological and the social sciences in unraveling the long and winding course of our seven-million-year evolutionary history?