ASU Research E-Magazine: A Walk Through Time

Kaye Reed is following in the footsteps of her ancestors. Literally.

Each summer, Reed trades her comfortable office at Arizona State University for a tent in the African wilderness. There, she and her colleagues spend their days plucking ancient fossils from cave walls and riverbeds. Some of these fossils have waited for almost 4 million years to be discovered. The fossils are providing scientists with clues to the mystery of human origins.

Reed is an assistant professor of anthropology at ASU and a research associate at the Institute of Human Origins. Her specialty is paleoecology, the study of prehistoric ecosystems. By examining the fossils of early animal communities, she can draw conclusions about the people who lived alongside those animals.

Currently, Reed is studying fossils from the Hadar region of Ethiopia. To find the fossils, Reed and her colleagues walked for miles under the scorching Ethiopian sun, scouring the banks of the dried-up Awash River.

Three million years ago, another woman walked the banks of the same river—and died there. Lucy, as she is fondly called, became an international celebrity when her bones were discovered in 1974 by Institute of Human Origins founder Donald Johanson. Lucy belonged to a previously unknown hominid species called Australopithecus afarensis, one of the earliest ancestors of modern humans.

As the 21st century looms, Reed traces the steps of Lucy’s kin and their descendents. By reconstructing the environment that surrounded these hominids—or human precursors—she hopes to learn what life was like before the dawn of recorded history.

Reed works to reconstruct environments that surrounded early humans. “I want to reconstruct habitat across time and then trace the evolution of various species,” she says.

The Hadar site is important to anthropologists because it contains fossils of two hominid species from different time periods. Australopithecus afarensis—Lucy’s species—dates back from 3.9 to 3 million years ago. Homo habilis, a more evolved hominid, roamed the area 2.5 to 1.6 million years ago.

The Awash River did most of the hard excavation work for Reed and her colleagues.

“We don’t have to dig down to find things. We just walk,” Reed explains. “In general, the older stuff is found along the river. It gets newer as you walk away. I’m looking at the different fauna from what we call the lower section and the upper section, to see the shift in mammalian communities between Australopithecus and Homo.”

Some of the animals she finds are rather surprising, like a six-foot-long otter. She also has found evidence of bears, which are rare in Africa. Determining what animals lived during a particular period can provide clues about how the hominids lived.

“Along the timeline, when you go from Australopithecus species to Homo species, you find a larger supply of grassland antelope,” Reed says. “These animals lived on the open grassland. There were lots of them, which meant more to hunt.”

The scientist says that more available game may have led to more cooperative hunting among hominids.

Mammal communities can provide clues about hominid communities in less direct ways, as well. By looking at animal fossils, Reed can make deductions about the vegetation and climate of the surrounding region. All are interdependent.

“All of these animals have teeth,” she explains. “And based on what they eat, their chewing design is different.” By making specific measurements, Reed can determine whether an animal ate grass, leaves, both, or whether it ate meat, meat and bones, or insects.

An animal’s bones hold even more clues. Bone structure can reveal how certain creatures moved. This in turn can provide information about the landscape. For example, tree climbing primates have different elbow structure than their earthbound cousins. When Reed finds the bones of tree-dwellers, she knows that the area once supported forests. She also knows that the area needed plenty of rain to support all those trees.

“The percentage of arboreal animals in modern sites can predict rainfall within a few hundred millimeters,” she says. “The thing I’d really like to know is the seasonality. In other words, how many months a year did it rain and not rain? The seasons in Africa are different than they are here. They’re defined by rain and no rain. The length of the non-rainy season must be very important for the animals.”

Armed with this information, Reed tackles broader questions about human evolution. For instance, why did primates begin walking on two legs? She believes climate is the key.

From 23 million to 5 million years ago, Africa was covered with forests populated by great apes. Somewhere during that period, the climate began to dry, and the forests began to shrink.

“Many of the apes that lived in these forests became extinct. One, at least, somehow developed the ability to go between forest patches on two legs and became an upright walker. Why do humans walk upright? That is a very big question. The answer has to be somewhere in that environmental shift,” Reed says.

Scientists have several hypotheses. One idea is that four-legged grassland apes began walking upright in order to hold tools. Reed disagrees. “My research has shown that in fact, there wasn’t any open grassland until about 1.8 million years ago. People were on two legs by 4.2 million years ago.

“In my own personal view, the ape that eventually became a biped was an arboreal animal, not a terrestrial quadruped. In other words, I don’t think the animal just stood up, because it was not advantageous to be on the ground and not run on four legs.”

Instead, Reed believes that an ape was forced, through loss of habitat, to come down from the trees and walk upright. In fact, some modern arboreal primates do walk on two legs when they are on the ground.—Diane Boudreau