Recent analysis of a Stone Age skeleton shows a dramatic increase in human brain size by about a quarter-million years ago.

The finding by Karen Rosenberg, chairperson and associate professor of anthropology at UD, and colleagues in China and the United States sheds new light on human evolution.

Rosenberg analyzed the fossil with Lü Zuné of Peking University in Beijing and Chris B. Ruff, director of the Center for Functional Anatomy and Evolution at the Johns Hopkins University School of Medicine in Baltimore. She says the female skeleton, which was found at a site called Jinniushan in northeastern China 22 years ago, shows that brain size relative to body size had increased dramatically from ancestors by the Middle Pleistocene, about 260,000 years ago.

“This fossil belonged to one person from a time and place we didn’t know very much about,” she says. “What we were really interested in was what could this person tell us about relative brain size [and] body shape, and we could look at all of that in this specimen.”

Rosenberg says the skeleton is exceptionally useful because it includes a nearly complete skull, vertebrae, a hip bone and an arm bone from a single human. These features enabled the scientists to evaluate skeletal evidence for body shape and relative brain size in one individual, rather than relying on samples from separate individuals from multiple regions, she says.

Because of its size, scientists initially thought the Jinniushan fossil was male, but Rosenberg, who specializes in the evolution of the human pelvis, says features of the skeleton were clearly female. Males in the region at that time were even larger because of adaptation to the cold climate, she says.

In humans, she says, the width of the pelvis and the shape of its bony opening are different in males and females. “In females, it has to be big enough for the passage of the baby’s head [through the birth canal],” she says. “In males, its main function is locomotion. Because of that, it is easy to look at the pelvis and see whether it’s female.”

In a report published Feb. 27 in Proceedings of the National Academy of Sciences, Rosenberg’s research team writes: “Because the Jinniushan specimen preserves both postcranial remains and a fairly complete skull, we have the unusual, if not unique, opportunity to examine the relationship between body size and cranial capacity in this Middle Pleistocene individual.

“Most estimates of brain size have been carried out by using brain size estimates and body size estimates from different specimens or have used body size estimates derived from cranial dimensions, which are less accurate and subject to possible circular reasoning.”

The team found that the fossil shows not only that the skeleton’s relative brain size fits the model of increasing brain size during the Middle Pleistocene throughout the human range, but also that the skeleton’s body proportions reflect a pattern of climatic adaptation that persists to the present day.

The report states that the Jinniushan specimen shows that humans living around the cold region, which now is in northeastern China near North Korea, had large, broad bodies with short limbs to enable them to retain more heat.

“As is typical of living cold-adapted peoples, her body size was very large, and her broad body shape also gave her a relatively low surface-to-volume ratio,” Rosenberg says of the skeleton. “Long linear bodies are adapted to get rid of heat and can be found in warmer climates, while short, stocky bodies are adapted to retain heat.”

 The researchers write that, “As expected, cold-adapted modern populations have relatively shorter ulnas [a bone in the forearm] for their trunk diameters than warm-adapted populations. Pre-Holocene humans from higher and lower latitudes follow the same pattern as modern humans.”

In addition to the reported findings, the Jinniushan fossil raises more questions about what life was like in the Pleistocene at the periphery of the human range, in a place where it was very cold and life would have been very difficult, Rosenberg says.

Rosenberg, who studied the skeleton in China and uses a replica of the pelvis to explain some of her findings, says her experience has benefited her students.

“The fact that I worked firsthand with the fossils is very important to the students; it makes it all real, which is especially important at a time when some people are questioning the scientific basis for evolution,” she says.

A graduate of the University of Chicago, Rosenberg received her doctorate in biological anthropology from the University of Michigan and joined the UD faculty in 1987. She studied cultural anthropology for her bachelor’s degree and says her interest in human evolution developed when she was a research assistant for a professor studying the oral anatomy of tadpoles.

“It’s the best,” Rosenberg says of the study of fossils. “It’s fantastic. I certainly don’t have any mystical feelings or anything like that, but it’s true that my training in anatomy and comparative anatomy enables me to see things that other people are not trained to see.”

She says the recent rapid development and widespread application of genetic science does not threaten traditional anthropology.

“Genetics provides some information about human variability, but it is not a direct record of human evolution,” she says. “Certainly, any correct reconstruction of human evolution and human history must explain both the fossil and genetic data, so, in that sense, they are not in competition with each other.”

 by Martin Mbugua