A genome-wide analysis of gene expression profiles in the brain provides further evidence that chimpanzees are more like humans than gorillas, demonstrating that chimpanzees are the evolutionary sister group of humans.

Wayne State University researchers measured gene expression in the anterior cingulated cortex (ACC) of human, chimpanzee, gorilla and macaque samples. The ACC is a specialized region of the brain's neocortex that is significantly involved in the regulation of emotional and cognitive behavior. The ACC participates in decision-making and shows increased activity when an individual is engaged in cognitive tasks.

The ACC profiles of chimpanzees are more like the human profiles than the gorilla, according to a March 2 article published in the Proceedings of the National Academy of Sciences (vol. 101, no. 9). Furthermore, histologically, clusters of spindle cell pyramidal neurons occur in the ACC of humans, in lesser numbers in chimpanzees, lesser yet in gorillas, least in orangutans, and not at all in other primates and mammals.

These brain features point to the evolution in the ancestry of humans and chimpanzees of the capacity for cognitive functioning, and supports placing chimps and humans on the same branch of the evolutionary family tree. The results challenge old-school evolutionary theorists who still hold that chimps and gorillas are more similar, as would be expected if human cognitive capacities were greatly different from any other animals.

Lead authors on the study are Monica Uddin, Ph.D., and Derek Wildman, Ph.D., both of whom studied under and collaborated with Morris Goodman, Ph.D., distinguished professor in the Wayne State University School of Medicine's Department of Anatomy and Cell Biology and the Center for Molecular Medicine and Genetics, and member of the National Academy of Sciences.

Dr. Goodman sparked great debate in 1962, when he originally asserted that chimpanzees and gorillas are genetically more closely related to humans than to other apes. Since then, he has shown that to be true through DNA sequencing and gene expression analyses. He believes in changing the primate order, proposing that all living apes should occupy the family Hominidae (which currently contains only humans), and that both humans and chimpanzees should occupy the genus Homo.

"The most parsimonious phylogenetic tree that can be constructed from our results demonstrates that humans and chimpanzees are closest relatives, not chimpanzees and gorillas. Also, simply in terms of degree of divergence, there are fewer character-state differences between humans and chimpanzees than between chimpanzees and gorillas," authors said.

Uddin et al. also report that chimpanzees and humans show distinctive patterns of up-regulation in particular genes: those involved in aerobic energy metabolism and those related to neuronal functions. This suggests greater neuronal functional activity and metabolic demand in the brains of chimps that is not present in the gorillas or macaques. "The chimpanzee lineage shows at least as much apparent regulatory evolution as does the human lineage," authors said.