Wayne State University study shows humans and elephants took similar evolutionary paths to develop large brainsApril 26, 2010
Morris Goodman, Ph.D., distinguished professor of anatomy in WSU's School of Medicine and resident of Oak Park, Mich., and co-author Derek Wildman, Ph.D., assistant professor of molecular medicine and genetics and of obstetrics and gynecology in WSU's School of Medicine and resident of Ferndale, Mich., were published in a study in a recent edition of the Proceedings of the National Academy of Sciences. The study showed that humans and elephants underwent a similar accelerated burst of evolution in a suite of genes responsible for aerobic energy production. Not seen in small-brained relatives of either humans or elephants, these gene adaptations appear to have played an instrumental role in the development of two of the largest brains in the animal kingdom.
The study was made possible by the expanding set of publicly available genomes. "In the genome age that we're in, there's a great opportunity to elucidate the evolutionary steps that resulted in the origin of humans," Goodman said.
"In the case of this study, we were trying to understand what genetic changes made this big brain of ours possible. A good starting point for answering a question like this is to take other species with the same trait and look for genetic adaptations we have in common."
Goodman's research group investigated the patterns of adaptive evolution in elephants, humans and a small-brained evolutionary relative of each: tenrecs, a hedgehog-like relative of elephants, and mice, a relative of humans.
The analysis, which encompassed about 6,000 genes from each lineage, investigated the ratio of nonsynonymous mutations - which are indicative of adaptive evolution - to synonymous or "silent" mutations. They found that humans and elephants both have a high ratio of non-synonymous mutations in a set of genes known as aerobic energy metabolism (AEM) genes. AEM genes control the production of aerobic energy, a process that occurs in mitochondria.
One likely hypothesis for these similar adaptations - also known as convergent evolution - is that increased production of aerobic energy would satisfy the huge amount of energy that the large brains of elephants and humans require. "The study strengthens the evidence for the importance of these genes in the development of a large brain," Goodman said.
The next steps in this line of research will include the investigation of the specific nucleotide and protein replacements that comprised this genetic evolution. It also opens the door for similar genetic studies comparing humans to mammals of other lineages. "It will be very interesting to see if other large-brained, social mammals such as dolphins have the same adaptations in AEM genes," Goodman said. "These studies, among others, will further elucidate the genetic evolutionary mechanisms that resulted in characteristics that define us as humans and shaped the animal kingdom as we know it."
Goodman's research team included participants from Wayne State University, University of Michigan, The George Washington University and Mount Sinai School of Medicine. To view the full paper, visit http://www.pnas.org/content/106/49/20824.long.
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