Now that the human genome has been sequenced, the hunt is on to find the genetic changes that led to the evolution of our own species, Homo sapiens. Researchers have recently found genes that may have endowed humans with larger brains and the ability to speak. Now a research group has uncovered evidence that another gene may have given the brains of apes, including humans, a major cognitive boost millions of years ago.
The gene, called GLUD2, encodes glutamate dehydrogenase (GDH)--an enzyme that helps recycle one of the brain's most important neurotransmitters, glutamate. But there are actually two types of GDH: the one coded by GLUD2, which is found mostly in nerve tissues, and a second type, coded by a gene called GLUD1, which is found in many different cells and performs a variety of functions.
In a paper published online on 19 September in Nature Genetics, Fabien Burki and Henrik Kaessmann, two genome researchers at the University of Lausanne in Switzerland, report that the brain-specific gene, GLUD2, is found only in apes and humans but not in Old World Monkeys, which only have GLUD1. Moreover, after the ape-human and Old World monkey lineages went their separate evolutionary ways about 23 million years ago, GLUD2 underwent a number of changes that may have enhanced its ability to recycle glutamate.The researchers concluded that GLUD2 arose when GLUD1 underwent a duplication and then was reinserted elsewhere in the genome. Although GLUD1 has remained the same in both apes and Old World Monkeys, GLUD2 underwent a number of alterations in its DNA sequence within the first few million years of its creation. Two of these, which are key to its greatly enhanced glutamate-recycling functions, are found in all apes including humans. Burki and Kaessmann conclude that GLUD2 "probably contributed to enhanced brain function in humans and apes by permitting higher neurotransmitter flux" and could have been important in the early evolution of increased cognitive capacities, including greater memory storage.This study "makes a very nice story," says molecular geneticist Wolfgang Enard of the Max Planck Institute for Evolutionary Anthropology in Leipzig, Germany. But he notes that the cognitive advantages conferred by GLUD2 would have been shared by all apes, meaning that their real significance might have been to help create the early brain improvements which served as the scaffolding on which human cognition was later built.