Genes that make us human

What is it that makes us human? This is no longer just a metaphysical question for philosophers to ponder over and debate but one that biologists are now able to seriously address in the laboratory. With the genomes of more and more organisms being sequenced, it is possible to compa re genomes and study what changes have occurred – or not occurred – over time. Such research is necessarily firmly rooted in evolutionary theory that Darwin first enunciated 150 years ago and which others have improved upon since. At one end of the spectrum, there are some 500 genes that humans share with all other living organisms, from bacteria and micro-organisms known as Archaea to fungi, plants and animals. Standing the test of time “The genes these organisms all share have withstood more than 2 billion years of the steady bombardment of mutation and stand out as threads of text whose sequence and meaning have not changed significantly despite the vast differences among the species that carry them,” observed geneticist Sean Carroll in his book “The Making of the Fittest.” In these “immortal” genes, natural selection has worked to prevent changes that would affect the structure of the vital proteins they generate. Comparative study At the opposite end of the spectrum, biologists can look at the changes that occurred in the primate lineage that led to the evolution of humans. A paper that has just been published in the journal Nature shows the kind of insights that can be derived by comparing genomes of different primates. A team of scientists led by Howard Hughes Medical Institute investigator and University of Washington geneticist Evan Eichler examined the genomes of humans, chimpanzees, orang-utans, and macaques. All these primates have descended from a common ancestral species that lived about 25 million years ago. The scientists looked specifically at the way segments of DNA in the genome of these animals had become duplicated. “Duplications are [an] important source of genetic variation associated with genetic disease and with the birth of novel gene and gene families within species,” said Dr. Eichler in an email. The researchers found that roughly 10 million years ago, a major genetic change occurred in a common ancestor of gorillas, chimpanzees, and humans, explained a press release issued by the Howard Hughes Medical Institute. Segments of DNA in its genome began to form duplicate copies at a greater rate than in the past, creating an instability that persists in the genome of modern humans. Genetic flexibility Such duplication also may be responsible for a genetic flexibility that has resulted in some uniquely human characteristics, the release added. “These dramatic changes in the genome have also created genome architectures that predispose to rearrangement and diseases such as autism, mental retardation and schizophrenia, observed Dr. Eichler in an email. “Since these duplications are so young, these specific forms of neurocognitive and neurodevelopmental diseases must also be quite young.”

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