About 8 million to 12 million years ago, the ancestor of great apes, including humans, underwent a dramatic genetic change. Small pieces of DNA replicated and spread across their resident chromosomes like dandelions across a lawn. But as these dandelion seeds dispersed, they carried some grass and daisy seeds additional segments of DNA along for the ride. This unusual pattern, repeated in different parts of the genome, is found only in great apes bonobos, chimpanzees, gorillas and humans.

I think its a missing piece of human evolution, said Evan Eichler, a geneticist at the University of Washington, in Seattle. My feeling is that these duplication blocks have been the substrate for the birth of new genes.

Over the past few years, scientists have begun to uncover the function of a handful of genes that reside in these regions; they seem to play an important role in the brain, linked to the growth of new cells, as well as brain size and development. In September, Eichlers team published a new technique for analyzing how these genes vary from person to person, which could shed more light on their function.

Clare McLean

Evan Eichler, a geneticist at the University of Washington in Seattle, proposes that bursts of genetic change in our great ape ancestors played a major role in ape and human evolution.

Much about the duplication process and its implications remains a mystery. Eichler and others dont know what spurred the initial rounds of duplications or how these regions, dubbed core duplicons, reproduced and moved around the genome.

Despite the duplication-linked genes potential importance in human evolution, most have not been extensively analyzed. The repetitive structure of the duplicated regions makes them particularly difficult to study using standard genetic approaches the most efficient methods for sequencing DNA start by chopping up the genome, reading the sequence of the small chunks and then assembling those sections like one would a puzzle. Trying to assemble repetitive sections is like trying to put together a puzzle made of pieces with almost the same pattern.

Because these regions are so complex, they are often ignored by conventional genome studies, and some regions still havent been fully sequenced, said James Sikela, a geneticist at the University of Colorado School of Medicine in Aurora. So not only are they important, they are unfortunately unexamined.

A Genetic Burst

In 2007, Eichler and his collaborators took on what seemed like a herculean task looking comprehensively at the repetitive stretches of the human genome. Previous studies had characterized individual regions, but Eichlers team employed new computational techniques and comparative genomics comparing DNA sequences from different species to examine the entire genome. Mathematical analysis published in Nature Genetics that year revealed a set of core duplicons stretches of DNA that appear over and over on a specific chromosome.

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A Missing Genetic Link in Human Evolution | Simons Foundation