"Long before it's in the papers"
January 27, 2015


“Junk DNA” may help explain human-chimp differences

Oct. 25, 2011
Courtesy of Georgia Tech
and World Science staff

For years, sci­en­tists thought an ex­plana­t­ion would soon turn up for the vast dif­fer­ences be­tween hu­mans and their clos­est rel­a­tives among the an­i­mals, such as chim­panzees. The dif­fer­ence must be in the genes, bi­ol­o­gists rea­soned.

But this hy­poth­e­sis ran in­to trou­ble when it lat­er emerged that hu­man and chimp genes are nearly iden­ti­cal. So what’s go­ing on? 

Re­search­ers say a ma­jor part of the ex­plan­ation for human-chimp diff­erences lies in re­gions of DNA out­side those tra­di­tion­ally con­sid­ered genes. (Image courtesy of Georgia Tech)

Re­search­ers at the Geor­gia In­sti­tute of Tech­nol­o­gy have now con­clud­ed that a ma­jor part of the an­swer lies in re­gions of DNA out­side those that are con­sid­ered genes. This ma­te­ri­al was called junk DNA only a few years ago be­cause it had no dis­cern­i­ble func­tion: un­like genes it does­n’t pro­vide code for pro­duc­ing pro­teins, the myr­i­ad mo­le­cules that do much of the day-to-day work of keep­ing the or­gan­ism tick­ing.

But much “junk DNA,” now more of­ten called non-coding DNA, does have an im­pact on how hard reg­u­lar genes work, as well as where and when they do so. That func­tion, more than the gene se­quences them­selves, may ac­count for many of the key dif­fer­ences be­tween us and our ev­o­lu­tion­ary rel­a­tives, the bi­ol­o­gists are now pro­pos­ing.

“Our find­ings are gen­er­ally con­sist­ent with the no­tion that the mor­pho­log­i­cal [phys­i­cal] and be­hav­ior­al dif­fer­ences be­tween hu­mans and chim­panzees are pre­dom­i­nately due to dif­fer­ences in the regula­t­ion of genes rath­er than to dif­fer­ences in the se­quence of the genes them­selves,” said Geor­gia Tech bi­ol­o­gist John Mc­Don­ald, who led the new re­search.

His team ex­am­ined a ma­jor type of non-coding DNA called retro­trans­posons, which com­prise about half of the genomes of both hu­mans and chimps. Retro­trans­posons, al­so called trans­pos­a­ble el­e­ments, are strips of DNA that over ev­o­lu­tion­ary his­to­ry in­sert them­selves in­to and de­lete them­selves from var­i­ous places in our the larg­er DNA struc­ture—a be­hav­ior not un­like that of cer­tain vi­rus­es.

Retro­trans­posons near genes are highly var­i­a­ble be­tween hu­mans and chimps and may ac­count for ma­jor dif­fer­ences be­tween the two spe­cies, Mc­Don­ald as­serts. The team’s find­ings are re­ported in the cur­rent is­sue of the re­search jour­nal Mo­bile DNA. “Trans­pos­able el­e­ments were once con­sid­ered ‘junk DNA’ with lit­tle or no func­tion. Now it ap­pears that they may be one of the ma­jor rea­sons why we are so dif­fer­ent from chim­panzees,” Mc­Don­ald said.

The re­search team ex­am­ined re­tro­trans­poson-rich ar­eas be­tween genes, known as ge­no­mic gaps, in both spe­cies and found that they’re sig­nif­i­cantly cor­re­lat­ed with dif­fer­ences in gene ac­ti­vity re­ported pre­vi­ously by re­search­ers at the Max Plank In­sti­tute for Ev­o­lu­tion­ary An­thro­po­l­ogy in Germany.

The Geor­gia Tech re­search­ers al­so said their own proj­ect was mo­ti­vat­ed by a 2009 study of theirs find­ing that peo­ple’s high­er can­cer rates rel­a­tive to chimps may have been a byprod­uct of ev­o­lu­tion­ary pres­sure for larg­er brains.

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For years, scientists thought an explanation would soon turn up for the vast differences between humans and their closest relatives among the animals, such as chimpanzees. The difference must be in the genes, biologists reasoned. But this hypothesis ran into trouble when it later emerged that human and chimp genes are nearly identical. So what’s going on? Researchers at the Georgia Institute of Technology have now concluded that a major part of the answer lies in regions of DNA outside those traditionally considered genes. This material was called junk DNA only a few years ago because it had no discernible function: it doesn’t provide code for producing proteins, the myriad molecules that do much of the day-to-day work of keeping the organism ticking. But much “junk DNA,” now more often called non-coding DNA, does have an impact on how hard regular genes work, as well as where and when they do so. That function, more than the gene sequences themselves, may account for many of the key differences between us and our evolutionary relatives, the biologists are now proposing. “Our findings are generally consistent with the notion that the morphological [physical] and behavioral differences between humans and chimpanzees are predominately due to differences in the regulation of genes rather than to differences in the sequence of the genes themselves,” said Georgia Tech biologist John McDonald, who led the new research. His team examined a major type of non-coding DNA called retrotransposons, which comprises about half of the genomes of both humans and chimps. Retrotransposons, also called transposable elements, are strips of DNA that over evolutionary history insert themselves into and delete themselves from various places in our the larger DNA structure—a behavior not unlike that of certain viruses. Retrotransposons near genes are highly variable between humans and chimps and may account for major differences between the two species, McDonald asserts. The team’s findings are reported in the current issue of the research journal Mobile DNA. “Transposable elements were once considered ‘junk DNA’ with little or no function. Now it appears that they may be one of the major reasons why we are so different from chimpanzees,” McDonald said. The research team examined retrotransposon-rich areas between genes, known as genomic gaps, in both species and found that they’re significantly correlated with differences in gene activity reported previously by researchers at the Max Plank Institute for Evolutionary Anthropology in Germany. The Georgia Tech researchers also said their own project was motivated by a 2009 study of theirs finding that people’s higher cancer rates relative to chimps may have been a byproduct of evolutionary pressure for larger brains in us.