One of the important question in the field of evolution is to understand : What makes us humans? It is well approved now that humans and chimpanzees originate from a common ancestor before diverging from each other some 6 million years ago.Charles Darwin was not provocative in saying that we descended from apes ,indeed we are apes in every form our long arms,habits,temperament and tailless bodies. In order to understand better what differs between humans with their closest living relatives chimps ,scientists have sequenced the genome of Chimpanzees in 2005 and found that humans are 96% similar to the great ape species.Despite similarities scientists have identified some differences in genome that could account for human specific traits. Infact even before the genome sequence it was generally agreed that difference between chimps and humans was due to changes in non protein coding sequences.( Read an excellent article on this by King and Wilson , 1975 )
DNA can be divided into two parts 1) Which codes for proteins 2) Non coding DNA .This second part of DNA regulate the first part,the coding region. Enhancers are the important part of DNA and are located usually away from the target gene.These enhancers decide when to activate a particular gene and in which tissue .As the genome sequence in all cells are same so there should be some control that proteins required for function of eyes are not expressed in Kidneys. Enhancers does this job to perfection with the help of some other proteins,prominent being the “transcripton factors”. When the DNA is looped ,these enhancers which lie far off from gene to be regulated comes in close proximity and Transcription factors/ other proteins (activators or repressors) bind to that region to regulate the gene ( either activate or shut off the gene)
In a bid to understand this primary question James Noonan of Yale University carried out research on a conserved non coding sequence (HACNS1) that evolved rapidly in humans and function as enhancer for a gene expressing in limbs.But the same region from chimps and rhesus macaque yielded only faint expression in the limbs of transgenic mouse embryos.

Udine. Bonobo Power!
Originally uploaded by Josef Knecht

This 546 base pair ( DNA is made up of A,T,G and C letters of code) conserved region have gained 16 mutations in humans since the ancestors of chimps and humans separated 6 million years ago.When the human like mutations were created into chimp version of enhancer, then chimp version started behaving like human enhancer by expressing strongly in limbs.So humans,by gaining changes in the enhancer region might create novel sites for transcription factors and hence modulation of gene expression ,a key aspect of evolution of new species. Changes like these might have been responsible for developments that allow humans to speak and walk upright.

These binding sites for transcription factors play a very important role in gene regulation ,just a change in base might lead to a complete different output. One base change can bring in repression of gene which was normally activated. Not only changes in regulatory regions but also the spacing between two transcription binding sites also influence the activity of regulatory regions as recently shown by Sean carroll’s group.

” There are many ways to tinker with them and get a different output” — Sean Carroll

Reference:

Prabhakar S, Visel A, Akiyama JA, Shoukry M, Lewis KD, Holt A, Plajzer-Frick I, Morrison H, Fitzpatrick DR, Afzal V, Pennacchio LA, Rubin EM, Noonan JP.
Human-specific gain of function in a developmental enhancer.
Science. 2008 Sep 5;321(5894):1346-50.

Update : Listen to what the authors talk about their discovery

Related Posts:

• Chimpanzees are not our ancestors
• Two new books from springer – Evolutionary Biology
• Provost Lecture – Carl Zimmer: Darwin, From Birth to Death
• The Ape that took over the World
• A talk by Sean B Carroll : Year of Darwin

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