Non-coding DNA drove human brain evolution by making nerve cells stickier

Most of our genome is made up of the poorly named ‘junk DNA’. New research shows that these sequences may have been vital in the evolution of human brains, by allowing our neurons to make better contacts with each other.

DNA, and little of it is 'junk'.Two months ago, a group of scientists found that the gene that has evolved fastest since our evolutionary split from chimpanzees is found in our so-called ‘junk DNA’. DNA is a code that tells our cells how to build their molecular workforce – proteins.

But the vast majority of our DNA sequence is never translated into proteins. While some considered this ‘junk’ DNA to be meaningless, recent research has shown that it makes important contributions to our most human of organs – our brains.

Now, Shyam Prabhakar and James Noonan at the Lawrence Berkeley National Laboratory have found further proof of the link between non-coding DNA and our mental evolution.

They studied over 110,000 stretches of DNA called ‘conserved non-coding sequences’ (CNSs), that are largely similar in a wide variety of animals. Of these sequences, 992 showed large numbers of changes that were specific to humans.

This number is much higher than would be expected if these DNA regions were drifting aimlessly in the evolutionary river. Their frequency is the mark of natural selection – these sequences must have changed for a reason.

To discover what this reason might have been, Prabhakar and Noonan looked at which genes these CNSs were in, and what they do in the body. They found that a large proportion of the genes in question were involved in the adhesion of neurons (nerve cells).

These genes are vital for the growth and development of our brains and allow neurons to make connections with each other, and with their surrounding framework of supportive cells.

The duo found a similar number of CNSs with chimpanzee-specific changes and many of these were also involved in nerve cell adhesion. But there was hardly any overlap between the chimp-specific and human-specific sequences.

Both lineages have developed nerve cells that make better contacts with each other, but have done so in separate ways using different genes.

It is possible that human and chimp brains have evolved different mental abilities to satisfy different evolutionary pressures. Identifying the precise role of the human-specific CNSs will help to test this possibility and it is the next big challenge facing Prabhakar and Noonan.

In the meantime, this research once again shows that non-coding DNA, far from being useless junk, was vitally important for the evolution of the human brain and its many unique abilities. Subtle changes in these sequences separate us from even our closest animal relatives.

Prabhakar, Noonan, Paabo & Rubin. 2006. Science 314: 786.
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