The concept of how intelligence is a heritable trait is a hot topic among geneticists; particularly for the evolutionary geneticists, who want to distinguish what makes us Homo sapiens (the taxonomic name for human beings) to our Homo ancestors, chimpanzees and other apes. Humans are unique for having such superior intelligence, with a brain of the volume of 1330 cm3 ; in contrast, that of a chimpanzee has a range between 282–500 cm3. What caused that?
A paper published just a few days ago by Cell found the gene that was associated with the human intelligence boom. NOTCH2NL genes are involved in the expansion of neurons (cells that are involved in transmitting information and found in abundance in the brain), and it was shown that there were versions of this gene that were present in humans, but not in other animals.
NOTCH2NL increases the number of cortical stem cells. These cells are special because they haven’t differentiated yet; this means they haven’t decided what cell they are yet, and what cell they end up being is dependent on what signalling occurs. As there are more of these cells, this delays the cells which ‘decide’ to be neurons, but the result of more of these stem cells is that there is a larger number of neurons and consequently bigger brains than our ancestors.
As you can see in the figure to the right (from Cell), NOTCH2NL is not found in Ancestral signalling and it is unique to humans. NOTCH2NL causes an increase in radial glia (cells capable of making neurons) activity, and the result is that more neurons can be generated.
What is also pretty awesome about finding genes linked to increased intelligence in humans is that they could be used to look at neurological disorders. Humans are prone to far more neurological problems than animals, and one of the obvious reasons behind this is that we have been able to extend our life expectancy to a point where the brain begins to malfunction. Looking at these genes would enable us to determine where brain development goes wrong, resulting in these disorders.
There have been numerous genes that have been shown to be implicated in human brain expansion, but this study was slightly different as they were able to monitor exactly HOW these genes caused such expansion. They may even be able to pinpoint the protein reactions that are responsible. The lab developed a method of growing neurons in a lab dish which allowed them to track more genes involved in brain expansion.
It was concluded that around 14 million years ago, an ancestral NOTCH2 gene was copied (and subsequently duplicated) during DNA replication. Although this gene was nonfunctional, so had no impact on gene expression, around 11 million years later an additional piece of NOTCH2 was inserted into this copy and the gene became functional. The function of this gene resulted in increased brain size in humans.
This finding has split the opinions of the scientists that discovered its role in evolution. Whilst some argue that there isn’t enough evidence for an evolutionary difference in gene function between humans and their ancestors, others say that it possible for some genes to have been more influential in brain growth and that possibly NOTCH2NL is one of these genes.