Young genes in young humans provide clue to brain evolution

Genes that other species do not possess may play a crucial role in making the human brain what it is. Until recently scientific consensus held that the different use of genes shared across most of the animal kingdom gave each species' brain its unique character. However this hypothesis may need some revision following a study led by Professor Manyuan Long of the University of Chicago.

Professor Long's team also report the as yet unexplained finding that these human-specific genes are particularly active during fetal development.

For the study, published in PLoS Biology, scientists merged a database containing information on how old genes are in evolutionary terms with data on when and where genes are expressed in humans and in mice. Genes are considered to be 'young' if they came into being shortly before an animal species or family emerged, and such genes will often be species or family-specific.

The researchers found that a higher percentage of young genes was expressed in the human brain compared with those of mice. Furthermore, the young human genes were particularly active in 'higher' brain structures — the very structures that are thought to be key in making humans' brains so much more powerful than animals'.

Human-specific genes were 'upregulated' (active at markedly higher levels) in the pre-frontal cortex - the 'thinking' part of the brain which is larger in humans. Older, primate-specific genes were upregulated in the neocortex, the area which houses the pre-frontal cortex. As far as our understanding of evolution goes, it would seem that these newer genes came into being at around the same time as the newer brain regions.

It may well be that these new genes were the spurs for the development and growth of the neocortex and pre-frontal cortex but as Dr Eric Vallender, a neurogeneticist at Harvard Medical School who was not involved in the study, told the Scientist: 'You always have the correlation versus causation question. But it's very consistent that these genes were all arising at the same time as these new anatomical structures that we know are very important in cognition and behaviour'.

Professor Long's team also looked at when — during human development, rather than over an evolutionary timescale — these new genes were expressed. Their discovery that the young genes were more likely to appear during fetal or infant development was unexpected.

'What's really surprising is that the evolutionary newest genes on the block act early', said study co-author Patrick Landback. 'The primate-specific genes act before birth, even when a human embryo doesn't look very different from a mouse embryo. But the actual differences are laid out early'.

However, the function of many of these new genes remains obscure. Fellow co-author Dr Maria Vibranovski said she hoped the work would 'open a window such that people will start working on these new genes to try to figure out what exactly [they do]'. Until then the question of why these genes are apparently so important in early development remains unanswered.