NOT A CHIMP The hunt to find the genes that make us human.
Jeremy Taylor
Oxford University Press, hbk
The prospect of reading the entire genome (DNA sequence) of a species once bordered on fantasy, but new technology quickly rose to the challenge.
With automated chemistry and clever computing techniques, the assembly of 3 billion information units (nucleotides) from short scraps of DNA has become close to routine.
The findings, though, have been full of surprises.
As superior humans we used to assume we had far more genes than simple species, perhaps about 150,000.
The shock reality is that we have fewer than 25,000, not many more than some dreary worms and insects.
Humans and chimps differ by some hundreds ofnon-equivalent genes, but most genes are similar to a surprising extent, about 98.4%.
Does that mean chimps are 98.4% human?
It depends on your (or the chimp's) point of view.
The obvious similarities are so compelling that non-human hominids have special protection under section 85 of our Animal Welfare Act 1999, and narrowly missed gaining quasi-human rights under the Act.
Leaving aside technical arguments as to how the similarity should be calculated, life's diversity, particularly among vertebrates, amounts to variations on a theme.
What developmental variations have endowed humans with their distinctive abilities including speech, language and reasoning?
Contrary to popular belief, DNA is not a construction diagram, but more a set of instructions on how a single fertilised cell shall divide repeatedly into a thousand-billion-cell adult.
A slight change in a key gene, which in turn may regulate a large cluster of genes, can direct the process down a different pathway and lead to huge differences in the adult, such as to distinguish a human from an ape.
Which are these crucial genes?
We cannot experiment with genes of humans (or hominids) to see what happens.
But exceedingly rarely, medical science stumbles upon an individual or family, sometimes an entire sub-population, in which nature has fiddled with a gene.
With their gracious co-operation, the consequences seen in persons may be traced to a microscopic change in their DNA.
The author relates fascinating real-life accounts of how aspects of language, speech, fine co-ordination and brain size have been narrowed down to DNA differences of a mere letter or two among 3 billion, usually through the knock-on effect upon a host of related genes.
In any case, to what extent is brain function truly different between humans and apes (and indeed other species)?
Which parts of the brain relate to which mental abilities?
The experimental approach is ethically limited to problem-solving tests and non-invasive scans, but once again the impossibility of invasive experiments has been circumvented by studying rare survivors of localised brain trauma.
Jeremy Taylor tells the story in a personalised and engaging manner, explaining the progress in accessible scientific detail but also impinging on a sadder side of science.
Where major advances will obviously flow from the serendipitous discovery of an unusual family, requiring the complementary expertise of clinicians, psychologists, neurologists, geneticists and molecular biologists, a certain amount of possessiveness and jealousy can creep in.
Perhaps there are genes for these failings, somewhere.
• Clive Trotman is a Dunedin science writer and arbitrator.
