Posted Mon Sep 10, 2007 10:00am AEST
Sprinters such as Jamaica's Asafa Powell have a different genetic make-up to endurance athletes. (AFP: Emiliano Grillotti)
Map: Sydney 2000 If you were a prehistoric human, would you prefer to able to sprint very fast for short distances or to jog comfortably for kilometres?
That is one of the questions thrown up by the so-called "gene for speed," known as ACTN3.
One of the most intriguing genes discovered, ACTN3 encodes a protein that governs metabolism in "fast twitch" muscle fibres, which generate force at high speed.
Around 18 per cent of the world's population has a truncated variant of the gene that blocks this protein.
The stubby variant, called R577X, is common among successful endurance athletes, previous research has found.
On the other hand, elite sprinters, who need explosive speed, are likelier to have the reverse - a functioning variant of ACTN3.
Keen to find out more, researchers led by Kathryn North, a professor at the Children's Hospital at Westmead in Sydney, created a batch of mice that had been engineered to lack ACTN3.
The "knockout" mice and ordinary mice with a functioning ACTN3 gene were put on a motorised treadmill, which spun ever faster until the luckless rodents were exhausted.
The easy winners in this endurance test were the knockout mice, which were able to run on average a third further than their counterparts.
The apparent reason for this: the loss of ACNT3's protein was compensated by a different protein, called alpha-actinin-2, which shifted muscle metabolism towards a smoother, more efficient, aerobic pathway.
As a result, fast-twitch leg muscles could be contracted again and again, without tiring.
Professor North's team also looked through genetic profiles from individuals of European and East Asian descent and found that there was remarkably little sign of mutation in the wider stretch of genetic code in the vicinity of R577X.
Such similarity is a telltale sign of what evolutionary experts call positive selection. Genes which help the fight for survival get lastingly incorporated in the human genome, whereas those that encumber it get weeded out.
In other words, the ability to run longer distances became a preferential trait that became incorporated into a wide swathe of Homo sapiens.
If so, the incorporation happened recently, on the long scale of human history.
According to Professor North's calculations, R577X took root among populations in central Europe around 15,000 years ago and in East Asia around 33,000 years ago.
The variant has not been incorporated in all of us, either because so little time has elapsed for this to happen or is being countered by selective pressures in favour of other genes, they speculate.