A new "fertility first" hypothesis published this week by a group of international experts in the American Journal of Human Biology, proposes that the global epidemic of Type 2 diabetes has its origins in the struggle, over millennia, to sustain human fertility in environments defined by famine.
A surprising and important implication for us in the modern world is that this hypothesis gives cause for optimism that the modern epidemics of diabetes and cardiovascular disease will diminish.
"If we are right, rapid human evolution should, in decades rather than centuries, reduce the prevalence of the conserved ancestral genes which underpin these conditions," said Associate Professor Stephen Corbett from the University of Sydney, one of the study's authors.
The key piece of evidence in support of this idea comes from a close look at a condition closely related to Type 2 diabetes, the Polycystic Ovary Syndrome (PCOS). This condition is globally the most common cause of ovarian infertility and its very existence is a paradox because evolutionary theory would predict that a genetically based condition which reduces fertility would quickly erase itself.
In women with PCOS fertility declines with increases in body weight and improves with weight loss. It is this property in particular which suggests that PCOS may be associated with a fertility advantage, rather than disadvantage in women who are undernourished. "We have called this the Fertility First genotype because for much of human history these genes, if present, enabled hungry and undernourished people to continue to reproduce in lean times," Associate Professor Stephen Corbett said.
However, since the Industrial Revolution, improvements in nutrition and increases in body weight for a human majority have flicked an evolutionary switch. "The long standing fertility advantage of this Fertility First genotype has been turned on its head, and it is now a disadvantage, and should become less common," he said.
The authors this paper are Associate Professor Stephen Corbett from the University of Sydney and Sydney West Area Health Service, Professor Tony McMichael from the Australian National University in Canberra and Professor Andrew Prentice from the MRC International Nutrition Group at the London School of Hygiene and Tropical Medicine.
Their article, Type 2 Diabetes, Cardiovascular Disease and the Evolutionary Paradox of the Polycystic Ovary Syndrome: A Fertility First Hypothesis goes on line this week in the American Journal of Human Biology 2009.