The most highly cited alternative to the thrifty gene hypothesis is the drifty gene hypothesis proposed by the British biologist John Speakman. This idea differs fundamentally from all the other ideas in that it does not propose any selective advantage for the obese state, either now or in the past. The main feature of this hypothesis is that the current pattern of obesity does not suggest that obesity has been under strong positive selection for a protracted period of time. It is argued instead that the obesity comes about because of genetic drift in the genes controlling the upper limit on our body fatness. Such drift may have started because around 2 million years ago ancestral humans effectively removed the risk from predators, which was probably a key factor selecting against fatness. The drifty gene hypothesis was presented as part of a presidential debate at the 2007 Obesity Society meeting in New Orleans, with the counter-arguments favouring the thrifty gene presented by British nutritionist Andrew Prentice. The main thrust of Prentice's argument against the drifty gene idea is that Speakman's critique of the thrifty gene hypothesis ignores the huge impact that famines have on fertility. It is argued by Prentice that famine may actually have only been a force driving evolution of thrifty genes for the past 15,000 years or so (since the invention of agriculture), but because famines exert effects on both survival and fertility the selection pressure may have been sufficient even over such a short timescale to generate a pressure for "thrifty" genes. These alternative arguments were published in two back-to-back papers in the "International Journal of Obesity" in November 2008.