It occurred about 23 million years ago, just before the hominoid superfamily that eventually produced humans branched off. Crucially, the timing approximately coincides with the development of full colour vision in Old World primates, thereby giving a major boost to the theory.
Zhang and colleague David Webb used a gene called TRP2 - unique to the pheromone pathway - to track the evolution of the system in primates. In humans, TRP2 has accumulated so many errors in its DNA sequence that is now a "pseudogene" that is no longer active.
It is also a pseudogene in Old World monkeys, such as the baboon and guereza, and apes including the chimpanzee and gorilla. But in New World monkeys, like the tamarin, squirrel monkey and saki, the team found TRP2 was completely functional and bestows a sharp ability to detect pheromones.
A computer simulation of random gene decay estimated the time of TRP2's shutdown at just over 23 million years ago. And this matches the estimated time at which male Old World monkeys developed the full colour vision that their New World cousins still lack.
In order to see the world in full colour, two copies of a colour vision gene are needed - one red, one green. The genes reside on the X chromosome, so female monkeys have long had full colour vision, while males have not.
But when male Old World monkeys gained a second colour vision gene, about 23 million years ago, it meant a new approach to mate selection was possible. Rather than pheromones, "sexual swellings" - gaudy, colourful patches of skin - could be used to signal female reproductive fitness and fertility.
"We think there might be some advantage to using a vision based signalling system, compared with a pheromonal one, because you can see colour from a distance," says Zhang. With pheromones, he notes, a mate has to close enough for the pheromones to drift through the air and be detected.
Zhang adds that red/green colour blindness is relatively common in humans compared with Old World monkeys and apes - with about seven per cent of male Europeans being colour blind. This might be because the need to very carefully select mates by visual means has become "relaxed" in modern humans, he suggests.
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