The quest to control fertility has been a long one, unsurprisingly given how it is often a matter of life or death for women. Many of the “folk contraceptives” used without scientific verification have been disastrously unsuccessful, but chemicals found in two of them have not only been found to prevent conception, but might have considerable advantages over modern hormonal-based pills.

Given how bizarre and improbable some of the substances pressed into service as contraceptives have been (crocodile dung probably works mainly by preventing people wanting to have sex entirely), it's not entirely surprising scientists often haven't taken them seriously. Yet our ancestors were not fools. Just because folk remedies are much less reliable than products that have passed clinical trials, doesn't mean they have no validity at all.

Dr Polina Lishko of the University of California, Berkeley, investigated whether there might be something to the claimed contraceptive effects of two plants, aloe and Tripterygium wilfordii, the wonderfully named "thunder god vine". She found both contain the steroid-like chemical pristimerin, which blocks sperm's calcium channel. This prevents progesterone from causing sperm's tails to shift from a steady to whip-like motion. Without this shift, sperm struggle to pass through the egg's outer layers to achieve conception.

"It doesn't kill sperm basal motility. It is not toxic to sperm cells; they still can move. But they cannot develop this powerful stroke, because this whole activation pathway is shut down.” Lishko said in a statement. Lishko also found a related plant molecule, lupeol, has similar effects. Lupeol is found in mangoes and olives and has previously been explored as an anticancer agent.

"Because these two plant compounds block fertilization at very, very low concentrations – about 10 times lower than levels of levonorgestrel in Plan B – they could be a new generation of emergency contraceptive we nicknamed 'molecular condoms,'" Lishko said.

In the Proceedings of the National Academy of Sciences, Lishko reports human hormones, including testosterone, estrogen, and hydrocortisone, showed similar effects, but more weakly. Nevertheless, the fact that hydrocortisone, a stress hormone, impedes sperm's passage helps explain why women are less likely to conceive when stressed.

There is some way to go before Lishko's work can be turned into a viable contraceptive, whether for taking before sex or in the hours immediately afterward. Concentrations of pristimerin and lupeol in wild plants are low, making extraction prohibitively expensive. Synthesization, or targetted breed for higher concentrations, are required.

No one wants to repeat the story of Silphium, a plant grown around the north African city of Cyrene, and traded around the Roman-era Mediterranean. Silphium was in such demand, particularly as a contraceptive and abortifacient, that it was overharvested, which probably accounted for its eventual extinction.