No one would dream of knowingly investing their hard-earned financial resources in a bank that did not take adequate security precautions. Similarly, plants which survive the winter or arid seasons by hoarding food supplies have evolved many different mechanisms for protecting their investments from thieving herbivores. Plants which produce storage organs full of carbohydrates must have seemed attractive to our hunter-gatherer ancestors and to early farmers in search of potential crops to cultivate. However, unlike fruit, which are ‘designed’ to be eaten because they have evolved as a method of seed dispersal, storage organs tend to be unpalatable or even toxic. For example wild and green potatoes contain poisonous alkaloids and cassava tubers are full of cyanide. Such, plant chemical defences have generally been a problem to those attempting to either domesticate or exploit species containing them. On other occasions man has discovered novel ways of utilising these compounds for his own ends; for example the tubers of yams have been harvested for their toxins to poison arrows, to poison in-laws, to catch fish, and to apply to other crops as an insecticide. In more recent times, large quantities of wild yam tubers have been collected commercially for an even more unusual purpose.
Yams grow wild all around the world, scrambling through other vegetation, with some vines twisting clockwise, and others anticlockwise, depending on species. Plants are usually single sexed, producing either all male, or all female flowers, but with both types producing large subterranean tubers. Wherever yams have occurred, they appear to have attracted the attention of man, with different species being domesticated in Asia, Africa and South America. The greater yam, the lesser yam, the Chinese yam and the bitter yam were all first cultivated in Asia; the yellow and white guinea yams and cluster yams are of African origin. The potato yam, which produces strange aerial tubers, occurs wild in both Asia and Africa, and appears to have been cultivated independently in both regions. In South America only the cush-cush yam was considered worthy of domestication but other species may have been collected from the wild. Even Britain has a native yam, called black bryony. Although it is now generally considered to be toxic, bryony tubers, measuring up to 60 cm, were eaten from prehistoric times, even to this day some French cookery books describe how to detoxify them, by prolonged soaking and boiling. The French name for the plant is herbe aux femmes battues, or literally the battered wives’ herb. Within the UK and the US the tuber most commonly called a yam is in fact the unrelated sweet potato, which is actually related to the ornamental morning glory.
The majority of the world’s yams are grown and consumed in the ‘yam belt’ of West and Central Africa, in the region east from Ivory Coast through Ghana, Togo, Benin and Nigeria to Cameroon. In spite of yam cultivation being labour intensive and yields being low, yams have been particularly valuable in the tropics, because they are easy to handle and store well for several months. Their keeping properties made them ideally suited for ships’ stores in the days of sail, in this manner yams were transported from Africa to the Americas as provisions onboard slave ships and similarly Asian yams were dispersed across the Indian and Pacific Oceans in the pre-European period.
Within the last one hundred years wild yams have been harvested for a new and unexpected purpose. The story dates from 1933 when the Eastman Kodak Company first isolated the human hormone progesterone. Huge quantities of cattle brains were required to produce a few grams of the desired compound. It was quickly realised that progesterone had great potential as a contraceptive, as it was found to halt ovulation in laboratory rabbits. The discovery that the hormone could be synthesised from various naturally occurring plant compounds stimulated a search, which was described by an American pharmacology newsletter as a ‘story warranting a Hollywood movie, full of intrigue, deception, scandal, corporate envy and trickery, bribery even violent harassment and murder’. Although 250, plants containing progesterone like compounds were identified, including: red clover, liquorice, fennel and soybeans, the first commercial synthesis of progesterone for use as an oral contraceptive was from diosgenin, derived from wild Mexican yams. This simple fact appears to have generated a plethora of stories and claims about the medicinal properties of wild yams.
Lotions and potions containing wild yam extracts or ‘natural progesterone’ are marketed as alternative medicines as a form of hormone replacement therapy, or to ease the discomforts of pre-menstrual tension, and even as contraceptives. Stories are told of South American Indians, who for centuries were able to regulate their own fertility by chewing on yams. The truth behind all these tales is that the compounds found within yam tubers are grazing deterrents and not active human hormones. Although yams have been used in the synthesis of the Pill, there are several complex chemical changes involved in the conversion of the naturally occurring diosgenin to progesterone and these are chemical changes that the human body is unable to perform for itself. If you were considering eating yams as a form of birth control, you may just as well have a box of liquorice all-sorts or rely on a lucky four leafed clover.