Scientists estimate that there are more than 400,000 species of plants on earth, at least half of which are edible for humans. Indeed, it is entirely possible that we are capable of eating 300,000 plant species. And yet we consume just a tiny fraction of that. Homo sapiens, the most cosmopolitan of species, one that thrives by virtue of being a generalist, eats only about 200 plant species. Remarkably, a mere three crops – maize, rice, and wheat – account for more than half of the calories and proteins that we derive from plants.
Strangely, there have been few attempts to explain why we consume so few of the species that are possible to eat. Taste is not the answer. Nor is nutritional value. The plants we eat have been enhanced by generations of selection, in which farmers have favoured those with the most palatability, the greatest nutritional value, and the highest yield. As much as one might hate broccoli, it is likely to be far tastier than most of the 300,000 alternatives. Wild plants taste like wild plants because they still are wild plants. But why is that?
In Guns, Germs, and Steel, the geographer and science writer Jared Diamond argues that the explanation for our limited menu lies within the plants themselves. Diamond argues that when agriculture was still in its infancy, our ancestors were remarkably efficient at identifying the very few species that were suitable for domestication – by which Diamond means not being poisonous.
The logic seems impeccable, and it is true that the vast majority of the planet’s 400,000 plant species do contain chemical defences (poisons) against grazing herbivores. But, unfortunately for Diamond’s theory, many of our most important crops are also stuffed full of toxins, to the extent that if they were introduced today, they would most likely be banned for human consumption.
Examples include the tomato, its relative the potato, and many other root crops, such as cassava, which contains cyanide; taro, which is full of oxalates; and yams, which produce defence chemicals that mimic female hormones. Indeed, many of the plants we love to grow and eat, including chilies, mustard, horseradish, and wasabi, are attractive precisely because they are packed with potentially harmful chemicals.
What distinguishes food crops from other plants has nothing to do with taste, nutrition, or whether they contain poisons. The plants we eat are atypical because of their particularly dull sex lives.
Many biologists believe that the reason there are so many species of flowering plants is that each has become dependent on a unique species of insects that coevolved to pollinate it. In other words, these are plants with elaborate sex lives. The more unusual the mechanism of insect pollination, the greater the genetic separation among plant populations becomes, almost as if they had evolved on different islands.
This explains why there are roughly 25,000 orchid species. Orchids are the kinky exhibitionists of the botanical world. Many of them have extremely elaborate flowers that have evolved to trick male bees or wasps into trying to copulate with them, thereby ensuring that they are regularly pollinated.
This explains why we don’t farm orchids for food. Seducing bees and wasps might work well for a few individual flowers, but it would never work on an agricultural scale. There would never be enough male wasps to pollinate an entire crop; and if there were, they would soon tire or wise up. Furthermore, because wasps are not found everywhere, an orchid crop could never be pollinated outside its original range.
Most food crops, by contrast, can be pollinated by different types of insects. They can be successfully cultivated around the world, using whatever insects are available to pollinate them. The most common crops of all – wheat, maize, and rice – are grasses that rely on the wind for pollination. Others, such as potatoes and yams, are spread vegetatively and are rarely propagated from seed at all, while some crops that would naturally be insect-pollinated, such as oilseed rape, become wind pollinated when they are grown on an industrial scale.
A more adventurous plant diet is possible. But it would have to accommodate the quirky sex lives of what we include in it.
John Warren is professor of Botany at Aberystwyth University and the author of The Nature of Crops. ©Project Syndicate, 2016. www.project-syndicate.org.