Part of the fun of hiking in the woods is the treat of coming across a bush chock-full of wild berries. Even those of us who are city dwellers know that some plants, like wild strawberries and blueberries, are safe to eat--they're sufficiently similar to their domesticated kin that we can easily recognize them, even though the wild berries are much smaller than those we grow as crops. More adventurous hikers may eat mushrooms too, although with caution, aware that many species can be poisonous. But not even the most ardent nut lover among us will eat wild almonds; their lousy taste keeps us away, which is fortunate, since just a few dozen of the wild nuts contain enough cyanide to kill us.
All the modern crops that we grow and eat today had to arise from just such wild plants. Until the advent of farming, every plant eaten by every human on Earth was wild. But what caveman ever got the idea of "domesticating" a wild plant, and how was the transformation accomplished? That question seems especially puzzling for the many crops, like almonds, whose wild progenitors are lethal or bad-tasting, or others, like corn, that look drastically different from their wild ancestors. After all, the first farmers didn't have any existing crop as a model to inspire them to develop other ones. They couldn't have been conscious that, whatever they were doing, there was a tasty result ahead.
Furthermore, why did those early farmers apparently find some plants so much easier to tame than others? Peas, for instance, were domesticated almost 10,000 years ago, but olives not until some 6,000 years ago, strawberries not until the Middle Ages, and pecans not until 1846. Even today many potentially valuable wild plants remain untamed; among them are oak trees, which many people have sought out for their acorns. Why did olive trees yield to Stone Age farmers while to this day oaks continue to outwit our best and brightest botanists?
In searching out the answers to these questions, I owe a special debt to Israeli geneticist Daniel Zohary, whose wonderful recently published book, Domestication of Plants in the Old World (written with Maria Hopf), is the latest word on the subject. Let's begin by looking at domestication from the plant's point of view. As far as plants are concerned, we humans are just one of thousands of animal species that unconsciously "domesticate" plants.
Like all animal species, plant species must spread their offspring to suitable areas where they can thrive and pass on their parents' genes. Young animals generally disperse by walking or flying. Because plants don't have that option, they must somehow hitchhike. Some plant seeds disperse by blowing in the wind or floating on water. Many other plant species, though, trick an animal into carrying their seeds, by enclosing them within a tasty fruit and advertising the fruit's ripeness by its color or smell. The hungry animal plucks and swallows the fruit, walks or flies off, and later defecates or spits out the seeds somewhere far from its parent tree. Seeds can thereby be carried for thousands of miles. It may surprise you to learn that plant seeds can resist digestion by the gut and still germinate out of feces. In fact, some seeds actually require passage through an animal's gut before they can germinate. One African melon species, for example, is so well adapted to being eaten by an aardvark that most of the melons grow on the animal's latrine sites.
Wild strawberries offer a good example of hitchhiking tactics. When strawberry seeds are still young and not yet ready to be planted, the surrounding fruit is green, sour, and hard. When the seeds finally mature, the berries turn red, sweet, and tender. The change in the berries' color serves as a signal to birds such as robins, which then pluck the strawberries, fly off, and eventually spit out or defecate the seeds.
Naturally, strawberry plants didn't set out with a conscious intent of attracting robins only when their seeds were ready to be dispersed. Nor did robins set out with the intent of domesticating strawberries. Rather, strawberry plants evolved through natural selection. The sweeter and redder the final strawberry, the more birds dispersed its ripe seeds; the greener and more sour the young strawberry, the fewer birds destroyed the seeds by eating berries before the seeds were ready.
Many other plants have fruits adapted to being eaten and dispersed by particular animals. Just as strawberries are adapted to birds, acorns are adapted to squirrels, and mangoes to bats. No one would describe this unconscious, evolutionary process as domestication; likewise, the early stages of crop evolution were unintentional also. They involved wild plants developing characteristics that attracted humans to eat and disperse their fruit. Like aardvark latrines, human latrines may have been a testing ground for the first crop breeders.
Latrines, though, were merely one of the many delightful places where our ancestors unintentionally sowed some wild seeds. As we know from our current practices, when we gather edible plants and bring them home, some inevitably spill, either en route or at our houses. Some fruit rots while still containing perfectly good seeds, and gets thrown, uneaten, into the garbage. Of the fruit that we actually take into our mouths, some contain seeds so tiny that they are always swallowed and defecated, but others have seeds large enough to spit out. Thus our ancestors' garbage dumps undoubtedly joined their latrines to form the first agricultural research laboratories.
At whichever such "lab" the seeds ended up, they tended to come from certain individual plants--namely, the individuals that humans preferred to eat for one reason or another. Eventually, when the first farmers began to sow seeds intentionally, they would sow seeds from those individual plants that they had chosen to gather.
What precisely were their criteria?
One, of course, was size--from your berry-picking days, you know it's not worth wading into a thorny, mosquito-infested thicket on a hot, humid day just to get some lousy little berries. As a result of such selection, many crop plants have much bigger fruits than their wild ancestors. Peas, apples, sunflowers, strawberries, and corn provide examples of colossal crops dwarfing their progenitors.
Another obvious criterion was taste. Many wild seeds evolved to be bitter, bad-tasting, or actually poisonous to deter animals from eating them. Thus natural selection acts oppositely on seeds and on fruits. Plants whose fruits are tasty get their seeds dispersed by animals, but the seed within the fruit has to be bad-tasting. Otherwise the animal would also chew up the seed, and it couldn't sprout.
Almonds provide a striking example of the evolution and de- evolution of bitter seeds. Most wild almond seeds contain an intensely bitter chemical called amygdalin, which breaks down to yield the poison cyanide. A snack of wild almonds can kill a person foolish enough to ignore the warning of the bitter taste. Since the first stage in unconscious domestication involves gathering seeds to eat, how on earth did domestication of wild almonds ever get started?
The explanation is that occasional individual almond trees have a mutation in a single gene that prevents them from synthesizing the bitter- tasting amygdalin. Such trees die out in the wild without leaving any progeny, because birds discover and eat all their seeds. But curious or hungry children of early farmers, nibbling wild plants around them, would also have sampled and noticed those nonbitter almond trees, and the non- bitter almond seeds are the ones ancient farmers would have planted, at first unintentionally in their garbage heaps, and later intentionally, by 3000 B.C., in their orchards. Lima beans, potatoes, and eggplants are among the many other familiar crops whose wild ancestors were bitter or poisonous, and of which occasional sweet individuals must have sprouted around the latrines and garbage heaps of ancient hikers.
Human hunter-gatherers also went after wild plants with fleshy or seedless fruits. Their preferences ultimately selected not only for big pumpkins and squashes weighing 700 pounds or more (the world's largest fruit), but also ones consisting of far more flesh than seeds. Cultivated bananas were selected long ago to be all flesh and no seed, thereby inspiring modern agricultural scientists to develop seedless oranges, grapes, and watermelons as well. Seedlessness provides a good example of how human selection can completely reverse the evolutionary purpose of a wild fruit, which is to serve as a vehicle for dispersing seeds.
Many plants were similarly selected for oily fruits or seeds. Among the earliest fruit trees domesticated in the Mediterranean world were olives, which were first cultivated around 4000 B.C., not just for edible fruit but, more important, to obtain olive oil. Thus crop olives are not only bigger but also oilier than wild ones. Ancient farmers selected sesame, mustard, poppies, and flax as well for oily seeds.
Finally, some plants were chosen for their fibers. In this way cotton came to be domesticated and used to weave textiles. The fibers, called lint, are hairs on the cotton seeds, and early farmers of both the Americas and the Old World selected cotton for long lint. In flax, the fibers come instead from the stem, and plants were selected for long, straight stems. We think of most crops as being grown for food, but flax is one of our oldest crops (domesticated by around 7000 B.C.). It furnished linen, which remained the main textile of Europe until it became supplanted by cotton and synthetics after the industrial revolution.
So far, all the changes I've described in the evolution of wild plants into crops involve characteristics that early farmers could actually notice, such as fruit size, bitterness, fleshiness, and so on. By consciously harvesting wild plants with those qualities, ancient peoples unconsciously dispersed the plants and launched them on the road to domestication.
In addition, though, there were changes that could not have involved such conscious choices, because in these cases the plant features being selected for were invisible. For instance, many wild plants have specialized mechanisms that scatter seeds but thereby make them unavailable to humans. A clear example involves peas, whose seeds--the peas we eat-- come enclosed in a pod. Wild peas have to get out of the pod if they are to germinate. To that end, pea plants evolved a gene that makes the pod explode, shooting the peas onto the ground.
But occasionally there are pods of mutant peas that don't explode. In the wild, the mutant peas would die entombed in their pod; only the popping pods would pass on their genes. But, conversely, the only pods available to humans to harvest would be the nonpopping ones left on the plant, making them the progenitors of crops. Thus, as soon as humans began bringing wild peas home to eat--spilling some, throwing spoiled ones away-- there was immediate selection for that single-gene mutant, even though hunter-gatherers were unaware of the difference between popping and nonpopping pods.
Another type of change was even less visible to ancient hikers and involved the speed with which certain seeds germinate. For annual plants growing in an unpredictable climate, it could be lethal if all the seeds sprouted quickly and simultaneously--all might be killed by a single drought or frost, leaving no seeds to propagate the species. Many annual plants, therefore, have evolved to hedge their bets by means of germination inhibitors, which make seeds initially dormant and spread out their germination over several years. In that way, even if most seedlings are killed by a bout of bad weather, some seeds will be left to germinate later.
A common bet-hedging adaptation is to enclose the seeds in a thick coat; this is the mechanism used by wild wheat, barley, peas, flax, and sunflowers, among many other plants. While such late-sprouting seeds still have the opportunity to germinate in the wild, consider what must have happened as farming developed. Occasional mutant individuals among wild plants lacked thick seed coats or other inhibitors of germination. Early farmers would have ensured good conditions by tilling and watering the soil, then sowing seeds. Mutant seeds that immediately sprouted would grow into plants whose seeds were then harvested and planted the next year. Seeds that did not immediately sprout yielded no harvest. Early farmers wouldn't have noticed the difference. But the result of this cycle of sow, grow, harvest, sow would have selected immediately and unconsciously for the mutants.
At the outset I mentioned that some plants were domesticated long ago, others not until the Middle Ages, while still others have proved immune to all our activities. How can we account for these vast differences in ease of domestication?
It turns out that the earliest Near Eastern crops--cereals and legumes such as wheat, barley, and peas, domesticated around 10,000 years ago--arose from wild ancestors that offered many advantages. They were already edible and productive in the wild. They were easily grown, merely by sowing or planting. They grew quickly and could be harvested within a few months of sowing, a big advantage to people still on the borderline between being nomadic hunters and settled villagers. They could be readily stored, unlike many later crops such as strawberries and lettuce. They were mostly self-pollinating, which meant that the crop varieties could pass on their own desirable genes unchanged, instead of having to hybridize with other varieties less useful to humans. Finally, their wild ancestors required very little genetic change to convert them into a crop: for instance, in peas, just a few mutations, such as the one for nonpopping pods.
A next stage in the Near East included the first fruit and nut crops, domesticated around 4000 B.C.; among these were olives, figs, dates, pomegranates, and grapes. Compared with the cereals and legumes, they had the disadvantage of not starting to yield food until at least three years after planting, and not reaching full production for as long as a decade. Thus, growing these crops was possible only for people who were committed to the settled village life and were no longer seminomadic. However, these early fruit and nut crops were still the easiest such crops to cultivate. Unlike later tree domesticates, they could be grown directly by planting cuttings or even seeds. Cuttings have the advantage that, once ancient farmers had found or developed a productive tree, all its descendants remained identical to it.
A third stage included fruit trees that proved much harder to cultivate, among them apples, pears, plums, and cherries. These trees cannot be grown from cuttings. It's also a waste of effort to grow them from seed, since the offspring of even an outstanding individual tree of those species are highly variable and yield mostly worthless fruit. Instead those trees must be grown by the difficult technique of grafting, developed in China long after the beginnings of agriculture. Not only is grafting hard work once you know the principle, but arriving at the principle itself must have required deliberate experimentation. The invention of grafting was hardly a matter of some nomad just relieving himself at a latrine and returning later to be pleasantly surprised by the resulting crop. Such problems delayed the domestication of these fruit trees until around classical times. From the outset, their cultivation was a highly conscious enterprise, carried out according to explicit rules that the Romans described in encyclopedic treatises.
By Roman times, almost all of today's leading crops were being cultivated somewhere in the world. The few later additions have remained of relatively minor importance. Yes, medieval monks did begin to cultivate strawberries and raspberries. Yes, modern agriculture has added other minor crops, notably some berries (like blueberries, cranberries, and kiwis) and nuts (like macadamias, pecans, and cashews). Naturally, modern plant breeders are still making improvements in ancient crops. But early farmers succeeded in at least initially domesticating most plants worth domesticating.
Still, our list of triumphs lacks many wild plants that, notwithstanding their value as food, we never succeeded in domesticating. Notable among these failures of ours are oak trees, whose acorns were the staple food of California Indians and a fallback food for European peasants in famine times from crop failure. Acorns are nutritionally valuable, being rich in starch and oil. Like many otherwise edible wild foods, acorns do contain bitter tannins, but acorn lovers learned to deal with tannins in the same way that they dealt with bitter chemicals in almonds and other wild plants: either by grinding and leaching the acorns to remove the tannins, or by harvesting acorns from the occasional mutant individual oak tree low in tannins.
Granted, oak trees aren't the simplest plant to cultivate, because they can't be started from cuttings. But that minor obstacle didn't prevent us from domesticating dozens of other species posing the same problem. Why then have we failed to domesticate such a prized food source? Why, for that matter, did we take so long to domesticate strawberries and raspberries? What is it about those plants that made them such a huge challenge?
As it happens, oak trees have three strikes against them. First, their slow growth would exhaust the patience of most farmers. Sown wheat yields a crop within a few months; a planted almond grows into a nut- bearing tree in three or four years; but a planted acorn may not become productive for a decade or more. Second, oak trees evolved to make acorns of a size and taste suitable for squirrels, which we've all seen burying, digging up, and eating acorns. Oaks grow from the occasional acorn that a squirrel buries and forgets. With billions of squirrels spreading acorns to virtually any spot suitable for oak trees to grow, we humans didn't stand a chance of selecting oaks for the acorns we wanted.
Finally, perhaps the most important difference between almonds and acorns is that bitterness is controlled by a single dominant gene in almonds but appears to be controlled by many genes in oaks. If an ancient farmer planted almonds from the occasional nonbitter mutant almond tree, the laws of genetics dictate that half the nuts from the resulting tree would be equally nonbitter. But if that same farmer planted acorns from a nonbitter oak, almost all the resulting acorns would still be bitter. That alone would kill the enthusiasm of any would-be acorn farmer who had defeated the squirrels and remained patient.
Just as squirrels gave us trouble with acorns, robins and other berry-loving birds thwarted our efforts to fully tame strawberries and raspberries. Yes, the Romans did tend wild strawberries in their gardens. But with billions of European thrushes defecating wild strawberry seeds in every possible place (including Roman gardens), strawberries remained the little berries that thrushes wanted, not the big berries that humans wanted. In addition, wild strawberries and raspberries are so abundant in Europe that early farmers could satisfy their berry needs merely by strolling over to the nearest bramble patch. Only with the rise of cities did we have to go to the work of cultivating berries in gardens, in order to satisfy berry-loving city dwellers. And only with the recent development of protective nets and greenhouses were we finally able to defeat the thrushes and redesign strawberries and raspberries according to our own standards.
Those standards eventually led to today's gigantic supermarket berries. But it's important to remember that the differences between those berries and their tiny wild ancestors arose out of natural variation among the wild plants themselves. Some of that variation, such as the variation in berry size, would have been readily noticed and thus consciously selected for by ancient farmers. Other variation would have been invisible to humans before the rise of modern botany. But no matter what criteria were used, the resulting evolution of wild plants into crops was an unconscious process. It followed simply and inevitably from our selecting among wild plant individuals.
In his great book Origin of Species, Charles Darwin didn't start with an account of natural selection, as you might expect. Darwin's chapter 1 instead is a lengthy account of how our domesticated plants and animals arose through artificial selection by humans.
"I have seen great surprise expressed in horticultural works," he wrote, "at the wonderful skill of gardeners, in having produced such splendid results from such poor materials; but the art has been simple, and as far as the final result is concerned, has been followed almost unconsciously. It has consisted in always cultivating the best-known variety, sowing its seeds, and, when a slightly better variety chanced to appear, selecting it, and so onwards."
Those principles of crop development by artificial selection still serve as our most understandable model of the origin of species by natural selection.