Pandora's Seed

by Spencer Wells

  Since the earliest days of hominid evolution, our ancestors had been seminomadic. This is because of the uncertainty inherent in being a hunter-gatherer—if the food supply dwindles in one place, you pack up and move to better foraging grounds. It was our ability to do this successfully that led our ancestors out of their homeland in Africa again and again over the past few million years. Homo erectus, who left around 1.8 million years ago, was simply following food, as was Homo heidelbergensis, who left around 500,000 years ago and gave rise to the Neanderthals in Europe. We are the descendants of a third wave of African hunter-gatherer migrants that left around 50,000 to 60,000 years ago, as I detailed in The Journey of Man. And throughout this long line of human evolution we were consistently seminomadic, staying in one place only as long as the pickings were good and moving on when they weren’t.

  This started to change toward the end of the Paleolithic period—the period that preceded the Neolithic—when all humans were hunter-gatherers. According to recent research carried out in Israel by archaeologist Dani Nadel and his colleagues, there is evidence for grain gathering and flour making at one of his sites near the Sea of Galilee dating back to 23,000 years ago. This would place that community within the early part of the time period allocated to the Kebaran culture, which preceded the Natufian in the Levant. The Kebarans were the link with the true hunter-gatherers of Middle Eastern prehistory—highly nomadic, shifting their settlements seasonally to follow the food and water supplies during the cold, dry terminal period of the last ice age. But even these mobile hunter-gatherers seem to have recognized the advantages of collecting and grinding wheat, albeit in much smaller quantities than the Natufians.

  When the last ice age ended, though, wheat expanded its range. Life became easier for our ancestors; food that had been difficult to obtain during the cold, dry conditions of the last ice age suddenly became more plentiful. This allowed these people to finally settle down in an area with large quantities of the easily gathered grain. Gathering wild wheat yields more calories of food for each calorie of energy invested than did early forms of agriculture, which made this grain a fabulously valuable food source for the Natufians. Moreover, it was a particular type of food resource that lent itself to long-term storage—a seed that could be stored dry for years. A couple of weeks of intensive grain gathering in the fall could yield enough wheat to feed a family for a year, if supplemented with nuts and game meat. Life was good, and they made the most of it by…well…doing what people do. They had babies.

  The hunter-gatherer way of life had limited the number of children people had as part of a complex feedback loop with the environment. If the population grew too large it was necessary to split and form two smaller groups, one of which would typically move on to new hunting grounds. The calorie-rich environment of the Fertile Crescent wild grain fields increased the region’s carrying capacity (the number of people the land could support), and the human population responded. Natufian settlements during this period expanded into villages of 150 people or more, complete with circular houses and storage pits. It was a radical shift in our relationship with nature, and it happened only because the Natufians could rely on a steady supply of grain from the territory where they lived.

  Then, suddenly, it all changed. That burst dam thousands of miles away in North America, setting in motion the Younger Dryas, brought a return of the long winter. The population of the Middle East was cast back into the ice age, but this time it had a strike against it: the people couldn’t move on to greener pastures. They had invested too much in their villages, the collective memory of the good times was probably still fresh in their minds (leave the village to return to the hardscrabble life of a nomad? Unthinkable!), and in all likelihood there were now too many people to return to life as nomadic hunter-gatherers. The Natufians were in a bind.

  Although Dryas refers to a cold-tolerant plant species, it could perhaps more aptly refer to the drying effect during these periods of global cooling, for this was the main result in the Middle East. As the land dried out, the wild grain retreated from the lowlands, remaining only in the higher mountain valleys, where it could get enough water. The Natufians had to travel farther and farther from their lowland settlements to gather enough to survive. This would have put tremendous pressure on the food supply, and probably resulted in an increased mortality rate in these people accustomed to a land of plenty. It was humanity’s first real encounter with Thomas Malthus’s conjecture that population growth will eventually produce more people than can be supported by the available food supply.

  Around this time we also see evidence for the extinction of megafauna, large mammals, such as the woolly mammoth and Irish elk in Europe, that would have formed part of the diet of these hunter-gatherers. While such extinction events had happened before, most notably in Australia when humans first arrived there around 50,000 years ago, as well as in North America with humanity’s arrival around 15,000 years ago, the clearest evidence of their occurrence in Europe and the Middle East comes at the end of the last ice age. This extinction event is further evidence that climate change and human population pressures were having a significant effect on food resources. A population that had been able to live sustainably during the warm period immediately following the end of the last ice age was now too large to be supported by the diminished resources of the Younger Dryas, and the animal species lost out.

  Then, sometime between 12,000 and 11,000 years ago, one of these stressed Natufians had a revolutionary idea. What if, instead of walking farther each day to gather food, they simply planted it close to the village? It was probably a woman, since women typically did the gathering in hunter-gatherer populations and thus had access to the seeds—and an incentive to reduce their gathering commute! Her first efforts must have been rewarded with admiration from the entire village, and the idea quickly spread. Virtually overnight humans had gone from being controlled by their food supply to controlling it.

  FIGURE 8: PATTERN OF MEGAFAUNA EXTINCTIONS ON THREE CONTINENTS, MADAGASCAR, AND NEW ZEALAND. IN EACH CASE THE EXTINCTION OCCURRED SOON AFTER THE ARRIVAL OF HUMANS.

  This course of events can actually be seen in the bones of the people who lived in the region at the time, making use of something called the strontium/calcium ratio. Strontium (Sr) is an element that accumulates in human bone, its level determined by its abundance in the groundwater of the region (as well as exposure to nuclear fallout, which has Sr-90 as one of its major constituents). Plants absorb strontium from the water as they grow, then pass it on to the animals that eat them. Thus, the higher the proportion of plant food in your diet, the higher your strontium levels. Natufian remains have a very high level of strontium during their intense gathering phase prior to the Younger Dryas; the level drops significantly as the wild stands of grain shrank and they turned to hunting to survive. The level then rises dramatically after the onset of domestication as the new culture took hold and plants made up a larger proportion of their diet.

  This change in our relationship with nature had an extraordinarily far-reaching impact on the future of humanity—it was about much more than just food. We’ll examine the early fallout from cultivation a bit later in this chapter, but for now we need to zoom out from our close focus on this one region. While this sequence of events was playing out in the Middle East, extraordinary things were happening in other places around the world. The Natufians were not alone in their early attempts to domesticate crops; cultivation seems to have been a global trend around this time. But in the days before mass media and the Internet, how did the seed of this revolutionary idea get planted in places as far afield as Mesoamerica, southern China, and New Guinea? And what does it reveal about the next stage in the development of agriculture—the crucial step from planting wild seeds to their domestication, coupled with selective breeding for desirable traits?

  PEAKS AND VALLEYS

  The Soviet botanist and geneticist Nikolai Vavilov led one of those lives that deserve to be a feature film. B
orn into a bourgeois merchant family in Moscow in 1887, he spent several years during his youth traveling and studying in Europe, returning home just in time for the Bolshevik Revolution. He became a prominent member of the Soviet regime, a member of the Supreme Soviet, and a recipient of the prestigious Lenin Prize. He created the Institute for Plant Industry in Leningrad (now Saint Petersburg), and headed it for nearly twenty years; today this institute and the Institute of General Genetics in Moscow both bear his name. Yet due to the bizarre rise of an agronomist named Trofim Lysenko in the 1930s and his pseudoscientific attacks on genetics and the basic rules of evolution (both championed by Vavilov), in 1943 this great scientist starved to death in a gulag, having been jailed by Stalin in 1940 for allegedly plotting to destroy Soviet agriculture.

  Vavilov was a hugely influential thinker on the origins of plant domestication, and the institute in Saint Petersburg still houses one of the world’s largest seed banks, created in an effort to preserve and study the diversity of cultivated crops from around the world. During the twenty-eight-month Siege of Leningrad, the workers managed to protect the collection from the city’s starving residents, who tried repeatedly to eat its contents, and it is still a major botanical resource to this day.

  In his extraordinarily influential work on domesticated plants, Vavilov described many primary centers of plant domestication. One was the Fertile Crescent, which we’ve just learned about. Other major centers were in China, Mesoamerica, and the Andes of South America. A wide range of places, but all are similar in one way: they are all mountainous regions. Why not coastal areas or prairies? Primarily because mountains serve as so-called refugia of biological diversity—places where species continue to thrive when the surrounding plains are too dry to sustain them, due to climatic shifts such as those that have occurred frequently throughout the past few million years. Because mountains draw more rainfall, they serve as relatively safe havens in times of climatic stress, so they are the places where genetic diversity is typically the highest. And high genetic diversity allows for the development of advantageous traits that can be selected for by humans, including seed retention (as opposed to a plant’s jettisoning its seeds when they are mature) and other characteristics that suit species’ use as food crops.

  Humans can’t live easily in high mountains—we tend to prefer lowlands, for climatic reasons—but plants advance and retreat, “breathing” in and out of the lowlands during wetter and drier phases. This provides us our first clue as to why domestication happened in all of these places at the same time.

  Mesoamerica, for instance, has given us many crops that are indispensable components of the modern diet: corn, tomatoes, beans, chilies, chocolate, vanilla, squash, pineapples, avocados, and pumpkins. Many were domesticated in the region of present-day Oaxaca, in southern Mexico, which has a rugged, mountainous terrain that has served to fragment human populations, resulting in a tremendous amount of cultural and linguistic diversity to match its botanical horn of plenty. Corn is far and away the most important Oaxacan crop, and evidence shows that it has been cultivated since around 10,000 years ago. There is some debate about corn’s botanical ancestor—its closest wild relative, teosinte (pronounced tay-o-SIN-tee), is so different in form that many scientists find it difficult to believe that one developed from the other—but not about its geographic origin.

  Domesticated corn later spread far from its Mexican homeland, reaching into North and South America over the subsequent 8,000 years, much as wheat and barley spread far from their origin in the Fertile Crescent. The spread of corn has been well documented from human remains in North America, where the sudden transition from hunting and gathering can be seen in the “carbon signature” in the bones, in a similar way to that in which strontium revealed the sequence of Neolithic events in the Middle East. This is because hunter-gatherers eat primarily what are known as C3 plants, which use carbon dioxide from the atmosphere to produce molecules with three carbon atoms as their energy store. Some 95 percent of the world’s plants are members of this C3 group, and it was the first to evolve, over 250 million years ago. A more efficient type of plant metabolism, known as C4, evolved more recently, within the past 65 million years. The C4 plants include mostly tropical grasses, such as corn, millet, and sugarcane, that store their energy in 4-carbon molecules.

  There is one other difference between C3 and C4 plants, and this is how this little foray into plant physiology fits into our story. The carbon atoms in the atmosphere—coming from your breath or the car exhaust from your morning commute—that these plants use to make sugars and starches aren’t all identical. There are several different variants of carbon, distinguished by their atomic anatomy. Most carbon molecules have 6 protons and 6 neutrons packed into each nucleus, for an atomic weight of 12 (6 + 6). However, rarer forms of carbon have 7 or even 8 neutrons packed in with their 6 protons, giving them atomic weights of 13 and 14. Carbon-14 is extremely rare, but its tendency to lose atomic baggage (an electron and an antineutrino, if you must know) in an effort to drop a bit of extra molecular weight makes it extremely useful as a way of dating once-living material. Carbon-13 doesn’t decay; it sticks around indefinitely and gives us another tool in our archaeological atomic arsenal. It turns out that C3 plants, for whatever reason, are picky and don’t like carbon-13, excluding it from their metabolic machinery. C4 plants don’t seem to care and will use whatever is available. This means that C4 plants have higher ratios of carbon-13 to carbon-12 than do C3 plants.

  So what does all of this mucking about in the world of carbon atoms mean? When people add C4 plants (like corn) to their diet, the ratio of carbon-13 in them also increases. By carefully measuring these carbon ratios in ancient bones, we can see when people started to eat C4 plants like corn. And when we do this in North America, examining bones from around the time that corn started to spread into a region, we can see a dramatic increase. While the ratios aren’t necessarily indicative of the actual amounts in a person’s diet (it’s unlikely that 75 percent of their calories came from corn, as Figure 9 might suggest), it does indicate the extraordinary shift in diet that accompanied the spread of agricultural “killer apps” like corn and wheat.

  FIGURE 9: THE SPREAD OF CORN INTO NORTH AMERICA, AS CHRONICLED IN ARCHAEOLOGICAL SITES FROM THE MIDWESTERN UNITED STATES. PRIOR TO THE INTRODUCTION OF AGRICULTURE, THE CARBON ISOTOPE SIGNATURE OF HUMAN SKELETONS IS FLAT, BUT IT INCREASES DRAMATICALLY AFTERWARD, A RESULT OF LARGE AMOUNTS OF THE C4 PLANT CORN ENTERING THE DIET.

  Similarly, rice seems to have been domesticated first in the mountains of southern China and northern India, where its wild ancestor Oryza rufipogon still grows. Through careful analyses of phytoliths, microscopic stonelike particles in plants that serve as a kind of species fingerprint and are preserved in the archaeological record, Zhijun Zhao of the Smithsonian Tropical Research Institute has found evidence that hunter-gatherers living on the Yangtze River in central China were eating rice around 13,000 years ago. With the onset of the Younger Dryas and the cooler temperatures in the Northern Hemisphere, however, the rice phytoliths disappear from the archaeological record, reappearing only around 11,000 years ago, when warmer and wetter conditions returned—and judging from changes in the phytoliths, these appear to have been cultivated. It seems that during the Younger Dryas the rice retreated back to a more hospitable environment, and humans—as in the Middle East and the mountain valleys of Oaxaca—were forced to start planting it to keep the grain in their diet.

  Thus, in the centers of domestication for the three main grain crops around the world, we see a similar interaction between hunter-gatherers and their local grains. Intensive foraging at the end of the last ice age, coupled with warmer, wetter conditions, led to specialized gathering of particular plant species and an increase in population. The onset of the Younger Dryas created a crisis in food supply, which forced these sedentary foragers to start cultivating grains that had previously been plentiful in the wild. The combination of a demographic expansion follo
wed by a climatic stress probably explains why we see the development of agriculture independently at the same time around the world. Cultivating food allowed these populations to survive the cold snap of the Younger Dryas, and when favorable conditions returned, agriculture was ready to take off. All that was needed was one final step: domestication.

  THE IMPORTANCE OF DUPLICATION

  Today Captain William Bligh’s name is synonymous with cruel leadership, but in fact he was quite a good naval commander. His fall to ignominy came from his tough treatment of his crew during a six-month sojourn in Tahiti in 1789, during which he tried to enforce a ban on “liaisons” with the local women. Ironically, he was motivated in large part by his concern for the Tahitians; Bligh didn’t want his crew to spread sexually transmitted diseases on the island. If they hadn’t spent so much time in Tahiti, the Bounty would probably be a footnote in naval history textbooks and history would have a different term for a cruel authoritarian. Unfortunately, Bligh couldn’t leave Tahiti any sooner because of the difficulty of cultivating breadfruit.

  Unlike most plants humans eat, breadfruit typically has no seeds (there are a few varieties that do, but they aren’t widely cultivated). The only way to propagate the plant is by air layering, a tedious process in which a small incision is made on a branch or stem and then wrapped with a rooting medium, like moss or soil. After a few weeks new roots will have grown from the incision and the branch can be removed and planted on its own, eventually yielding a new, independent plant. Two botanists from the Royal Botanic Gardens at Kew accompanied Bligh on his voyage in order to perform this time-consuming task. It was the only reason for the Bounty’s multiyear journey—Bligh was supposed to deliver as many of the plants as possible to British colonies in the West Indies in order to feed the burgeoning slave population there. Breadfruit, despite the tedious business of propagation, is a calorie-rich and easily grown food source.