Domestication: Nature’s Greatest Success

The words “The New Food Revolution” were blazoned across the cover of a recent issue of National Geographic as part of their “Feeding 9 Billion” campaign – the projected human population that demographers prophesize reaching by 2050. The slogan is intended to raise awareness of the urgent need to alter humanity’s food production industry, but it caught the attention of archaeologists concerned with understanding how humanity reached this point. Understanding the past informs the future. During the late Pleistocene, as the continental ice sheets retreated and floral communities resettled the northern landmasses, human population, globally, reached a record high of a couple million. Demographers, such as Ester Boserup and economists such as Amartya Sen, have effectively argued that the main driver of human population growth is food availability. They noted that the Industrial Revolution, the Green Revolution, and the Neolithic Revolution were, effectively, agriculture-based demographic shifts. Domesticated plants and animals fed the growing populations of the ancient world, and increasingly more productive cultivation systems have continued to feed the demographic uptick of the modern world. Therefore, understanding how humanity fed the Neolithization of the ancient world is key to understanding how the human population reached this point.

Domestication is one of the most heavily discussed areas of research in the biological and social sciences. The questions of how and why plants became domesticated have fascinated scholars since Darwin popularized the debate in 1868. Over much of the past century this debate has been framed as: Why did people do this? The answers to the great why question have largely fallen into two camps, the push or pull followers. The push arguments suggest that people were forced to innovate out of necessity, whether as a response to demographic pressure, diminishing resources, or climate change. The pull arguments suggest that resource abundance allowed humans to innovate. After nearly a century of back and forth entrenched arguments, it seems more likely that both camps of scholar are wrong. A few scholars are now reframing the ‘why’ question and setting aside the assumptions of human innovation. Increasingly more data are illustrating that the earliest steps in the process of domestication took two to three millennia and that humans were not consciously driving this gradual process. In reframing the question, anthropologists and biologists have shed a century’s worth of assumptions and are taking a novel approach to the discussion of plant domestication. In reframing the plant domestication debate, I have recently published a series articles, which collectively exemplify this critical approach to domestication theory. These articles and the research they entail are set to culminate in a book on domestication, which I hope to publish early in 2022.

archaeological tell

Figure 1: An archaeological tell site on the edge of the Bukhara Oasis, with agricultural fields in the foreground. The key to understanding increasing human population and urbanization is the intensification of agricultural systems in the ancient world. Archaeology holds the clues to understanding what brought humanity into the modern world we all live in today.

My research, in many ways, resurrects the groundbreaking synthesis from three and a half decades ago by the biologist, David Rindos, suggested that plant domestication arose out of mutualistic relationships between humans and the ancestors of modern crops. He also cautiously theorized that the need for plants to spread their seeds was an important factor in this process. I am building on his seminal ideas with novel archaeobotanical data and the advances of the genomics revolution. In doing so, I have the opportunity to break free of the normal parameters of the push and pull models and to dive into the fossil record in an attempt to understand how plants evolve ways to spread their seeds. Every plant needs to disperse its seeds away from the parent plant. If an apple does not fall far from the tree, then it will be overshadowed by the parent tree and the seedlings will not survive. Likewise, plants cannot run from local environmental changes, and they cannot move into an area to recolonize after a fire or flood. But all plants have evolved ways to move their offspring. In studying the ways that crop ancestors dispersed their seeds, I hope to better understand how plants evolved in the fields of early farmers.

apples

Figure 2: A visualization of apple domestication, which occurred through the hybridization of four separate wild apple species. Much of the diversity that you see in apples today arose over the past couple centuries.

In an article recently published in Frontiers in Plant Science, I explored the domestication of apples as a case study for understanding the domestication of arboreal crops. A sugary fruit is an evolutionary adaptation for dispersing seeds. Plants will invest a considerable amount of energy into enticing an animal to consume the fruits and deposit the seeds in a new area, effectively allowing for the mobility of that tree. Smaller fruits, such as cherries, have evolved to be consumed by smaller animals, often birds. However, larger fruits, such as an apple or a peach, evolved to entice larger animals to consume their fruit and spread their seeds. In many cases, these plants evolved larger fruits to attract a megafaunal mammal that no longer exists. By studying the fossil record for plants, it is clear that many unrelated species of plants evolved larger fruits over the past 40 million years in order to recruit larger animals to disperse their seeds. By better understanding the evolution of larger fruits in the wild, scholars can more effectively study the evolution of larger fruits in cultivated orchards (Figure 2).

The early traits of domestication in fruit trees, including larger more sugary fruits and higher yields, are all linked to a mutualistic relationship between that plant population and humans for the purpose of dispersing seeds. In the case of the apple, the tree evolved larger fruits to entice humans, who have, in turn, dispersed its seeds around the globe. It is easy to see that all of the traits of domestication in the apple originally evolved for seed dispersal in the wild; however, it may be harder to envision how a more durable ear of wheat or a thinner shell on a quinoa grain are tied into seed dispersal. In an article published in Trends in Plant Science, I recently summarized all of the earliest traits of domestication that archaeobotanists can clearly see in the remains of plants that they recover from early archaeological sites. I then discussed the ways the ancestors of these crops dispersed their seeds. All of the early traits of domestication were linked to seed dispersal in the wild plant, and, in many cases, the domestication of the plant resulted in an inability of that plant to disperse its seeds without humans. Therefore, the earliest step toward domestication in all of these plants is best thought of as an evolution of new traits or a severing of old traits in order to better entice humans to disperse their seeds.

grasspea

Figure 3: Most wild progenitors of peas and other legumes have pods that explode when the seeds are ripe, sending the seeds flying away from the plant. The first step towards domestication in crops, such as the grass pea (pictured), would have been to evolve non-dehiscent pods so that farmers did not lose all their seeds.

Interestingly, scholars already recognize that the domestication of cereals and legumes is an evolutionary change in the plant, whereas they lose the ability to disperse their seeds without humans. The domestication of cereals (wheat, barley, rye, oats, and rice) has received the greatest scholarly focus of any group of plants, and the earliest trait of domestication in this group of plants is a strengthening of the rachis, the small stems that hold the individual grains to the plant (Figure 3). In the wild, these plants dispersed their seeds by a mechanical process of the seed shattering off the ear when it is ripe (Figure 4). In a similar way, wild legume plants dispersed their seeds through a mechanical process of explosive dehiscence. When the seeds reach maturity the pod pops open and sends the seeds away from the parent plant. In all of these cases, the earliest traits of domestication are the same traits that allowed the plant to spread its seeds in the wild, in the same way that the sugary fruit of the apple allowed the tree to spread its seeds. Evolutionary ecologists refer to the process of two unrelated organisms evolving the same or similar traits as parallel evolution. The toughening of the rachis in cereals and the pods in large-seeded legumes – peas, grass peas, chickpeas, lentils, fava beans, and kidney beans – are two sets of parallel traits. Scholars recognize that plants evolved similar or parallel traits in response to cultivation and many researchers call the collection of these traits the domestication syndrome.

Scholars have been quick to explain the parallel evolution of tough rachises and non-shattering pods as a result of similar selective pressures imposed by humans. But few scholars have discussed the parallel evolution of larger and sweeter fruits in relationship to seed dispersal. The evolution of larger fruits in orchards also parallels the evolution of larger fruits in the wild to entice megafaunal mammals to spread the seeds within the fruits. In the same way that plants evolved tough rachises to assist humans in spreading their seeds, plants have evolved larger fruits in order to better entice humans.

wild barley

Figure 4: The brittle rachis of cereal crops, such as in this wild barley, is the most widely studied and recognized trait of plant domestication. The plants evolved to have harder rachises to retain their seeds through harvesting.

There is one other broad category of plants that evolved domestication traits very early by humans. In addition, to plants with fleshy fruits and large-seeded grasses and legumes, there were small-seeded annual plants that evolved domestication traits in the Early Holocene. These small-seeded annuals include, quinoa, amaranth, millets, buckwheat, and all the ancient lost crops of North America. Scholars have almost completely overlooked the process of seed dispersal in the ancestors of these crops. Additionally, there are two important mysteries involving their domestication that have stumped researchers for decades. The first is why people targeted these crops in the first place, seeing that they tend to grow thinly across the landscape. Unlike wild wheat and barley, which naturally grows in dense easily-harvested stands, the wild relatives of these small-seeded plants are rarer and not easily collected by foragers. The second mystery is why the specific traits of an increase in seed size, a thinning of the seed coat, and a breaking in dormancy were the first traits of domestication. In a recent article in Nature Plants, coauthored with Natalie Mueller, I attempted to answer both of these mysteries by discussing the evolutionary adaptations for seed dispersal in the wild relatives of these plants. I noted that the wild relatives have evolved a suite of traits that allowed them to spread their seeds by means of grazing animals. The plants evolved rapid annual growth, no defensive chemicals or thorns and they display their seeds on top of the plant. They have essentially evolved to be the perfect food for a cow or bison. Additionally, the first three traits of domestication in these plants are the traits that allowed the seeds to be dispersed by animals. The restrictive digestive system of an ungulate grazing animal rarely allows seeds larger than 2 mm to pass through the system. Therefore, the small size of these seeds (including the quinoa on your salad) is part of the suite of traits that allowed for this form of seed dispersal. Additionally, heavy herd animal grazing removed plants that did not evolve to be consumed and concentrates the progenitors for these crops. As Dr. Mueller and I demonstrate, heavy grazing of herd animals concentrates these plants into dense easily-harvested stands that would have been attractive to early foragers. In this regard, we hypothesize why these three traits were the first traits of domestication, why early foragers would have targeted these plants in the past, and we show that these plants evolved in response to human seed dispersal in the same way that all other early crops evolved.

The leveling of the rainforests in the Amazon to plant soybeans and the monocropping zones of North American maize and European rapeseed reflect the success of the ability for humans to disperse seeds. The plants that evolved special traits to utilize this human seed-dispersal service have taken over the planet. The higher yields and great grain surpluses that have resulted from the mutualistic relationship between humans and these plants has fed the demographic shifts of the past ten thousand years and brought us to the brink of nine billion. Stepping away from the simplistic push and pull arguments of plant domestication and focusing on using parallel examples of evolution among plants in the wild allows scholars to better study domestication. Likewise, understanding how and why these plants evolved helps explain how humanity became what it is today.

cotton field

Figure 5: A field of cotton in Uzbekistan, where this water-demanding crop is growing in the arid desert. The intensification of farming systems has paralleled human population growth over the past ten millennia.

References Cited

Spengler, Robert N., III (2020) Anthropogenic Seed Dispersal: Rethinking the origins of plant domestication. Trends in Plant Science. 25(4):340-348.

Spengler, Robert N., III (2019) Origins of the Apple: The Role of Megafaunal Mutualism in the Domestication of Malus and Rosaceous Trees. Frontiers in Plant Science. 10(617):1-18.

Spengler, Robert N., III and Natalie Mueller (2019) Grazing Animals Drove Domestication of Grain Crops. Nature Plants. 5:656–662.