A note at the top, especially for my academic friends: this is a simplified discussion of domestication of plants for the purposes of an upcoming presentation on the history of gardening. The dates and theories have been rounded off for the amount of time and target audience.
Until about 10,000 years ago, all humans on the planet were hunter-gatherers. This means they traveled across the landscape in small groups or bands feeding themselves from, well, hunting animals and gathering plants. This doesn’t mean that our ancestors were unsophisticated: they certainly had a deep knowledge of the world around them, biology, weather, etc. Their social systems, languages, and interactions were every bit as complex as ours, just in another world entirely. But then about 10,000 years ago, glaciers receded and the Holocene began — a geological epoch characterized by warmer temperatures than the previous periods. Note that this change happened over thousands of years instead of decades, as we’re seeing now (much of the data for this discussion was summarized by Larson et al. 2014).
As temperatures warmed, local biological zones shifted, moving polewards, and people had to adapt. Archaeologists have developed hypotheses to explain what happened next: the invention of agriculture. We don’t have the space to look at each one in this context, but some of them are Childe’s oasis hypothesis (humans, plants, and animals compressed into close quarters led to domestication), Braidwood’s hilly flanks hypothesis (plants ripe for domestication found on the hilly flanks of the Fertile Crecent became staples for locals), and Hayden’s feasting model (social status raised by giving feasts, so domestication gave more food for feasting). But what I’ll be describing below is a hybrid of the Sauer, Binford, and Flannery demographic hypothesis and Rindos’s evolutionary hypothesis (a summary of each).
Imagine a band of hunter gatherers ranging across the land. They know where the best spots are to hunt animals and gather plants in each season. Instead of traveling across great distances and coming to new locations, they keep coming back to seasonal camps (called seasonal sedentism). Over time, they would encourage the growth of plants that they ate and used around these camps. This might be as simple as chopping down saplings and trampling unwanted plants to give their food sources more sunlight. Over generations, our band might begin selecting for the best of their plants: chopping down trees that don’t produce the biggest, sweetest nuts or fruit and pulling out the vines that produce more bitter vegetables or tubers, for example. Each year the band would come back to this location and over time, their selected plants produced more and more.
Maybe one year, grandma gets sick or is just too old to travel to the winter camp, so she and part of the band decides to stay at the summer or fall camp. By this time they can rely on the plants they’ve selected to provide much of their calories. The hunters might still visit the winter camp, but they’re likely to circle back to the rest of their group. Over time, this develops into a permanent settlement and the people rely primarily on their domesticated plants.
Most discussions of domestication discuss grains and legumes: wheat, rye, oats, corn, rice, lentils, peas, beans, etc., but evidence suggests that these plants took thousands of years to become fully domesticated. On the other hand, vegetables and fruits might have made the transition much more quickly.
To understand why, we have to look at what it means for a plant to be domesticated. Plants selected for human use share a suite of features. They have reduced chemical or physical defenses (less bitter, less spiky, thinner rinds) and bigger edible parts. Many of them develop synchronous maturation, that is, the fruits all ripen together for easier picking. In grains and legumes the seeds are retained by the stalk or pod, unlike wild cousins, which use the wind to disperse seeds that break off easily.
Why, then, would fruits and vegetables be domesticated more quickly? The reason may well be that ancestral gardeners were pickier and more active in selecting the best produce than the early farmers who were out gathering grains and legumes in large numbers in the grasslands. Think about it: a gardener is visiting his or her plot every day or two to weed and manage the plants. S/he easily sees the best-producing individuals. On-site taste testing can quickly eliminate the bitter from the sweet. The thorny ones can be avoided or chopped down. Those plants that mature together are more likely to be gathered.
In the fields, on the other hand, seeds from a target species are gathered with less discrimination. Seed gatherers only perform one selection event each year (i.e., harvest). They’ll gather a wider variety of that species each year. They won’t necessarily sort the best from the middling, so a greater diversity of genes get passed on to the next year.
While much of the academic work focuses on seeds and their domestication, gardens might have been the real hotbeds of plant selection.
Larson, Greger, Dolores R. Piperno, Robin G. Allaby, Michael D. Purugganan, Leif Andersson, Manuel Arroyo-Kalin, Loukas Barton, Cynthia Climer Vigueira, Tim Denham, Keith Dobney, Andrew N. Doust, Paul Gepts, M. Thomas P. Gilbert, Kristen J. Gremillion, Leilani Lucas, Lewis Lukens, Fiona B. Marshall, Kenneth M. Olsen, Chris Pires, Peter J. Richerson, Rafael Rubio De Casas, Oris I. Sanjur, and Mark G. Thomas. 2014. “Current Perspectives and the Future of Domestication Studies.” Proceedings of the National Academy of Sciences of the United States of America 111, no. 17: 6139-146. www.jstor.org/stable/23772467.