This month I've been reading and writing a lot about the agricultural transition. As most things, the more you learn about something the more you realize how little is actually known. I thought I knew the history of the domestication of corn, say, but that was before I actually read anything about it.
For at least the last two million years of the evolution of the genus Homo, and for maybe even longer than that, reaching back into Australopithecus times, our ancestors foraged for food, collecting seeds, roots, fruits, eggs, and scavenging or hunting for meat and fish. This came to an end in much of the world some 10-12,000 years ago.
Since intention isn't preserved in the archaeological record, the reason for this will probably never be known, but possible explanations include population pressure; perhaps that foraging could no longer feed growing populations, or climate change; the end of the last glacial age, and the warming of the climate, which among other things may have brought changes in the availability of familiar wild foods.
Why it happened at generally the same time around the world is a curious thing; first in the Fertile Crescent of the Near East, then in Mesoamerica, followed by Southeast Asia, although there is evidence that plants were being domesticated in Ecuador at about the same time as they were in the Fertile Crescent. The fact that the transition to agriculture happened pretty much simultaneously everywhere suggests that global climate change was in fact that driver, but that's hard to demonstrate.
In any case, as far as is known today, plant domestication happened in 24 different regions, and in 13 of those regions grain crops were the primary focus.
|Centers of plant domestication; The nature of selection during plant domestication|
Michael D. Purugganan & Dorian Q. Fuller, Nature 457, 843-848(12 February 2009)
Eight plants were domesticated in the Fertile Crescent; einkorn and eimmer wheat; barley; lentils; peas; chickpeas and bitter vetch, and flax. Rice was domesticated in Asia, potatoes and beans in South America, maize, and bottle gourd and squash in Mesoamerica, with many more as the agricultural transition progressed.
Dogs were probably the first domesticated animal, about 35,000 years ago, from wolves, followed by cats near the time of the agricultural transition, but it's possible that both these animals were attracted to people and domesticated themselves. Cats may have been attracted to settlements because rodents would have lived in and around stored grain.
Goats were probably the first animals domesticated for food, in the Fertile Crescent, and then cattle, sheep, pigs, and horses. These then were dispersed, along with agriculture, into Europe and North Africa. Alpaca and llamas were domesticated in South America, chickens in southeast Asia, and, again, many more over time.
How is all this known? In part, molecular genetics has revealed some of the relationships between wild and domesticated forms of each of these plants and animals, but for animals, bones found at early Neolithic settlement sites have told much of the tale. However, the tale they tell isn't always clear -- it was thought early on that size of the animals indicated domestication, until archaeologists realized that all animals had gotten smaller through the Paleolithic, including humans. And, sex and age of animals in piles near settlements was thought to be a sign, if the animals were primarily female, and the males were young, until it was realized that this demographic profile could just as credibly indicate that the males had been hunted. And so on.
Teosinte to maizeThe only way paleoanthropologists can interpret what they find is through a contemporary lens. E.g., we know that males aren't of use to a modern dairy farmer so probably weren't to Neolithic farmers either, which could explain the predominance of female sheep or goats in piles of bones. Many animals did get smaller when domesticated, so size could legitimately be a signature of domestication. For some grains, the primary difference between wild and domesticated forms are that the seeds of the domesticate don't 'shatter', or fall to the ground when blown by a breeze or brushed by an animal, or when the grain is harvested. But domesticated grains lose the shattering trait, and that's a huge advantage when it comes to harvesting and storing the grain.
|Teosinte and maize; Nicolle Rager Fuller, National Science Foundation|
Corn is another matter. The wild form of maize is teosinte, which was domesticated about 9000 years ago. But, teosinte doesn't look much like maize. It has many branches, it's not as tall, it has tiny ears, the seeds are encased in hard nut-like shells and, unlike maize, they mature serially rather than simultaneously. Molecular comparison of the genomes of modern wild teosinte and domesticated maize show basically five genetic differences between the plants, and these are responsible for the transition from teosinte to corn.
But still, the question was why was this plant a candidate for domestication at all, given its less than ideal characteristics for farming? And even for foraging, if the seeds from this scrappy plant didn't ripen all at once, and the shells were difficult to open. Early farmers couldn't see into the plant's future, weren't thinking ahead to a maizeier form of the plant. Presumably they were just planting the seeds, assuming they'd get more of the same. So why try domesticating it?
Well, it turns out that modern teosinte may not be what teosinte looked like 10,000 years ago, when the temperature was 3-4 degrees lower than it is now, and there was less carbon dioxide in the atmosphere. Anthropologist Dolores Piperno and colleagues published a paper in Quaternary International last year reporting a an experiment with teosinte that might be the explanation. They decided to grow teosinte in a greenhouse under early Neolithic environmental conditions, so the built a greenhouse and supplied teosinte plants with the growing conditions they assumed were true at the beginning of the Neolithic. And they found that even with the same genome that now builds a leggy, short plant with lots of tiny cobs and nut-like kernels, changing growing conditions produced plants that were in fact maizier.
The plants had a single tall stem, the ears were topped with the tassels we’re familiar with from modern corn, and they had a few short branches with ears, and seeds that matured all at once rather than sequentially. The seeds were exposed, too, rather than encased in the hard seedcase. With fewer branches, and seeds that were easily visible, all maturing at once, teosinte would have been much easier to harvest for early Neolithic farmers, and it might not have been such a leap to produce maize.
Complicating evolutionary just-so stories
If Piperno and her coauthors are right, this means that modern versions of wild plants don't necessarily represent their ancestral selves. The reason for this, they suggest, is ‘phenotypic plasticity,’ which is characteristic of many organisms as they respond to different environmental conditions with changes in appearance.
It's also important because phenotypic plasticity is the norm, which makes evolutionary sense since environments frequently change, and organisms that can't adapt may well be doomed. But, adaptive stories are so tempting.