Exploring the mysteries of domestication


May 2, 2014 |

We all think we have a rough idea of what happened 12,000 years ago when people at several different spots around the globe brought plants under cultivation and domesticated animals for transport, food or fiber. But how much do we really know?

Recent research suggests less than we think. For example, why did people domesticate a mere dozen or so of the roughly 200,000 species of wild flowering plants? And why only about five of the 148 species of large wild mammalian herbivores or omnivores? And while we’re at it, why haven’t more species of either plants or animals been domesticated in modern times?

If nothing else, the tiny percentages of domesticates suggests there are limitations to human agency, and that it almost certainly is not true that people can step in and completely remodel through artificial selection an organism shaped for millennia by natural selection.

The small number of domesticates is just one of many questions raised in a special issue of the Proceedings of the National Academy of Sciences published online April 21.

The issue is the product of a 2011 meeting of scholars with an interest in domestication at the National Evolutionary Synthesis Center, a nonprofit science center jointly operated by Duke University, the University of North Carolina at Chapel Hill and North Carolina State University.

Of the 25 scholars at the conference, two were from Washington University in St. Louis: Arts & Sciences’ Fiona Marshall, PhD, professor of archaeology, who studies animal domestication, and Kenneth Olsen, PhD, associate professor of biology, who studies plant domestication.

Both Marshall and Olsen are currently engaged in research on the crumbling margins of domestication where questions about this evolutionary process loom the largest.

Marshall studies two species that are famously ambivalently domesticated: donkeys and cats. Olsen studies rice and cassava and is currently interested in rice mimics, weeds that look enough like rice that they fly under the radar even when rice fields are handweeded.

Both Marshall and Olsen contributed articles to the special PNAS issue (seeThe story of animal domestication retold and Genetic study tackles mystery of slow plant domestications) and helped write the introductory essay that raises the big questions confronting the field.

“This workshop was especially fun,” said Olsen, “because it brought together people working on plants and animals and archeologists and geneticists. I hadn’t really thought much about animal domestication because I work primarily with plants, so it was exciting to see the same problem from a very different perspective.”

How much of it was our doing?
Many of our ideas about domestication are derived from modern experience with animal breeding. Anyone familiar with the huge variety of dog breeds, all of which belong to the same subspecies of the gray wolf, has some appreciation of the power of selective breeding to alter appearance and behavior.

But what about self-fertilizing or wind-pollinated plants, or for that matter, domesticated animals accidentally or deliberately bred with wild relatives?

Recent evidence that cereal crops, such as wheat or barley, evolved domestication traits much more slowly than had been thought has led to renewed interest in the idea that selection during domestication may have been partly accidental.

Charles Darwin himself drew a distinction between conscious selection, in which humans directly select for desirable traits, and unconscious selection, where traits evolve as a byproduct of natural selection in crop fields or from selection on other traits.

“The big focus right now is how much unintentional change people were causing environmentally that resulted in natural selection altering both plants and animals,” said Marshall.

“We used to think cats and dogs were real outliers in the animal domestication process because they were attracted to human settlements for food and in some sense domesticated themselves. But new research is showing that other domesticated animals may be more like cats and dogs than we thought.

“Once animals such as donkeys or cattle were caught,” Marshall said, “the changes humans sought to make were pretty minimal. Really it just came down to culling a few of the males and breeding all of the females.”

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Even today, she points out, African pastoralists can afford to kill only four out of every 100 cows or they run the risk that drought and disease will wipe out the entire herd. “So I think outside of industrialized societies or special situations, artificial selection was very weak,” she said.

“In the donkeys and other transport animals, it’s not affiliative [tame] behavior the herders want,” Marshall said. “What they care about more than anything else is that their animals stay alive.”

So artificial selection is acting in the same direction as natural selection, or maybe pushing even harder, because humans often place animals in harsher conditions than natural ones.

“The comparable idea for plants,” said Olsen, “is the dump heap hypothesis, originally proposed by Edgar Anderson, a botany professor here at Washington University. The idea is that when people threw out the refuse of plant foods, including seeds, some grew and again set seed, and in this way people inadvertently selected species they were eating that also did well in the disturbed and nutrient-rich environment of the dump heap.”

“Cultivation practices play a huge role in selection,” said Olsen. “Traditionally in Southeast Asia, many different varieties of rice were grown simultaneously in a given field. It was a bet-hedging strategy,” he said, “that ensured some plants would survive and produce seed even in a bad season.” So it wasn’t people selecting the crop plants directly so much as people changing the landscape in ways that altered the selection pressure on plants.

How best to time travel
Questions about the original domestication events are difficult to answer because plants and animals were domesticated before humans invented writing, and so figuring out what happened has been a matter of making do with the limited evidence that has survived.

The problem is particularly difficult for animal domestication because what matters most is animal behavior, which leaves few traces. In the past, scientists tried measuring bones or examining teeth, looking for age or size differences or pathology that might plausibly be related to animals living with people.

“Sometimes there aren’t morphological shifts that are easy to find or they’re too late to tell us anything,” Marshall said. “We’ve gone away from morphological identifiers of domestication, and we’re going with behavior now, however we can get it. If we’ve got concentrations of dung, that means animals were being corralled,” she said.

Olsen, on the other hand, seeks to identify genes in modern crop species that are associated with domestication traits in the plant, such as an erect rather than a sprawling architecture. The techniques used to isolate these genes are difficult and time consuming and may not always penetrate as deeply into the past as scientists had once assumed because present-day plants are only a subset of the crop varieties that may have once existed.

So both Marshall and Olsen are excited by recent successes in sequencing ancient DNA. Ancient DNA, they say, will allow hypotheses about domestication to be tested over the entire evolutionary time period of domestication.

Another only recently appreciated clue to plant domestication is the presence of enriched soils, created through human activities. One example is the terra preta in the Amazon basin, which bears silent witness to the presence of a pre-Columbian agricultural society in what had been thought to be untouched forest.

By mapping distributions of enriched soils, scientists hope to better understand how ancient people altered landscapes and the effects that had on plant communities.

“It is really clear,” Marshall said, “that we need all the different approaches that we can possibly get in order to triangulate back. We’re using all kinds of ways, coarse-grained and fine, long-term and short, because the practical implications for us are quite great.”

After all, the first domestications may have been triggered by climate change at the end of the last ice age — in combination with social issues.

As a result, people abandoned the hunter-gatherer lifestyle they had successfully followed for 95 percent of human history and turned instead to the new strategies of farming and herding.

As we head into a new era of climate change, Marshall said it would be comforting to know that we understood what happened then and why.

23 Responses to Exploring the mysteries of domestication

  1. Abigail Dunkley 14027853 May 7, 2014 at 12:36 pm #

    I do agree that it is indeed necessary to understand past motives for plant and animal domestication, so that humans today can learn from past actions and techniques. Although domestication has proved to be beneficial, it is important to not abuse the various techniques. As mentioned in the above article, domestication became widely used, leading from a hunter-gathering era to one of farming and herding. Domestication of plants and animals, therefore, became important to human life- from cultivating plant species to breeding and training animals for specific roles. However, where must the line be drawn? Today, animals are often bred excessively for financial purposes, so much that animal rights are abused. If one was to look at the lifespan of a chicken, particularly in Southern Africa, it is estimated to be 42 days. Dogs are often bred to sell. Pigs remain in 1×2 metre enclosures until day of slaughter. When will the line be drawn? As the the article claims: humans are pushing artificial selection; so hard that they are placing plants and animals in harsher environments than natural ones. Humans need to remember that they are animals too. Treat the environment the way it was supposed to be treated, or it will soon be all gone.

  2. u14102669 May 6, 2014 at 5:24 am #

    An interesting and thought provoking article, unfortunately lacking in detail. One only managed to fully appreciate some of the topics briefly mentioned if the links in the text were followed.
    It seems obvious that domestication is human led, as the species seemingly has the ability to select and plan towards breeding goals, however, the understanding of selection and inheritance is a largely modern development.
    I do believe that proper study of the long term physical and behavioral changes involved in domestication needs to focus on these more “accidental” domestication, or “co-domestication” processes (as postulated between humans and dogs actively evolving together for instance; http://news.nationalgeographic.com/news/2013/03/130302-dog-domestic-evolution-science-wolf-wolves-human/, http://www4.uwsp.edu/psych/s/275/Science/Coevolution03.pdf)
    In the end, we are trying to decode multiple processes that took generations, of both domesticator and domesticatee, in cultures often more alien to our own than we can imagine, in multiple places around the world with many, many species of both plants and animals.
    I suspect we can not begin to grasp the complexity of the question from the blog article in front of us, but it serves as a good general introduction into new areas of study within the field.

  3. 14005302 May 5, 2014 at 12:33 pm #

    Pets, or as they mentioned it, domesticated animals, are on of my favorite interest from when I can remember!
    After reading this blog I finally got to understand the reasons for many domestication of animals in the past towards domestication today. In the past the humans had to rely more animals and thus did it came that more animals was domesticated, for instance how farmers uses dogs to herd the cattle and sheep, horses and donkeys was for the most of the population the only way of transportation, and farmers had to personally fed the cattle and sheep where now a days the
    are many machinery etc.
    Today most of the domesticated animals are pets to the human-being.

    Although the blog was interesting, it was still a little bit to vague (as mentions in some of the comments) and left me wanting more information.
    Like they mentioned in the blog, this topic is still to be research and there is still many aspects that is to be discovered that will hopefully answered everyone’s, in this case the bloggers, unanswered questions.

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