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April 21, 2014
Washington University in St. Louis
New research on domestication raises more questions than it hasanswered. Scientists have outlined some of the key questions that havebeen raised about this pivotal event in human history.
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More questions than answers as mystery of domestication deepens
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Credit: Brian Hare/Duke University [Click to enlarge image]
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Perhaps the most famous experiment in domestication is a project in Russia thatturned silver morphs of the wild red fox into tamer and more dog-like silver foxes injust 40 generations. But the silver foxes were kept in cages on a fox farm wherethey were sheltered and fed and illicit liaisons with wild foxes were thwarted. Howrepresentative was this experiment of prehistoric domestication events?
e all think we have a rough idea of what happened 12,000 years agowhen people at several different spots around the globe broughtplants under cultivation and domesticated animals for transport, food
or fiber. But how much do we really know?
Recent research suggests less than we think. Forexample, why did people domesticate a mere dozen orso of the roughly 200,000 species of wild floweringplants? And why only about five of the 148 species oflarge wild mammalian herbivores or omnivores? Andwhile we're at it, why haven't more species of eitherplants or animals been domesticated in modern times?
If nothing else, the tiny percentages of domesticatessuggests there are limitations to human agency, andthat it almost certainly is not true that people can stepin and completely remodel through artificial selectionan organism shaped for millennia by natural selection.
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an organism shaped for millennia by natural selection.
The small number of domesticates is just one of manyquestions raised in a special issue of the Proceedingsof the National Academy of Sciences published onlineApril 21.
The issue is the product of a 2011 meeting of scholarswith an interest in domestication at the NationalEvolutionary Synthesis Center, a nonprofit sciencecenter jointly operated by Duke University, the University of North Carolina at ChapelHill 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 studiesanimal domestication, and Kenneth Olsen, PhD, associate professor of biology, whostudies plant domestication.
Both Marshall and Olsen are currently engaged in research on the crumbling marginsof domestication where questions about this evolutionary process loom the largest.
Marshall studies two species that are famously ambivalently domesticated: donkeysand 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 arehandweeded.
Both Marshall and Olsen contributed articles to the special PNAS issue and helpedwrite the introductory essay that raises the big questions confronting the field.
"This workshop was especially fun," said Olsen, "because it brought together peopleworking on plants and animals and archeologists and geneticists. I hadn't reallythought much about animal domestication because I work primarily with plants, so itwas 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 frommodern experience with animal breeding. Anyone familiar with the huge variety of dogbreeds, all of which belong to the same subspecies of the gray wolf, has someappreciation of the power of selective breeding to alter appearance and behavior.
But what about self-fertilizing or wind-pollinated plants, or for that matter, domesticatedanimals accidentally or deliberately bred with wild relatives?
Recent evidence that cereal crops, such as wheat or barley, evolved domesticationtraits much more slowly than had been thought has led to renewed interest in the ideathat selection during domestication may have been partly accidental.
Charles Darwin himself drew a distinction between conscious selection, in whichhumans directly select for desirable traits, and unconscious selection, where traitsevolve as a byproduct of natural selection in crop fields or from selection on othertraits.
"The big focus right now is how much unintentional change people were causing
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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 domesticationprocess because they were attracted to human settlements for food and in somesense domesticated themselves. But new research is showing that otherdomesticated animals may be more like cats and dogs than we thought.
"Once animals such as donkeys or cattle were caught," Marshall said, "the changeshumans sought to make were pretty minimal. Really it just came down to culling a fewof the males and breeding all of the females."
Even today, she points out, African pastoralists can afford to kill only four out of every100 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 selectionwas very weak," she said.
"In the donkeys and other transport animals, it's not affiliative [tame] behavior theherders want," Marshall said. "What they care about more than anything else is thattheir animals stay alive."
So artificial selection is acting in the same direction as natural selection, or maybepushing even harder, because humans often place animals in harsher conditions thannatural ones.
"The comparable idea for plants," said Olsen, "is the dump heap hypothesis, originallyproposed by Edgar Anderson, a botany professor here at Washington University. Theidea is that when people threw out the refuse of plant foods, including seeds, somegrew and again set seed, and in this way people inadvertently selected species theywere eating that also did well in the disturbed and nutrient-rich environment of thedump heap."
"Cultivation practices play a huge role in selection," said Olsen. "Traditionally inSoutheast Asia, many different varieties of rice were grown simultaneously in a givenfield. It was a bet-hedging strategy," he said, "that ensured some plants would surviveand produce seed even in a bad season." So it wasn't people selecting the crop plantsdirectly so much as people changing the landscape in ways that altered the selectionpressure on plants.
How best to time travel Questions about the original domestication events are difficultto answer because plants and animals were domesticated before humans inventedwriting, and so figuring out what happened has been a matter of making do with thelimited evidence that has survived.
The problem is particularly difficult for animal domestication because what mattersmost is animal behavior, which leaves few traces. In the past, scientists triedmeasuring bones or examining teeth, looking for age or size differences or pathologythat 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
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tell us anything," Marshall said. "We've gone away from morphological identifiers ofdomestication, and we're going with behavior now, however we can get it. If we've gotconcentrations of dung, that means animals were being corralled," she said.
Olsen, on the other hand, seeks to identify genes in modern crop species that areassociated with domestication traits in the plant, such as an erect rather than asprawling architecture. The techniques used to isolate these genes are difficult andtime consuming and may not always penetrate as deeply into the past as scientistshad once assumed because present-day plants are only a subset of the crop varietiesthat may have once existed.
So both Marshall and Olsen are excited by recent successes in sequencing ancientDNA. Ancient DNA, they say, will allow hypotheses about domestication to be testedover the entire evolutionary time period of domestication.
Another only recently appreciated clue to plant domestication is the presence ofenriched soils, created through human activities. One example is the terra preta in theAmazon basin, which bears silent witness to the presence of a pre-Columbianagricultural society in what had been thought to be untouched forest.
By mapping distributions of enriched soils, scientists hope to better understand howancient 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 canpossibly get in order to triangulate back. We're using all kinds of ways, coarse-grainedand fine, long-term and short, because the practical implications for us are quitegreat."
After all, the first domestications may have been triggered by climate change at theend of the last ice age -- in combination with social issues.
As a result, people abandoned the hunter-gatherer lifestyle they had successfullyfollowed for 95 percent of human history and turned instead to the new strategies offarming and herding.
As we head into a new era of climate change, Marshall said it would be comforting toknow that we understood what happened then and why.
"The Modern View of Domestication," a special issue of PNAS edited by GregerLarson and Dolores R. Piperno, resulted from a meeting titled "Domestication as anEvolutionary Phenomenon: Expanding the Synthesis," held April 7-11, 2011, that wasfunded and hosted by the National Evolutionary Synthesis Centre (National ScienceFoundation EF-0905606) in 2011.
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The above story is based on materials provided by Washington University in St.Louis. The original article was written by Diana Lutz. Note: Materials may be edited forcontent and length.
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Washington University in St. Louis. "More questions than answers as mystery ofdomestication deepens." ScienceDaily. ScienceDaily, 21 April 2014.<www.sciencedaily.com/releases/2014/04/140421093738.htm>.
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Journal References:
1. Fiona B. Marshall, Keith Dobney, Tim Denham, and José M. Capriles. Evaluatingthe roles of directed breeding and gene flow in animal domestication.PNAS, April 21, 2014 DOI: 10.1073/pnas.1312984110
2. Andrew N. Doust, Lewis Lukens, Kenneth M. Olsen, Margarita Mauro-Herrera, AnnMeyer, and Kimberly Rogers. Beyond the single gene: How epistasis andgene-by-environment effects influence crop domestication. PNAS, April 21,2014 DOI: 10.1073/pnas.1308940110
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