• (Discover Magazine) 
  • Which came first the chicken or the egg? That's easy: Eggs. Microfossil embryos in primitive eggs from southern China are about 600 million years old. 
  • The chicken, however, now the world's most common domestic animal, dates back a mere 4,000 or so years. Most researcher believe that Gallus gallus, the red jungle fowl, was first domesticated in East or Southeast Asia. 
  • In 2014, a study ruffled feathers with claims that bones from northern China showed the wild Gallus gallus had been chick-enized into Gallus gallus domesticus about 10,000 years ago. 
  • But critics suggested the 2014 paper really laid an egg: The bones purported to be from chickens were, they claimed, from pheasants and possibly even dogs. 
  • The word domestication is itself contentious. Some definitions focus on genetic changes due to selective breeding; others describe it as humans removing an animal from its natural environment to control its entire life cycle. 
  • A 2015 study offered a broader definition: a mutually beneficial relationship, over generations, between one organism influencing reproduction and care of another. 
  • The study didn't specify humans as the domesticator because these relationships can exist beyond our sphere, such as numerous species of ants that farm fungus or herd aphids. 
  • Although researchers traditionally used archaeological evidence to determine when our species domesticated another, DNA can also show when a population went from wild to mild. 
  • By comparing ancient and modern DNA from a domesticated animal as well as its nearest wild relatives, researchers can identify when specific genetic mutations associated with domestication arose. 
  • Some of the earliest and most common genetic markers for domestication involve changes in the endocrine system that make an animal less fearful of humans. 
  • In 1868, Charles Darwin was the first to document a collection of physical and behavioral traits seen in domestic animals, particularly mammals, but not their wild relatives. 
  • It wasn't until 2014, however, that researchers offered a single explanation for the phenomenon of floppy ears, smaller teeth, tameness and other "domestication syndrome" traits: a mild deficit in neural crest cells. 
  • In vertebrate embryos, neural crest cells (NCCs) form along the dorsal side, or crest, of the neural tube (the proto-central nervous system). 
  • NCCs spread through the embryo as it develops, and directly or indirectly affect a range of other functions, from the adrenal gland, which controls the fear response, to pigmentation, which can differ between domesticated and wild population: the speed of different between the two populations: the speed of development. Domesticated animals, develop more slowly, so their NCCs spread at a slower pace, having minimal or no effect on the function they typically target. It's that deficit that results in domestication syndrome. 
  • Subsequent genetic studies have supported the NCC deficit hypothesis, most recently an April report in Science on the sequencing of 14 ancient horse genomes up to 4,100 years old; that's close to the estimated dawn of domesticated horses some 5,500 years ago. 
  • Sheep were domesticated twice as long ago as horses, with goats at 10,500 years ago, pigs and hump-less cattle both about 10,300 years ago hot on their heels. 
  • Dogs have the distinction of being the first animal humans domesticated and the only one before the advent of agriculture. A 2016 paleogenetic study found two doggie domestication events' perhaps 14,000 years ago: one in Europe and a second in East Asia. The latter population eventually spread west and replaced the former. 
  • There's genetic evidence for a similar double domestication in cats, too. In June, a study in Nature Ecology & Evolution found the African wildcat Felis silvestris lybica was first domesticated regionally in the Near East before 4,400 B.C. 
  • But a second domestication in ancient Egypt, before the first millennium B.C., proved more successful: This later lineage spread rapidly around the world via land and sea trade routes and eventually took over the Internet. 


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