Thursday, November 26, 2009


The answer to the age-old riddle is biologically obvious

In March 2006, on the occasion of the release of Chicken Little on DVD, Disney convened a panel to put an end to the long-standing riddle: Which came first, the chicken or the egg? The verdict was unanimous. “The first chicken must have differed from its parents by some genetic change [that] caused this bird to be the first ever to fulfill our criteria for truly being a chicken,” said John Brookfield, an evolutionary biologist at the University of Nottingham in England. “Thus the living organism inside the eggshell would have had the same DNA as the chicken that it would develop into, and thus would itself be a member of the species of chicken.” What we recognize as the DNA of a chicken exists first inside an egg. Egg came first. Yet despite the unified front of the three-person panel—David Papineau, a philosopher of science, and Charles Bourns, a chicken farmer, agreed in spirit with Brookfield’s analysis—the question is at best incomplete, at worst misleading. If we take “chicken” to mean a member of Gallus gallus domesticus (a subspecies of junglefowl that evolved in Southeast Asia and has been domesticated for perhaps 10,000 years), we could ask at what point the first member of this species appeared (and whether it was in bird or egg form). Yet speciation is not a process that happens in an instant or in an individual. It takes generations on generations of gradual change for a group of animals to cease interbreeding with another group; only then can we say that speciation has occurred. Viewed in this way, it does not make sense to talk about the first chicken or the first egg. There was only the first group of chickens—some of whom, presumably, were in egg form. And if one relaxes the species qualification, then the race is not even close. Invertebrates as simple as sponges rely on some form of egg for reproduction, which means that eggs probably predate the Cambrian explosion in biodiversity of 530 million years ago. Fish and amphibians lay gelatinous eggs; ancestors of reptiles and birds laid the first shelled eggs 340 million years ago, and that innovation, which allowed their eggs to survive and mature on dry land, enabled the rise of land vertebrates long before the first rooster crowed.

Source of Information : Scientific American September 2009

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