鸡下蛋，蛋孵鸡，这周而复始的循环到底从哪里开始？科学家们提出了一种解释--鸡由鸟进化而来，鸟由恐龙进化而来，而恐龙从蛋里孵出来。那就是先有蛋喽？让我们一起来看个究竟吧。

　　Chicken or egg? Like a hall of mirrors at the carnival, each attempt at an answer just leads to another question. If the chicken came first, then didn't it hatch from an egg? And if the egg came first, wasn't it laid by a chicken? It's one of those questions that seem unanswerable.

　　Scientists agree on where chickens came from: In a sense, human beings invented them, just like they invented cows and pigs and other domesticated animals on Old MacDonald's Farm.

　　If chickens were interested in tracing their family trees, they would need to bone up on some DNA research done in Japan. Every chicken that ever lived can trace its ancestors, say researchers, to a particular subspecies of Red Jungle Fowl in Thailand.

　　The male Red Jungle Fowl looks a lot like a storybook rooster. But the Jungle Fowl isn't identical to a farm chicken. Unlike chickens, female Red Jungle Fowls have no combs. Another Jungle Fowl peculiarity: After mating season, males replace their bright red and orange ruff with a crop of dull, blackish feathers called "eclipse plumage."

　　Scientists think the first domestic chickens were bred from Red Jungle Fowls more than 8,000 years ago in the region now divided into Thailand and Vietnam. People bred chickens first for cockfighting contests, later for eggs and meat.

　　So the first official "chicken" pecked its way out of an egg laid by a bird that was not-quite-a-chicken. Depending on how you look at it, the egg--or the wild chicken--came first.

　　In creating the domestic chicken--and coming up with some 175 varieties--human beings also created a world where chickens rule the roost: There are more chickens than any other kind of domesticated bird on Earth.

　　And where did birds come from? Scientists think that a group of egg-laying feathered dinosaurs were probably the ancestors of today's birds. So if it weren't for dinosaurs, there wouldn't be any Jungle Fowl OR chickens.

　　We've solved the riddle of where chickens came from. But there's still the question of where eggs came from.

　　Scientists say eggs--handy miniature incubators of life, nutrients already packed inside--evolved more than 1 billion years ago, in the oceans of Earth. When land animals evolved about 250 million years ago, their eggs had a tough covering to retain moisture on dry land. Egg-layers like amphibians, reptiles, and insects flourished. The first "land eggs" pre-dated chickens by about 249,992,000 years.

　　So "the egg" may be one answer to the old riddle, but here's another, if a little longer: The chicken came after the bird, the bird came after the dinosaur, the dinosaur came after the egg. And the egg came long after the first single-celled bacteria, the prokaryotes, evolved in the oceans, some 3.5 billion years ago.

　　bone up on: 专心致志于

　　comb: 鸡冠

　　incubator: 孵卵器