英文阅读：鸡生蛋，蛋生鸡？
关于这个问题的讨论似乎是经久不衰，可答案又不是那么容易得出的。那么，咱们就接着讨论讨论吧。This question appears regularly in the question file, so let's take a shot
at it.
In nature, living things evolve through changes in their DNA. In an animal
like a chicken, DNA from a male sperm cell and a female ovum(卵) meet and combine
to form a zygote(受精卵) -- the first cell of a new baby chicken. This first cell
divides innumerable times to form all of the cells of the complete animal. In
any animal, every cell contains exactly the same DNA, and that DNA comes from
the zygote.
Chickens evolved from non-chickens through small changes caused by the
mixing of male and female DNA or by mutations(突变) to the DNA that produced the
zygote. These changes and mutations only have an effect at the point where a new
zygote is created. That is, two non-chickens mated and the DNA in their new
zygote contained the mutation(s) that produced the first true chicken. That one
zygote cell divided to produce the first true chicken.
Prior to that first true chicken zygote, there were only non-chickens. The
zygote cell is the only place where DNA mutations could produce a new animal,
and the zygote cell is housed in the chicken's egg. So, the egg must have come
first.
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