Actually... according to a video we saw in science
( i may or may not be reapeating what Lebis and pyro said)
According to scientists, comets repedetly pelted the earth etc.
One day a HUGE comet crashed into the earth and sorta buried itself inside. The chunk of the earth floated off and later became teh moon.
Im pretty sure the comet didnt fuse with the moon.
As for the chiken and egg, I believe the egg came first as a reptile egg. possibly t he archeapthrox or however you spell his name ( the repltile bird dinosaur creature) laid an egg and some extreme conditions helped deform the egg creating a misshapened reptile bird which inheirited some of the archeapthrox's flyijg ability and beak. Over time the "chicken" probably evolved to meet its habitat and became what we came to know as chicken after being caught by farmers and domesticated
I personally have read nothing on the egg and chiken thing, above is my theroy i developed moments ago