http://www.madsci.org/posts/archives/sep2000/967827843.Zo.r.html

Message:
All placental mammals have navels, which are simply scars left at the site where the umbilical cord was attached to the body. While the point is subject to much debate, there is evidence that several other amniotes hatch from their eggs still attached to their egg blood vessels through an allantoic stalk homologous to the umbilical cord. Hence, birds and reptiles may be considered to have "navels" (or at least something homologous to a navel) as well.

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this is the only kind of info I can find online..I don't have a question about navels--I am curious about the things we call bellybuttons. I want to know when/why humans started knowing to make their babies have them.

(forgot to thank you for links, Bee. Thanks!)