Why do humans have belly buttons and mammels don't?

Question:

Why do humans have belly buttons and mammels don't?

Since we're mammels we're all connected to our mothers in the womb, so why is that we as humans are the only ones with belly buttons?

Additional Details

5 days ago
I suddenly just thought of this when my dog rolled over for a belly rub and I realized she was (as to be expected) 'belly button-less'.

Best Answer - Chosen by Asker: 5 days ago
I suddenly just thought of this when my dog rolled over for a belly rub and I realized she was (as to be expected) 'belly button-less'.