I saw this short story in an anthology years ago. At least 20 years old, probably more.
This was a humorous short story about a child of human parents who was born as a tiny earth, complete with landform and weather, which then rapidly evolved tiny dinosaurs and finally primates, to the fascination of the baby's brother and parents. There were some amusing swipes at the government's attempts to take the baby away from the parents, and other social & political responses to this bizarre event. Baby had a post-hippie type of name, perhaps Zenobia?