So far as I know, childbirth is generally painful in only one of the millions of species on Earth: human beings. This must be a consequence of the recent and continuing increase in cranial volume... Childbirth is painful because the evolution of the human skull has been spectacularly fast and recent.
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About Carl Sagan
Carl Sagan was a 20th-century American scientist and science communicator. Carl Edward Sagan was an American astronomer, planetary scientist and science communicator. Initially an assistant professor at Harvard, Sagan later moved to Cornell, where he was the David Duncan Professor of Astronomy and Space Sciences and directed the Laboratory for Planetary Studies. Read more on Wikipedia →