Our probable ancestors, Homo erectus and Homo habilis -now extinct- are classified as of the same genus (Homo) but of different species, although no one (at least lately) has attempted the appropriate experiments to see if crosses of them with us would produce fertile offspring.
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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 →