The bones of ancient hunters unearthed in Morocco are the oldest known specimens of the human species, potentially pushing back the clock on the origin of modern Homo sapiens, scientists announced Wednesday.

Found among stone tools and the ashes of ancient campfires, the remains date from about 300,000 years ago, a time when the Sahara was green and several early human species roamed the world, the scientists said. That makes them about 100,000 years older than any other fossils of Homo sapiens—the species to which all people...