Planet Earth

Human Skull Begins to Fill in 20,000-Year Gap in Fossil Record

80beatsBy Sophie BushwickAug 22, 2012 3:15 PM

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Back in the day, in the northern part of modern-day Laos

, an early modern human died and its corpse washed into a nearby cave. Sure, it doesn’t sound like a particularly noteworthy event. But researchers dated the remains of this human's skull to at least 46,000 years ago, making it the oldest modern human ever discovered in Southeast Asia. Scientists discovered the skull fragments back in 2009, but have only this week published their findings in the Proceedings of the National Academy of Sciences

. Features not found in any earlier specimens of the Homo genus, such as the absence of a brow ridge

on the skull's frontal bone

, mark it as a modern human. The remains had been hidden under layers of sediment that had washed into the cave over the years. Because there were no signs of a dwelling or of a ceremonial burial, the body to which the skull once belonged was probably swept into the cave from outside as well. Luminescence dating

, which measures the energy that crystals in the dirt can store, told the researchers that the last time the sediment layer around the skull experienced sunlight or heat---before it drifted into the dark cave---was between 46,000 and 51,000 years ago. And they calculated the skull’s age with uranium-thorium dating

, a radiometric

 technique often used on materials that retain uranium, like bones and teeth. The skull was about 63,000 years old, indicating that it lay outside the cave for a while before it washed in along with the sediment on which it sat. This time window makes the new fossil at least 20,000 years older than modern human fossils previously uncovered in the same area, illuminating a blank period in the fossil record and providing evidence for the commonly accepted “Out of Africa” theory

of human development. How exactly Homo sapiens spread across the globe is still under debate, but according to this hypothesis, modern humans evolved in Africa and then quickly migrated out of the continent. This skull demonstrates that modern humans had reached Southeast Asia by about 60,000 years ago, a migration pattern that aligns with archaeological and genetic evidence

, as well as the Out of Africa theory's predictions. Image courtesy of Laura Shackelford / University of Illinois at Urbana-Champaign

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