Showing Their Age- page 2 | History | Smithsonian
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(Cheryl Carlin)

Showing Their Age

Dating the Fossils and Artifacts that Mark the Great Human Migration

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(Continued from page 1)

Because the hominid skulls and other artifacts found at Herto could not be directly dated—the organic material had long since been fossilized—the researchers instead performed their analysis on volcanic rock that was embedded in the sandstone near the fossils. The rock was about 154,000 to 160,000 years old, making the skulls the oldest Homo sapiens remains yet to be found.

Engraved ocher stones, Blombos Cave, South Africa
Age: ~77,000 years old
Method: thermoluminescence dating

An excavation of a seaside cave in South Africa revealed two objects that were clearly manmade—pieces of ocher stone etched with a crisscross pattern. Neither the stones nor the rock in which they were buried were volcanic in origin, though, so the researchers chose another method for determining their age: thermoluminescence.

As in argon-argon dating, the thermoluminescence clock also begins with the last time that a rock was heated to a high temperature. The extreme heat eliminates electrons stored in certain crystals—such as quartz and feldspar—within the rock. Over time, the crystals trap electrons produced by trace amounts of radioactive atoms found in the environment. By reheating the rock, scientists can release the stored energy, which is given off as light and called "thermoluminescence." The intensity of the light indicates how long it has been since the rock was last heated.

Like the Herto skulls, the ages of the carved ocher stones from Blombos Cave could not be directly determined. However, in the same rock layer as the ochers were pieces of burnt stone, which were likely the same age as the ochers and ideal for thermoluminescence dating. The burnt stone, it was revealed, was about 77,000 years old, which made the ochers some of the oldest pieces of abstract design to be discovered.

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About Sarah Zielinski
Sarah Zielinski

Sarah Zielinski is an award-winning science writer and editor. She is a contributing writer in science for Smithsonian.com and blogs at Wild Things, which appears on Science News.

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