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Lucy and other members
of her species, Australopithecus afarensis, lived between 3.9 and 3.0 million
years ago. They are believed to be the most ancient common ancestor, or "stem"
species, from which all later hominids sprang.
How do we know when they lived? Estimating the age of hominid
fossils is usually a painstaking, two-part process, involving both "absolute" and
"relative" dating. "Absolute" dating means finding a specific age for an object.
A sample of volcanic ash, for instance, can be given an absolute date of 3.18
million years old. "Relative" dating involves comparing one object to others to
build a chronology. Scientists currently don't have a technique for dating fossils
like Lucy directly, but they can assign these fossils relative dates based on the
age of layers of volcanic ash found above and below them.
The Laetoli footprints are rare treasures in the record of human
ancestry. They are fossils captured in volcanic rock that can be given an absolute
date. (Any remnant of the past, not just bones, can be considered a fossil.) 3.6
million years ago, a volcano now called Sadiman puffed out a cloud of ash that
blanketed the surrounding area. A light rain then turned the ash into a sort of
cement that recorded thousands of tracks of antelopes, rhinos, guinea fowl, and
monkeys, as well as the footprints of our ancestors.
Volcanic rock -- like the trail at Laetoli -- can be dated by a
method called potassium-argon dating. Hot, newly erupted lava and ash contain a
form of the chemical element potassium (called potassium-40) that is radioactive.
Over time, potassium-40 changes, or decays, into a different material, called
argon-40. By comparing the ratio of potassium to argon, scientists gauge how long
this natural clock has been ticking. The age of volcanic rock and ash can be
"pinpointed" to within roughly twenty thousand years -- a mere moment in Earth's
4-billion-year history.
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