It was in large part due to the desire to understand the age of the Olduvai hominid remains that pioneering attempts were made to date geologically young materials using the K-Ar method.

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"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.

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.

Recent advances in K-Ar geochronology, specifically the Ar dating has been a major factor in this success.

This grain-discrete method now permits precise and accurate ages to be measured on single grains and, thus, contaminating grains can be eliminated.

In the past two decades, particularly, discoveries of our fossil ancestors have been made in unprecedented numbers and diversity.

Detailed studies of these fossils provide new insights into human evolution, such as the origin of locomotion and cultural activity, and the evolution of the brain, among many other complex features that have come to define humanity.

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. Estimating the age of hominid fossils is usually a painstaking, two-part process, involving both "absolute" and "relative" dating.

Over time, potassium-40 changes, or decays, into a different material, called argon-40.