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Palaeontology was once limited to digging up fossils
and trying to deduce their age with inaccurate
methods. However, fossil analysis improved
dramatically in the 1960s, with the advent and
refinement of two techniques: radiometric dating and
stratigraphy. The first radiometric method was also
known as carbon-14 dating, and it was usable for
specimens younger than 50,000 years. Later,
potassium-argon dating revolutionized the field by
enabling scientists to detect the radioactive decay of
elements found naturally in rocks and soil surrounding
much older fossils. Stratigraphy, which is the study of
rock layering, actually was developed well before the
1960s, but that was the decade scientists began to
better understand how geological conditions, earthen
layers, and fossil records all relate. The resulting
refinement of biostratigraphy, i.e., the study of the
complete life of a stratum of earth, allowed scientists
to determine the environment and lifestyle of human
ancestors based on fossilized flora and fauna found
within the same layer as the hominine fossils. Since
the 1960s, DNA testing has come to be used widely.
As all living organisms have the same genetic code,
scientists can use DNA variations as a molecular
clock. After splitting with a common ancestor, each
generation develops a constant rate of genetic
mutations. The molecular clock allows scientists to
calculate how long ago the split occurred based on
the number of differences between species. The
method is now helping scientists map the routes that
humans took out of Africa.