[Illustration: _Fossil skull of Zinjanthropus, nearly 2,000,000 years
old, discovered in 1959 by Dr. L. S. B. Leakey in Olduvai Gorge.
Accurate dating of this earliest human ancestor was possible by using
the potassium-argon method._]
One of the most talked-about age measurements in recent years was the
determination of the unexpectedly great age of fossil ancestors of man,
found by the British anthropologist, Dr. L. S. B. Leakey, in Olduvai
Gorge in Tanzania. The measurements were made by Garniss H. Curtis and
Jack F. Evernden at the University of California in Berkeley by the
potassium-argon method. The age came out a little less than 2 million
years, about twice as old as it “should be” in the view of many
scientists. Human remains of such great antiquity had never been found
before, and much doubt was raised about the validity of the figures.
Time periods as short as two million years are not easy to measure by
potassium-argon. The amount of argon produced in that time is extremely
small, and contamination by argon from the air is a serious problem.
Still, the measurements were repeated, the rocks were studied again, and
the result did not change: The fossils were still about 2 million years
old.
In cases like this, one tries to find some other method to check the
results in an independent way. After many attempts it was discovered
that the same rock strata dated by potassium-argon also contained some
pumice—a porous volcanic glass—and that this glass was suitable for
uranium fission-track dating. The measurements were made in the General
Electric Research Laboratory. What was the result? Just about 2 million
years!
When such altogether different techniques give the same number, one can
have some confidence that the number is exact. It would be difficult to
imagine a disturbance in nature that would cause these unrelated methods
to give the same wrong number—in both cases by a factor of two. The
double check simply means the Olduvai man is 2 million years old. There
is not much doubt about it.