FOSSIL DATING
A Better Test
In the 1950s important advances were made in scientists' ability to date fossils accurately. While radioactive dating techniques had been developed before 1950, the new radioactive-potassium dating system devised during the decade was much more reliable.
Decaying Isotopes
Radioactive dating is possible because all naturally occurring material contains small amounts of radioactive isotopes, which are maintained at a predictable ratio to nonradioactive elements in the same material. When an animal dies, the nonradioactive traces remain stable during decay, but the radioactive traces diminish at a steady rate over a very long period of time. Long after an organism's death, the amount of radioactive element remaining in dead tissue can be carefully measured and compared to nonradioactive material to determine how long it has been since the organism's death.
Carbon Dating
Standard radioactive dating processes such as carbon dating, discovered in 1948, will not work for most fossils, though. Fossilized remains contain very little carbon, if any, from the original organism. The only methods of dating fossils in the early 1950s were crude. A scientist might date a new specimen based on knowledge of the age of other specimens in the area. Or the scientist might know how deep the new fossil was when recovered and make an estimate about the age of the remains by guessing how long it took layers of earth to form over it. Such estimates were known not to be very accurate.
Uranium Dating
A group of geologists and physicists at Berkeley were using radioactive dating to measure uranium traces so they could determine the age of ancient rocks and fossils. The half-life of the uranium isotope (the time it takes to lose half its radioactivity by conversion to lead) being measured is 4.5 billion years. This system works well for dating ancient rock, but a method based on a quicker rate of disintegration had to be found for dating more-recent formations, such as animal fossils, which are unlikely to be old enough for the the uranium disintegration to be measurable.
Potassium Dating
The Berkeley group turned to radioactive potassium as a dating measure. Like radioactive uranium, radioactive potassium is found in small amounts in rock. It forms argon (a gas) and has a half-life of only 1.31 billion years. So it was theoretically possible to measure the small amounts of radioactive potassium converted to argon over a period of a few million years. The measurements were still difficult, but by using very laborious extraction methods and a highly sensitive monitoring device called a mass spectrometer, the Berkeley group succeeded in developing an acceptable method.
PILTDOWN MAN
In 1908 the fossilized remains of a man were tound in Piltdown, Sussex, in the south of England. Because these fossils, which came to be called the Piltdown Man, were found near the remains of mammals known to have lived in the Lower Pleistocene age (also known as the Ice Age), scientists, who back then had no reliable method of fossi1 dating, assumed that the Piltdown Man was of that era himself. Some scientists were dubious, but Piltdown advocates successfully argued that their man had lived between 200,000 and 1 million years ago.
In 1952 scientists tested the Piltdown Man using the fluorine method of fossil dating. Fluorine accumulates in fossils from groundwater at a predictable rate, so by measuring the fluorine content, assumptions about age can be made. Modern scientists found that the jawbone of the Piltdown Man contained bones of two different ages. X rays confirmed that the Piltdown Man's jaw had been altered within the century. He was a hoax. His oldest parts were no more than 50,000 years old, making Piltdown a recent link, and a scientifically insignificant one, in the chain of evolution.
Source:
Alexander Hellemans and Bryan Brunch, The Timetable of Science (New York: Simon & Schuster, 1988): 512.
Recent Developments
Not every fossil bed contains enough naturally occurring radioactive potassium to make the technique useful. After the Berkeley method was announced, similar methods of accurate dating have been developed. Since the pioneering work by the Berkeley
group, dating by radioactive decay has become the standard in anthropology and archaeology.
Source:
John Reader, Missing Links: The Hunt for Earliest Man (Boston: Little, Brown, 1981).
