"Some of the most highly publicized applications of the racemization method have been to human skeletal remains found along the coast of California. One, called the Del Mar man, was dated by this method at 48,000 years. Another, the skeleton of a female found in an excavation near Sunnyvale, appeared to be even older, a startling 70,000 years!
These ages created quite a stir not only in the public press but especially among paleontologists, because no one had believed that man was in North America that long ago. Speculation arose that man could have wandered across the Bering Strait from Asia as much as a hundred thousand years ago. But how certain were the dates turned out by this novel method?
To answer this, tests were made by a radioactive method involving intermediate decay products between uranium and lead that have half-lives suitable for this range. This gave ages of 11,000 years for the Del Mar skeleton and only 8,000 or 9,000 for the Sunnyvale. Something was wrong.
The big uncertainty in racemization ages is the unknown thermal history of the specimen. As mentioned above, the rate of racemization is extremely sensitive to temperature. If the temperature goes up by 25 degrees Fahrenheit (14° C), the reaction goes ten times as fast. How could anyone know what temperatures the bones could have been exposed to so many years in the past? How many summers might they have lain bare under a hot California sun? Or might they even have been in a campfire or a forest fire? Besides the temperature, other factors have been found to affect the rate greatly, such as the pH (degree of acidity).
One report says: “Amino acids in sediments show an initial rate of racemization almost an order of magnitude (tenfold) faster than the rate observed for free amino acids at a comparable pH and temperature.”
Even that is not the end of the story. One of the Sunnyvale bones was tested for radiocarbon, both by the counting of beta particles from decaying atoms and by the newer atom-counting method. These gave roughly concordant values. The average was only 4,400 years!
What can we believe? Obviously some of the answers are terribly wrong. Should we put more confidence in the radiocarbon date, since there is longer experience in using it? But even with it, different samples from the same bone varied from 3,600 to 4,800 years. Perhaps we should just admit, in the words of the scientist quoted previously, “Maybe all of them are wrong.”"
Awake 1986 9/22
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