How Old ? - A Better Answer

Throughout this book the age of the Irvington fossils deposits is given as approximately 1.3 million years. This is a new estimate established by scientists at the University of California. The Irvington deposits were previously thought to be between 500,000 and 800,000 years old. The new calculation is the result of a new method of dating ancient rocks.

Bob Bray in 1950 uncovering  fossils from the T-2 site in Irvington.

This method, called the potassium-argon (K40/Ar40) method of dating, enables scientists to determine the age of some rocks so accurately that the possibility of error amounts to only about 1 percent. It has been in use since about 1950 and makes possible a more accurate and detailed ordering of past events both physical and biological.

For scientist attempting to discover the evolutionary development of living things, this method of dating is extremely important. The method confirms to a remarkable degree certain earlier estimates of the ages of rocks formed in the Cenozoic era. These estimates were based on studies of (1) how layers of rock and fossils within them were deposited, (2) how deeply rocks and fossils were buried by other sediments, and (3) how various animals developed through evolutionary change.

The Cenozoic fossil-bearing strata on which K40/Ar40 dates are based have not been greatly disturbed; that is, they have not been severely broken by faults or greatly distorted by folding. (Folds are commonly formed by great pressures that squeeze the strata horizontally.) The strata are in the order of original deposition. But without knowing (within that small percent of error, at least) how much older is the stratum underneath than the one above, there is no way to measure the time it too for evolutionary changes to occur in the animals that are now buried there.

The ability to closely clock evolutionary changes in living things is of special importance to the pale ecologist because this also allows him to estimate the evolution of environments. For example, for several million years the first horses were browsers, as evidenced by the structure of their teeth. This means that shrubs and trees were part of their environment. Eventually, horses became grazers (as are modern horses). Grasses, then, must have become important features of their environment. If it is possible to closely date the evolutionary change from browsers to grazers, it thus becomes possible to date the development of the vegetation upon which these horses lived.

Furthermore, the age of any animal fossil at a particular location determines the age of all fossils found with it in the same stratum. (Unless, of course, some came from an earlier cycle of erosion.) By use of the potassium-argon method, scientists have been able to show that similar types of animals lived in various locations at the same time. This means that the environment of these widespread populations were also similar.