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280,000 years for sample MB8, a mastodon tooth, from El Horno. The El Horno site is at a lower stratigraphic level than any of the Hueyatlaco layers, and contained tools similar to those of Unit I, the lowest tool-bearing layer at Hueyatlaco. One wonders what remains of human culture might be found at levels lower, and hence older, than El Horno.

Szabo reported that he used calculations based on both open and closed systems in obtaining the above uranium series test results. Nevertheless, some scientists have suggested that these dates are in error because uranium and its decay products may have migrated into or out of the samples over the course of their interment to a greater extent than Szabo supposed. Cynthia Irwin-Williams, who originally discovered the tools, suggested that the real age of the samples should be around 25,000 years. But careful study of the data supplied by Szabo, who performed the uranium series tests, appears to rule out the hypothesis that migration caused falsely old dates.

There are two ways that a falsely old age can be obtained by uranium series dating—outward migration of uranium or inward migration of byproducts. If uranium has migrated out, this will result in a higher ratio of byproducts (thorium or protactinium) to uranium in a sample, and hence a greater than normal age for the sample. If byproducts (thorium or protactinium) have migrated in, that will, of course, also result in a higher than normal ratio of byproducts, and hence a greater age for the sample. This latter alternative is, however, highly unlikely since both thorium and protactinium are virtually insoluble in water.

Furthermore, thorium 230, the isotope produced by the decay of uranium 234, is always accompanied in nature by the far more common isotope thorium 232. So let us suppose that the Hueyatlaco bone samples are in fact very young. Let us also suppose, although it is quite unlikely, that thorium 230 and thorium 232 have migrated into the bone, giving a falsely old age. In this case, one would expect to find a low ratio of thorium 230 to thorium 232, because thorium 232 is more common than thorium 230. But it was reported (Szabo et al. 1969, p. 243) that the ratio of thorium 230 to thorium 232 in the samples under consideration was “unusually high,” which indicates that virtually all the thorium 230 measured in the samples was produced by the decay of uranium 234.

We have thus established that the uranium byproducts thorium and protactinium most probably did not migrate into the samples. That means that the hypothesis of a falsely old age depends on uranium migrating, or leaching, out of the samples.

In order to investigate the possibility of uranium leaching out of a sample, one of us (Thompson) analyzed two of several possible models—one in which leaching takes place at the end of the period of burial and one in which leaching is continuous throughout the period of burial. We shall now briefly discuss the results of these calculations.

Let us first consider the model in which leaching of uranium took place at the end of the period of burial. Taking bone sample MB3, we assumed, as claimed by Cynthia Irwin-Williams, that its real age is only 25,000 years instead of roughly 245,000 years. Then we computed the amount of leaching that must have taken place in order to give a date of 245,000 years for sample MB3, using the ratio of protactinium to uranium 235.

Sample MB3 was also originally dated at roughly 245,000 years using the ratio of thorium to uranium 234. So when we plugged the leaching factor for uranium 235 into the uranium 234 series equations we expected that the ratio of thorium to uranium 234 would yield a date of 25,000 years. Here we assumed that uranium 234 and uranium 235 are chemically identical (as atomic theory says they are) and that any leaching process would affect them equally. But the ratio of thorium to uranium 234, when calculated using the standard equations for radioactive decay, yielded an age of 52,451 years instead of 25,000 years. This result calls into question