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the Early and Middle Pleistocene formations where they were found (Oakley 1974, p. 257; 1975, p. 151). But Oakley (1974, p. 257) pointed out that “in volcanic areas (particularly under tropical conditions) fluorine analysis does not provide a reliable method of relative dating.” If this is so, one wonders why he ran the tests. Nevertheless, the results he reported are consistent with the hypothesis that the human bones at Kanam and Kanjera are as old as the faunal remains at those sites. Of course, the agreement in fluorine content might, as Oakley suggested, be the result of uneven fluorine absorption in a volcanic, tropical environment. But then again, it might not.

Oakley (1974, p. 257) found that a Kanjera 4 skull fragment showed just a trace of nitrogen (0.01 percent), while a Kanjera 3 skull fragment showed none. Neither of the two animal fossils tested showed any nitrogen. The presence of “measurable traces” of nitrogen in the Kanjera 4 skull fragment meant, said Oakley (1974, p. 258), that all the human fossils were “considerably younger” than the Kanjeran fauna.

But certain deposits, such as clay, preserve nitrogen, sometimes for millions of years (Appendix 1.1.2). So perhaps the Kanjera 4 fragment was protected from nitrogen loss by clay. In any case, the Kanjera 3 fragment, like the animal samples, had no nitrogen. It is possible that this human bone was younger than the animal bones, and lost its nitrogen fairly quickly. But the test results do not dictate this interpretation—the bones could be the same age.

As shown in Table 11.2, the uranium content values for the Kanjera human fossils (8 – 47 parts per million) overlapped the values for the Kanjeran fauna (26–216 parts per million). This could mean they were of the same age.

But the human bones averaged 22 parts per million while the mammalian fauna averaged 136 parts per million. To Oakley (1974, p. 257), the substantial difference between the averages meant that “the Kanjera hominids, although fossilized (Upper Pleistocene?), are considerably younger than the Kanjeran faunal stage (Middle Pleistocene).” Similar uranium contents results were obtained at Kanam. The Kanam mandible had 4–12 parts per million eU O , while the Kanam fauna had 60–214 parts per million (Oakley 1975, p. 151). “The low radiometric values of the Kanam jaw fragment strongly suggest that it is younger than the Kanam fauna,” said Oakley (1975, p. 151).

While the uranium content values, as reported, are consistent with the Kanjeran and Kanam faunas being older than the human bones, there are reasons for caution. The values reported for the Kanjeran fauna—26, 131, 146, 159, and 216 parts eU3O8 per million—vary widely. The highest value is 8.3 times greater than the lowest, although the bones are supposedly of the same general age. Also, the uranium content of the Kanjera 3 human fossils ranged from 8 to 42 parts per million, differing by a factor of 5 in a single individual. The high and low values for the Kanam fauna vary by a factor of 3.5, and for the Kanam jaw itself by a factor of 3. This reinforces our observation (Appendix 1.2.4) that the rate at which a bone absorbs uranium depends on many highly variable conditions— such as the concentration of uranium in the groundwater, the rate of groundwater flow, and the nature of the surrounding sediment. Also, different kinds of bone (and, apparently, even different parts of the same bone) may absorb uranium at greatly different rates. All of this tends to reduce the value of uranium content as a relative age indicator.

Oakley himself pointed out: “the distribution of uranyl ions in ground-water, like that of fluorine ions is subject to very considerable variation from place to place . . . it appears that fossil bones of Upper Pleistocene or early Holocene age in Kugata near Mount Homa [close to Kanam] not only contain more fluorine than bones of Lower Pleistocene age at Kanam, but are more radioactive on account of adsorbed uranium” (Tobias 1968, p. 181).

Leakey reported that some of the Kanjera human skull fragments, now classified