[1] It is important that the limitations of the radioactive carbon “dating” system be held in mind. As the statistics involved in the system are used, there are two chances in three that the “date” of the sample falls within the range given as plus or minus an added number of years. For example, the “date” for the Jarmo village (see chart), given as 6750 ± 200 B.C., really means that there are only two chances in three that the real date of the charcoal sampled fell between 6950 and 6550 B.C. We have also begun to suspect that there are ways in which the samples themselves may have become “contaminated,” either on the early or on the late side. We now tend to be suspicious of single radioactive carbon determinations, or of determinations from one site alone. But as a fabric of consistent determinations for several or more sites of one archeological period, we gain confidence in the “dates.” HOW THE SCIENTISTS FIND OUT So far, this chapter has been mainly about the people who find out about prehistoric men. We also need a word about _how_ they find out. All our finds came by accident until about a hundred years ago. Men digging wells, or digging in caves for fertilizer, often turned up ancient swords or pots or stone arrowheads. People also found some odd pieces of stone that didn’t look like natural forms, but they also didn’t look like any known tool. As a result, the people who found them gave them queer names; for example, “thunderbolts.” The people thought the strange stones came to earth as bolts of lightning. We know now that these strange stones were prehistoric stone tools. Many important finds still come to us by accident. In 1935, a British dentist, A. T. Marston, found the first of two fragments of a very important fossil human skull, in a gravel pit at Swanscombe, on the River Thames, England. He had to wait nine months, until the face of the gravel pit had been dug eight yards farther back, before the second fragment appeared. They fitted! Then, twenty years later, still another piece appeared. In 1928 workmen who were blasting out rock for the breakwater in the port of Haifa began to notice flint tools. Thus the story of cave men on Mount Carmel, in Palestine, began to be known.