The best physical data about ancient American Indian cultures came in 1924 with the discovery of projectile points embedded in the bone of ancient Pleistocene megafauna, especially ancient mammoths and bison. Most early archaeologists put ancient Native Americans occupation of America at a mere 5,000 years. To find humans involved in the hunting of 'ice age' mammals pushed back the dates at least 10,000 years or to about 15,000 years ago. However, the potential for land bridge openings could go back as far as 100,000 years. Again the physical evidence consistently produced sites with dates of ca.12,500 years ago. Some recent sites have given dates of 19,000 or earlier, but the accuracy of the dates and the validity of artifacts are controversial. Here is a summary of these cultures and representative sites. Dates are years ago or B.P.