All life forms exchange carbon 14 gas with their environment. When they die, this process stops, and the C14 in their bodes begins to decay at a half-life rate of 5730 years. Radiocarbon dating is accurate up to 50,000 years ago. Radiocarbon dating was invented in the 1940s. Since then, scientists have discovered “wiggles” in the radiocarbon curve, because atmospheric carbon has fluctuated over time. The notation cal BP means the date has been calibrated to account for the wiggles. BP - Before Present is the dating convention used for radiocarbon dating. The present is taken to be 1950, the date when radiocarbon dating first began to be used, and before atomic bomb testing interfered with atmospheric carbon. —- Source: https://www.thoughtco.com/archaeological-dating-cal-bp-meaning-3971061