In the early 1960s, scientists compared carbon-14 dates with a sequence of dates derived from the tree rings of sequoias, and later from the bristlecone pine (which is the oldest know living species). It was discovered that dendrochronological dates and carbon-14 dates are agreed well until 640 A. D., and generally back to the time of Christ, but from the first century back to approximately 10,000 B. C., the carbon-14 dates tended to be more recent.