[JimSEA]

Radiometric dating, explained well in a Wikipedia entry. "Radiometric dating (often called radioactive dating) is a technique used to date materials such as rocks, usually based on a comparison between the observed abundance of a naturally occurring radioactive isotope and its decay products, using known decay rates." A limiting factor on fossils as old as those discussed in this article is that there is not a lot of rock preserved from that far back.

[thackney]

Using what isotope for those time frames?

The precision of a dating method depends in part on the half-life of the radioactive isotope involved. For instance, carbon-14 has a half-life of 5,730 years. After an organism has been dead for 60,000 years, so little carbon-14 is left that accurate dating can not be established. On the other hand, the concentration of carbon-14 falls off so steeply that the age of relatively young remains can be determined precisely to within a few decades.