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The methods are all based on radioactive decay: The first radiometric dates, generated about 1920, showed that the Earth was hundreds of millions, or billions, of years old.

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Older dates may change by a few million years up and down, but younger dates are stable.For example, it has been known since the 1960s that the famous Cretaceous-Tertiary boundary, the line marking the end of the dinosaurs, was 65 million years old.Early geologists, in the 1700s and 1800s, noticed how fossils seemed to occur in sequences: certain assemblages of fossils were always found below other assemblages. Since 1859, paleontologists, or fossil experts, have searched the world for fossils.In the past 150 years they have not found any fossils that Darwin would not have expected.A key point is that it is no longer necessary simply to accept one chemical determination of a rock’s age.

Age estimates can be cross-tested by using different isotope pairs.

If the fossils, or the dating of the fossils, could be shown to be inaccurate, all such information would have to be rejected as unsafe.

Geologists and paleontologists are highly self-critical, and they have worried for decades about these issues. D., is a vertebrate paleontologist with particular interests in dinosaur origins and fossil history.

Repeated recalibrations and retests, using ever more sophisticated techniques and equipment, cannot shift that date. With modern, extremely precise, methods, error bars are often only 1% or so.

The fossil record is fundamental to an understanding of evolution.

These demonstrate that, of course, we do not know everything (and clearly never will), but we know enough.