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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.New discoveries have filled in the gaps, and shown us in unimaginable detail the shape of the great ‘tree of life’.Darwin and his contemporaries could never have imagined the improvements in resolution of stratigraphy that have come since 1859, nor guessed what fossils were to be found in the southern continents, nor predicted the huge increase in the number of amateur and professional paleontologists worldwide.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.Since then, geologists have made many tens of thousands of radiometric age determinations, and they have refined the earlier estimates.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.

Age estimates can be cross-tested by using different isotope pairs.The field relationships, as they are called, are of primary importance and all radiometric dates are evaluated against them.For example, a geologist may examine a cutting where the rocks appear as shown in Figure 1.All these labors have not led to a single unexpected finding such as a human fossil from the time of the dinosaurs, or a Jurassic dinosaur in the same rocks as Silurian trilobites.Paleontologists now apply sophisticated mathematical techniques to assess the relative quality of particular fossil successions, as well as the entire fossil record.

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