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.These demonstrate that, of course, we do not know everything (and clearly never will), but we know enough.Today, innovative techniques provide further confirmation and understanding of the history of life.Other critics, perhaps more familiar with the data, question certain aspects of the quality of the fossil record and of its dating.These skeptics do not provide scientific evidence for their views.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.
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.
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.
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.
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