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.
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.
This conclusion was based upon carefully designed and conducted experiments that compared the ratios in rock samples of parent elements to daughter elements ( some of which would have been from radioactive decay of the parent, some of which may have been present in the sample at the time of formation).Current understanding of the history of life is probably close to the truth because it is based on repeated and careful testing and consideration of data.The rejection of the validity of fossils and of dating by religious fundamentalists creates a problem for them: Fossil sequences were recognized and established in their broad outlines long before Charles Darwin had even thought of evolution.Another possibility is (as in the case of the potassium - argon - K-Ar method) that because the daughter element is gaseous, it would escape from the rock when the rock was molten.
Once the rock cooled, the gaseous daughter would be trapped in the rocks crystal structure and could no longer escape.
For this reason, the tests were designed to account for those possibilities.