Our understanding of the shape and pattern of the history of life depends on the accuracy of fossils and dating methods.
All rocks and minerals contain tiny amounts of these radioactive elements.
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.
Say for example that a volcanic dike, or a fault, cuts across several sedimentary layers, or maybe through another volcanic rock type.
Radioactive dating is a method of dating rocks and minerals using radioactive isotopes.