Biologists actually have at their disposal several independent ways of looking at the history of life - not only from the order of fossils in the rocks, but also through phylogenetic trees. Relative dating is done by observing fossils, as described above, and recording which fossil is younger, which is older.
The discovery of means for absolute dating in the early 1900s was a huge advance.
Results from different techniques, often measured in rival labs, continually confirm each other.
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.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.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.
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.