The final quarter of the 20th century saw the development of molecular phylogenetics, which investigates how closely organisms are related by measuring how similar the DNA is in their genomes.
Molecular phylogenetics has also been used to estimate the dates when species diverged, but there is controversy about the reliability of the molecular clock on which such estimates depend.
The science became established in the 18th century as a result of Georges Cuvier's work on comparative anatomy, and developed rapidly in the 19th century. Use of all these techniques has enabled paleontologists to discover much of the evolutionary history of life, almost all the way back to when Earth became capable of supporting life, about .Areas less dense than surrounding stones, easily pass X-rays, causing darkening of the image. This dinosaur footprint in cretaceous limestone on the banks of the Paluxy River is approximately 30 cm (one foot) across, and located with similar tracks.Early excavations in this area revealed human footprints inside the dinosaur tracks leading to rumors that the human prints had clawslater excavations uncovered human footprints up to 64 cm long (25 inches) next to the dinosaur printsthis discounted the doubters. This confirms that dinosaurs occupied the arbitrary and spurious "cretaceous" period of geological history.The term itself originates from Greek παλαιός, palaios, i.e. As knowledge has increased, paleontology has developed specialised sub-divisions, some of which focus on different types of fossil organisms while others study ecology and environmental history, such as ancient climates.Body fossils and trace fossils are the principal types of evidence about ancient life, and geochemical evidence has helped to decipher the evolution of life before there were organisms large enough to leave body fossils.