 Paleontology. Paleontology or Paleontology is the scientific study of life that existed prior to, and sometimes including, the start of the Holocene epigraphy eleven thousand seven hundred years before present. It includes the study of fossils to determine organisms evolution and interactions with each other and their environments their paleoecology. Paleontological observations have been documented as far back as the 5th century BC. The science became established in the 18th century as a result of Georgian's work on comparative anatomy, and developed rapidly in the 19th century. Paleontology lies on the border between biology and geology, but differs from archaeology in that it excludes the study of anatomically modern humans. It now uses techniques drawn from a wide range of sciences, including biochemistry, mathematics, and engineering. 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 3.8 billion years ago. As knowledge has increased, paleontology has developed specialized subdivisions, 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. Estimating the dates of these remains is essential but difficult. Sometimes adjacent rock layers allow radiometric dating, which provides absolute dates that are accurate to within 0.5%, but more often paleontologists have to rely on relative dating by solving the gypsy puzzles of biostratigraphy. Classifying ancient organisms is also difficult, as many do not fit well into the linean taxonomy that is commonly used for classifying living organisms, and paleontologists more often use clodistics to draw up evolutionary family trees. 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.