We provide “hard” minimum and “soft” maximum age constraints for 30 divergences among key genome model organisms; these should contribute to better understanding of the dating of the animal tree of life.Calibrating the tree of life has long been the preserve of paleontology but its place has recently been usurped completely by molecular clocks.In doing this, we argue that paleontological data do not provide actual age estimates for divergence events, but they can provide rather precise minimum constraints on the calibration of molecular clocks, and much looser maximum constraints.The evidence of a “hard” lower bound (minimum constraint) and a “soft” upper bound (maximum constraint) provided from paleontology can then be fed into a molecular clock analysis. Traditionally, very small numbers of calibration dates have been employed and these have been selected for utility and have rarely been defended.By the end of the 19th century, geologists had used these principles to put together an outline of the geological history of the world, and had defined and named the eons, eras, periods, and epochs of the geologic time scale.They did not know how many thousands, millions, or billions of years ago the Cambrian period began, but they knew that it came after the Proterozoic Eon and before the Ordovician Period, and that the fossils unique to Cambrian rocks were younger than Proterozoic fossils and older than Ordovician ones.Fossils found in China since the 1990s have provided new information about the earliest evolution of animals, early fish, dinosaurs and the evolution of birds and mammals.
As a "historical science" it tries to explain causes rather than conduct experiments to observe effects.
Fossil data are fundamental to molecular clock methodology, providing the key means of clock calibration, but their commonplace use is far from satisfactory.
We consider the utility and qualities of good calibration dates and, on that basis, we propose a number of well-supported dates, and give ages based on the best current information.
What would the geologist be able to deduce about the world we live in, based on what was left in the strata?
Stratigraphy started to become a formal science due to the work of a man who published under the name Nicolaus Steno in the 17th century.