For example, if fossils of B date to X million years ago and the calculated “family tree” says A was an ancestor of B, then A must have evolved earlier.
It is also possible to estimate how long ago two living branches of a family tree diverged by assuming that DNA mutations accumulate at a constant rate.
Particularly useful are index fossils, geographically widespread fossils that evolved rapidly through time.
The principle of radiocarbon dating is simple: the rates at which various radioactive elements decay are known, and the ratio of the radioactive element to its decay products shows how long the radioactive element has existed in the rock.
This rate is represented by the half-life, which is the time it takes for half of a sample to decay.
Together with stratigraphic principles, radiometric dating methods are used in geochronology to establish the geological time scale.
Beds that preserve fossils typically lack the radioactive elements needed for radiometric dating (” radiocarbon dating ” or simply “carbon dating”).
There are several different methods for estimating the ages of fossils, including: Paleontologists rely on stratigraphy to date fossils.
Stratigraphy is the science of understanding the strata, or layers, that form the sedimentary record.
This is difficult for some time periods, however, because of the barriers involved in matching rocks of the same age across continents.
Family-tree relationships can help to narrow down the date when lineages first appeared.
Misleading results can occur if the index fossils are incorrectly dated.