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.
These are chemical elements, like carbon or uranium, that are identical except for one key feature -- the number of neutrons in their nucleus.
Fossils are generally found in sedimentary rock not igneous rock.
Sedimentary rocks can be dated using radioactive carbon, but because carbon decays relatively quickly, this only works for rocks younger than about 50 thousand years.
Until this century, relative dating was the only technique for identifying the age of a truly ancient object.
By examining the object's relation to layers of deposits in the area, and by comparing the object to others found at the site, archaeologists can estimate when the object arrived at the site.