Radiometric dating relies on the properties of isotopes.These are chemical elements, like carbon or uranium, that are identical except for one key feature -- the number of neutrons in their nucleus.How do scientists know the bones are really 68 million years old?Today's knowledge of fossil ages comes primarily from radiometric dating, also known as radioactive dating.
Helium ages, however, tend to be too low because the gas escapes from the rock.
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.
The Geologic Time Scale is up there with the Periodic Table of Elements as one of those iconic, almost talismanic scientific charts.
Long before I understood what any of it meant, I'd daydream in science class, staring at this chart, sounding out the names, wondering what those black-and-white bars meant, wondering what the colors meant, wondering why the divisions were so uneven, knowing it represented some kind of deep, meaningful, systematic organization of scientific knowledge, and hoping I'd have it all figured out one day.