But for humans whose life span rarely reaches more than 100 years, how can we be so sure of that ancient date? Even the Greeks and Romans realized that layers of sediment in rock signified old age.
But it wasn't until the late 1700s -- when Scottish geologist James Hutton, who observed sediments building up on the landscape, set out to show that rocks were time clocks -- that serious scientific interest in geological age began.
Early geologists, in the 1700s and 1800s, noticed how fossils seemed to occur in sequences: certain assemblages of fossils were always found below other assemblages. Since 1859, paleontologists, or fossil experts, have searched the world for fossils.
In the past 150 years they have not found any fossils that Darwin would not have expected.
But this sediment doesn't typically include the necessary isotopes in measurable amounts.
Fossils can't form in the igneous rock that usually does contain the isotopes.
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.
To determine the ages of these specimens, scientists need an isotope with a very long half-life.Geologist Ralph Harvey and historian Mott Greene explain the principles of radiometric dating and its application in determining the age of Earth.As the uranium in rocks decays, it emits subatomic particles and turns into lead at a constant rate.Measuring the uranium-to-lead ratios in the oldest rocks on Earth gave scientists an estimated age of the planet of 4.6 billion years.Segment from A Science Odyssey: "Origins."Geologists have calculated the age of Earth at 4.6 billion years.