The relative time scales are determind by the distribution of the living organisms which have survived as fossils.
The dating of geological periods is based on the precise measurements of the concentrations of radioactive elements in rocks and of the products of their radioactive decay.
Modern techniques allow smaller and smaller samples to be analysed. As a result the age of rocks and of the Earth can be estimated with increasing accuracy.
What fossils tell us about the Past
Darwin realised that extinctions are a likely result of evoultion by natural selection. Extinct species have failed to adapt to changing environment of competition from other species.
According to data, the background extinction rate works out at around eight families of species becoming extinct per million years. Calculation of this figure depends on which species you include.
The Great Dying
A extinction event that happened 250 million years ago which wiped out whole groups of organisms…