In the geological time scale, periods are the third largest interval used to divide the Earth's history into segments of time. The year 2015 is in the Quaternary Period which began 1.6 million years earlier.
Periods are divided by significant changes inferred from rock formations. The life forms present are the defining feature of most periods. Most sea life appeared during the Cambrian Period. The Triassic Period contained the first dinosaurs while the Cretaceous Period ends with the extinction of all dinosaurs. The Tertiary Period brought the appearance of recognizable ancestors of modern mammals. The first modern humans did not appear until the beginning of the Quaternary Period.