Event Geologic Time Period Era Man appears[4] (2 million years ago) Quaternary CENOZOIC Tertiary Extinction of the dinosaurs (70 million years ago) Cretaceous MESOZOIC Mammals appear (130 million years ago) Jurassic Triassic Oldest known reptiles (300 million years Permian PALEOZOIC ago) Pennsylvanian Mississippian Devonian Silurian Ordovician Cambrian First abundant life in the sea (animals without backbones) (550 million years ago) Algae and other microorganisms (1,900 Proterozoic PRECAMBRIAN million years ago) Archean Oldest rocks in North America (2,800 million years ago) First hint of life (bacteria?) (3,100 million years ago) Oldest rocks (3,300 million years old) Formation of Earth’s core (4,500 million years ago) THEORY OF NUCLEAR AGE DETERMINATION We can think of the nucleus of an atom as a sort of drop—a bunch of NEUTRONS and PROTONS held together by very strong short-range forces. These elementary particles within a nucleus are not arranged in any fixed or rigid array, but are free to move about within the grip of these forces. These motions may be quite violent, but for most NUCLIDES found in nature, the nuclear forces are powerful enough to keep everything confined; thus the nuclei of these atoms hold together, and are said to be stable. If any one nucleus of a given ISOTOPE is stable, then all others are also stable, because what is true for one atom of a given kind is true for all others of the same kind.[5] Some nuclides, both man-made and natural, are unstable, however. Their nuclei are in such violent turmoil that the nuclear forces cannot always hold them together, and various bits and pieces fly off. If we were to try to predict when one particular unstable nucleus would thus disintegrate, however, we could not succeed, because the instant any specific decay (or disintegration) event will occur is a matter of chance. Only if a large number of unstable nuclei of one kind are collected together can we say with certainty that, out of that number, a certain proportion will decay in a given time. It turns out that this proportion is the same regardless of any external conditions.