▫ What primarily defines life is the will to live. Life blossomed into a plethora of forms, each vectored by adaptive impulses to the immediate environment. As life diversified, interdependence among species intensified in both competitive and cooperative relationships.
▫ Earth’s 4.55-billion-year history is divided into a nested hierarchy of eons, eras, periods, epochs, and ages: eons have eras, eras comprise periods, and so on.
▫ Each of the 4 eons are characterized by major life stages: Hadean (4.54–3.8 BYA): before life; Archean (3.8–2.5 BYA): before an oxygenated atmosphere; Proterozoic (2,500–542 MYA): early life; and Phanerozoic (542 MYA–now): life that leaves lots of fossils.
▫ The end of each geological era is marked by a major mass extinction event.