- The Precambrian Era spans from 4600 to 540 million years ago and is characterized by the emergence of early life forms such as anaerobic prokaryotes, photosynthetic prokaryotes, and multicellular algae, with life existing exclusively in marine environments.
- The Paleozoic Era (540–245 million years ago) begins with the Cambrian Explosion, which introduced diverse invertebrates like trilobites and brachiopods. It includes the Ordovician, Silurian, Devonian (Age of Fishes), Carboniferous (coal formation, first seed plants), and Permian periods, culminating in Earth’s largest mass extinction, wiping out 95% of marine species.
- The Mesozoic Era (245–65 million years ago) is divided into the Triassic (rise of dinosaurs, cycads, conifers), Jurassic (dinosaurs dominant, first birds, Pangaea breaks apart), and Cretaceous (dinosaurs still dominant until a mass extinction event eliminates them along with 50% of plant and animal species; angiosperms become dominant).
- The Cenozoic Era (65 million years ago to present) comprises the Tertiary (flourishing of angiosperms, insects, placental mammals, and rapid mammalian evolution) and Quaternary periods (climate cooling, ice ages, appearance of mastodons, mammoths, and Homo sapiens 200,000 years ago; climate began warming 20,000 years ago).
Parent Tip: Review the logic above to help your child master the concept of geologic time scale worksheet.
