Evolutionary History of Life - Earliest History of Earth

Earliest History of Earth

History of Earth and its life -4500 — – -4000 — – -3500 — – -3000 — – -2500 — – -2000 — – -1500 — – -1000 — – -500 — – 0 — Hadean Archean Protero
-zoic Phanero
-zoic Eo Paleo Meso Neo Paleo Meso Neo Paleo Meso Ceno ← Solar System formed ← Impact formed Moon ← ? Cool surface, oceans, atmosphere ← Late Heavy Bombardment ← ? Earliest evidence of life ← Oxygenation of atmosphere ← Earliest multicellular organism ← Earliest known fungi ← Earliest known cnidarians ← ? Cambrian explosion ← Earliest land invertebrates and plants ← Earliest land vertebrates ← Earliest known dinosaur ← Extinction of non-avian dinosaurs Scale:
Ma (Millions of years)

The oldest meteorite fragments found on Earth are about 4.54 Ga; this, coupled primarily with the dating of ancient lead deposits, has put the estimated age of Earth at around that time. The Moon has the same composition as Earth's crust but does not contain an iron-rich core like the Earth's. Many scientists think that about 40 Ma later a planetoid struck the Earth, throwing into orbit crust material that formed the Moon. Another hypothesis is that the Earth and Moon started to coalesce at the same time but the Earth, having much stronger gravity, attracted almost all the iron particles in the area.

Until recently the oldest rocks found on Earth were about 3.8 Ga, leading scientists to believe for decades that Earth's surface had been molten until then. Accordingly, they named this part of Earth's history the Hadean eon, whose name means "hellish". However analysis of zircons formed 4.4 Ga indicates that Earth's crust solidified about 100 Ma after the planet's formation and that the planet quickly acquired oceans and an atmosphere, which may have been capable of supporting life.

Evidence from the Moon indicates that from 4 Ga to 3.8 Ga it suffered a Late Heavy Bombardment by debris that was left over from the formation of the Solar System, and the Earth should have experienced an even heavier bombardment due to its stronger gravity. While there is no direct evidence of conditions on Earth 4 Ga to 3.8 Ga, there is no reason to think that the Earth was not also affected by this late heavy bombardment. This event may well have stripped away any previous atmosphere and oceans; in this case gases and water from comet impacts may have contributed to their replacement, although volcanic outgassing on Earth would have supplied at least half.