Life in the Archaean and Proterozoic eras
Earth’s history is divided into eras: Archaean (the most ancient), Proterozoic (early life), Palaeozoic (ancient life), Mesozoic (middle life) and Cenozoic (new life). The age of rock layers is found from the decay of radioactive elements (radiometric dating). The age of the Earth is about 4.54 billion years; its first half-billion years (4.54–4.0 billion years ago) is often separated as the Hadean. In the Archaean (about 4.0–2.5 billion years ago) there was no oxygen in the atmosphere, volcanoes were many and oceans had formed; fossils are few because the layers were altered by heat and pressure; the oldest traces of life are bacteria and their stromatolites (about 3.5 billion years ago) and photosynthesising cyanobacteria. Photosynthesis released oxygen, which later (about 2.4 billion years ago) accumulated in the atmosphere (the Great Oxidation Event). In the Proterozoic (about 2.5 billion – 539 million years ago) oxygen rose and aerobic organisms spread; about 2 billion years ago cells with a nucleus – eukaryotes – appeared (mitochondria and chloroplasts being the result of symbiogenesis); sexual reproduction and then multicellularity developed (red algae about 1.2–1 billion years ago). At the end of the Proterozoic (the Ediacaran, about 575–539 million years ago) soft-bodied multicellular animals appeared: cnidarian-like, flatworm- and annelid-like forms; bilateral symmetry – having front and back, dorsal and ventral sides – was a major aromorphosis that made movement easier and gathered sense organs in the head. The ozone layer prepared conditions for moving onto land. In some periods the Earth is thought to have been almost fully covered with ice (the “Snowball Earth” hypothesis).
“Time tape”: on a 4.5 m tape, 1 m = 1 billion years; the starts of the Archaean, Proterozoic, Palaeozoic and later eras are marked.