For billions of years, Earth's most advanced life was a slimy microbial mat
Earth is about 4.54 billion years old, and clear signs of life appear by 3.7 billion. Yet animals with left and right sides only show up around 555 million years ago. Most of life's story is microbes quietly transforming the planet, including filling the air with the oxygen we now breathe.
The oldest undisputed evidence comes from 3.7-billion-year-old rocks in western Greenland, which hold stromatolites and carbon with a biological signature. Claims go further back: possible traces in 4.1-billion-year-old Western Australian rock, and fossil microbes from Quebec's Nuvvuagittuq belt perhaps 4.28 billion years old, though such tiny rod shapes can also form without life. Between about 4 and 3.8 billion years ago a late heavy bombardment of leftover debris probably pounded the planet, but microbes living underground could have ridden it out.
Early life was dominated by mats of bacteria and archaea. Around 3.5 billion years ago cyanobacteria began photosynthesising, releasing oxygen as waste. Once that oxygen had reacted with everything it could at the surface, it accumulated in the air in the Great Oxygenation Event about 2.4 billion years ago. Complex cells with internal compartments appear by 1.85 billion years ago, probably born when an archaeon and an oxygen-using bacterium merged; the bacterium became the mitochondrion. Multicellular life followed around 1.7 billion years ago.
Animals diversified in the Cambrian explosion, with early vertebrates by about 525 million years ago. Land plants, descended from green algae, appear around 470 million years ago, and early trees such as Archaeopteris may even have helped cause the Late Devonian extinction by drawing down carbon dioxide and flushing nutrients into seas. The Permian–Triassic extinction 252 million years ago wiped out most complex species, opening the way for dinosaurs, whose non-bird members vanished 66 million years ago, letting mammals expand.
Species living today make up less than one percent of all that have ever existed. Around 1.8 million have been named, while one estimate puts the total at a trillion, mostly microbes. Chemistry explains why all of them are built on carbon and water: silicon, carbon's nearest neighbour, forms few complex stable molecules, and ice floating on water lets aquatic life survive winter beneath it.
Source: History of life
