A brief account of evolution

The first cellular life appeared about 2000 million years ago (mya). How non-cellular aggregates of giant molecules became cells with membranes is not known.

Some early cells could release O₂, probably by a reaction like the light reaction of photosynthesis, where water is split using captured solar energy. Single-celled forms slowly became multicellular.

Invertebrates had formed and were active by about 500 mya. Jawless fish probably arose around 350 mya, and seaweeds with a few plants were probably present around 320 mya.

Plants were the first organisms to invade land, and were widespread there when animals followed.

About 350 mya, fish with sturdy fins could walk onto land and return to water. These lobefins gave rise to the first amphibians. In 1938 a Coelacanth, a lobefin believed extinct, was caught in South Africa.

No specimens of the first amphibians survive, but they were ancestors of today's frogs and salamanders. Amphibians gave rise to reptiles, whose thick-shelled eggs do not dry up in the sun; turtles, tortoises and crocodiles are their modern descendants.

For the next 200 million years or so reptiles of many shapes and sizes dominated. Giant ferns (pteridophytes) fell and slowly formed coal deposits. Some land reptiles returned to water as fish-like reptiles probably around 200 mya (for example Ichthyosaurs).

The land reptiles were the dinosaurs; the biggest, Tyrannosaurus rex, stood about 20 feet tall with huge dagger-like teeth. About 65 mya they suddenly disappeared; the true reason is unknown (climate change, or evolution into birds, or something in between). Small reptiles of that era still live.

The first mammals were shrew-like, with small fossils. Mammals were viviparous, protecting the unborn young inside the mother, and better at sensing and avoiding danger; when reptiles declined, mammals took over.

Continental drift: when South America joined North America, its horse-, hippopotamus-, bear- and rabbit-like mammals were overridden by North American fauna, while Australia's pouched mammals survived for lack of competition. Some mammals, such as whales, dolphins, seals and sea cows, live wholly in water.