life begins on earth

After originally condensing from cosmic dust and gas by gravitational attraction, the earth would have been almost homogeneous and relatively cool. However its continued contraction of these materials caused them to heat up, as did the radioactivity of some of the heavier elements. In the next stage of its formation, as the earth became hotter, it began melting under the influence of gravity. This caused the differentiation into crust, mantle, and core, with the lighter silicates moving upwards to form the mantle and crust, and the heavier elements, mainly iron and nickel, sinking downwards towards the centre of the earth to form the core.

By volcanic eruption, light, volatile gases and vapours continually escaped from the mantle and crust. Some of these, mainly carbon dioxide and nitrogen, were held by the earth's gravity and formed the primitive atmosphere, while water vapour condensed to form the world's first oceans. Soon the Earth had cooled and an atmosphere had developed.

Microscopic living cells, neither plants nor animals, begin to reproduce and flourish in Earth's many volcanic environments. From here many creatures begin to evolve over a long period of time; in the first place mainly flat-worms, jellyfish and algae. By 570 million years before the present, large numbers of creatures with hard shells had appeared. The first mammals evolved from a class of reptiles with mammalian traits, such as a segmented jaw and a series of bones that make up the inner ear.

It then appears that an asteroid or comet may have slammed into the northern part of the Yucatan Peninsula in Mexico. This world-wide cataclysm brought to an end the long age of the dinosaurs, and allowed mammals to diversify and expand their ranges. Eventually our ancestors speciated in Africa from apes.

- Tt, ASh (ed), Fx (ed)