Fossils - the Evidence for Evolution

On the previous pages we looked at a very brief history of the way in which life on Earth began, and how it developed to the stage where the first invertebrate animals - animals without backbones - began to appear on land.

In the following pages we shall examine some of the many ways in which the higher animals developed, and look at their successes and failures. This, however, is a convenient point at which to see how we find out about the ancient world.

Three main kinds of rocks

All our information about prehistoric animals comes indirectly from rocks. Three main kinds of rocks make up the outermost layer or 'crust' of the Earth.

Igneous rocks are those which have solidified from the molten state, either deep down in the crust, for example granite, or at the surface as a result of the lava coming from volcanoes, for example basalt.

Sedimentary rocks are formed from thick layers of particles worn or eroded from the land and usually carried by rivers to the sea, where the material sinks to the bottom and is compressed. Chalk, limestone and clay were formed in this way.

Metamorphic rocks (metamorphic means 'changed in form') are those rocks which result from the alteration of igneous or sedimentary rocks, either by heat or by the enormous pressures inside the Earth. Slate, schist, gneiss and marble are metamorphic rocks.

Solidified lava (now igneous rock) at Playa De las
Americas, Tenerife

How fossils are formed

We learn about prehistoric times by studying the sedimentary rocks and, in particular, the fossils these often contain. The word 'fossil' comes from a Latin word meaning 'dug up'.

Usually when plants or animals die they decay and disappear. Sometimes, however, a plant or animal, when it dies, is quickly covered by a layer of mud or sand. Under these conditions the harder parts remain, though the softer parts soon decay, and over thousands, millions or even tens of millions of years, water containing dissolved material salts seeps through them. The minerals gradually replace the once-living tissues of the plant or animal, turning them to stone, but preserving their shape and often parts of their insides as well.

In the case of an animal, the fossil will probably be of its bones or its teeth or shell. If the fossil is part of a plant, we may have the stem or the leaves or fruit. Less often, a fossil can be something which an animal has produced such as a footprint, bite-mark, burrow or even its droppings.

How fossils are buried in rock

Eventually more layers of sediment are deposited on the mud or sand in which the plant or animal was buried. The lower layers are under huge pressures and turn into rock, most commonly clay, sandstone or limestone.

Many of the fossils formed long ago were in layers of sediment in shallow water. Later, the Earth's crust wrinkled and folded, and sometimes layers once under water were pushed up into mountains. The wind, rain and other agents of weathering then started to wear down these rocks until some of the fossils were uncovered.