600 – 500 million years ago

Year forty-one

Shells, skeletons and fish

Shells and skeletons evolve.

It was just after Earth's 40th birthday (remember we are pretending that Earth is 46 today) that animals developed hard shells and skeletons for the first time. All the main groups of creatures that are found living in the seas today (as well as some that are not) quickly evolved. Soon there were sponges, corals and shell fish related to mussels, cockles and winkles.

Things like sea urchins crawled over the sea floor and, a bit later, crinoids (or 'sea lilies') evolved. Trilobites (distant relatives of crabs and woodlice) scurried over the sea bed or burrowed into it looking for food. Some were spiny, some were smooth, some had large eyes, some were blind and all moulted their shells as they grew larger, in the same way as crabs do today.

Some species existed for millions of years, but others become extinct shortly after they evolved. One type of shell fish called Lingula first evolved at this time and is still found living today in burrows near to the shore. It has hardly changed in millions of years. However, other types of creature soon became extinct, like the trilobites, graptolites and peculiar creatures such as spiny Hallucigenia, with seven pairs of legs, and Opabinia, with its clump of five eyes.

But there were still no animals living on the land. There was still insufficient oxygen and ozone in the air to stop those rays from space.

Fish — the first vertebrates

Fish-the first vertebrates.

The oldest known fish evolved just before Earth's 42nd birthday (about 500 – 400 million years ago). They were related to lampreys, jawless fish that still live today. Instead of a backbone they had a 'notochord' of gristly cartilage. However, by the time Earth was 42 years old, fish with backbones had evolved from those with notochords. True vertebrates had appeared.

Fish soon spread throughout the world. Some lived in the sea and others managed to adapt to living in the fresh waters of rivers and lakes.

Fish with backbones were to become very important. One group, called crossopterygians, were 'lobe-finned fish'. They had very strong fins (which evolved into legs) and a sort of lung so it could survive out of water (and lung fish still live in Africa, South America and Australia today). From these fish, the first land vertebrates, the amphibians, were eventually to evolve.

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