Paleozoic Era

541 million years ago, primitive multi-cellular life forms in our oceans began an incredible diversification which we call the 'Cambrian explosion'. By the end of the era, life had colonised the land, and all the major animal groups had evolved, including fish, insects, reptiles, birds and mammals.

The Cambrian 'explosion' was a period when early multicellular animals diversified rapidly into a large range of complex creatures that filled the oceans and fossil records for the first time. Typical fossils include trilobites, echinoderms (pre-starfish), Most of the modern-day animal phyla (major categories) appeared at this time, including chordates, the group from which vertebrates evolved, arthropods (the family that covers insects, spiders, lobsters and centipedes today), annelids (worms & leeches) and molluscs. Ozone layer forms.

The Ordovician Period - Invertebrates, particularly molluscs and arthropods dominated the oceans, but primitive fish were gaining a foothold. Mosses may have been the first plants to colonise land, accelerating the conversion of the CO2-rich atmosphere to oxygen, causing global cooling. Oceans temperatures plummeted from a sizzling 45 Celcius to ice-age levels by the end of the Ordovician Period, and a mass-extinction.

The Silurian Period - Greenhouse gasses warmed the air again, and warm shallow seas returned. On land, forests of moss spread along rivers and lakes, and the first 'vascular' plants - land plants with roots and leaves - appeared. The first land invertebrates included predatory millipedes and a spiders. In the seas, the first bony fish appeared, as well as a sea scorpion the length of a crocodile.

The Devonian Period - 'the age of fish' saw great diversity and development of many modern features, jaws, cartilage or bone skeletons, and the first sharks. Shellfish and corals also dominated, including the first ammonites. On land, dead moss created the first soil, and taller plants, including the first trees began to rise above the carpet of primitive land-hugging plant life. The close relationship between plants and insects has its origins in this Period.

The Carboniferous Period - This was the age of coal formation, when vast forests covered the land. Atmospheric oxygen peaked at nearly double today's levels. Whilst amphibians became well-established on land, there were also giant scorpions the size of a cat, dragonflies the size of a seagull, and at 7ft, the world's longest ever millipede. Amphibians declined as the world cooled, but the first amniote eggs allowed reptiles to thrive away from water. These would later evolve into dinosaurs, birds and mammals. The period ended with an ice age, falling sea levels, a collapse in rainforests and amphibians, and survival of the reptiles.

The Permian Period - As the ice age receded very slowly, much of the land became vast desert. Conifers came to dominate much of the surviving forests. Reptiles continued to diversify, into turtles, pre-dinosaurs, and earliest mammals. The Period ends with the great Permian mass extinctions, during which up to 70% of land and 95% of all marine species became extinct.