500 – 400 million years ago

Year forty-two
The land turns green

The land turns green

For billions of years, oxygen and ozone produced by bacteria and plants had been gradually building up in the atmosphere. Only now, when Earth was about 42 (about 400 million years ago), was there enough to stop the harmful radiation from the sun reaching the Earth's surface. Living things had been protected by the water in which they lived, but as time passed, they were able to live at shallower and shallower depths and now they began to leave the protection of the water and emerge onto the land.

Small plants first colonised the marshy edges of the river estuaries and lakes. They remained close to the water's edge at first, but some time later, they were able to spread inland. They gradually got bigger and by the time Earth was 43 years old, great forests of tree-sized ferns grew. These were the forests that eventually turned into coal.

The oldest plants appear to be distant relatives of mosses, liverworts and hornworts (a group of plants called bryophytes), but only their 460 million year old spores have been found. New plants grew from these spores but only when they fell into very wet soil and for this reason plants had to live close to water. Fossil stems and branches of one of the earliest land plants, called Cooksonia, have been found in Wales. They had special cells and roots to draw up water from the soil and for this reason they are known as 'vascular plants'. They didn't have leaves, but they had spores in small pods on their branches.

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