The solution came from other forms of life that had emerged on land. A few small, worm-like creatures probably emerged from the sea at about the same time as the earliest plants and mosses, about 420 million years ago.
What tempted them ashore had something to do with the rising quantities of oxygen in the air. Oxygen levels had steadily increased over millions of years until they levelled off at the beginning of the Cambrian Period, about 530 million years ago. Then there was a "blip" that occurred between 400 and 200 million years ago. This blip was caused by the luscious green forests now covering the land. Plants and trees dramatically increased the amount of oxygen in the atmosphere. The effect was a bit like lining the shores with sweets.
Life in the seas just couldn't resist the temptation to come ashore for a taste. When the first sea creatures came crawling out they found that the adjustment wasn't too hard, with all that extra oxygen in the air to give them a boost. Today about 21 per cent of the air we breathe is oxygen. But 350 million years ago, with the arrival of the carboniferous forests, oxygen levels shot up perhaps to as much as 35 per cent. The chart gives a sense of how oxygen levels have risen, and of the resulting effects on the development of life-forms.