About 400 million years ago, an event of earth shattering importance happened—plants which had hitherto been confined to oceans invaded land. The all-important chemical process, photosynthesis, had been around in some form as early as 2.5-3 billion years ago in bacteria. Early cells had acquired these food producing bacteria and formed what were the first plant cells. Evolutionary pressures on the first land plants then resulted in the first trees with roots and leaves. This was about 350 million years ago, but the real innovation came about somewhat later, when the seed makes it first appearance. This simple evolutionary innovation enabled plants to colonise land and form vast forests.