单项选择题

B

Life on land probably began about 430 million years ago, though it has existed in the water for perhaps much as 3 000 million years. When we think of the first life on land, we probably think of strange animals coming out of the oceans, but, in fact, no animals could have been living if plants had not been on land first. Plants had to be on land before animals arrived. They supplied the first land animals with the surrounding and food necessary, since the plants are the only form of life that is able to get and store energy. The first plants to exist out of the water were probably certain kinds of algae (海藻) which were followed by other plants that grew close to the ground and needed water in which to reproduce. Once their move to land had been made, however, evolution (进化) took place quickly. By the end of 100 million years, plants had developed their roots (根), and some had got tree-like forms since height was very important in gaining sunlight. About 300 million years ago, much of the world was covered with forests of huge trees. In most ways they were like modem trees. They had roots, leaves, wood, but mostly they had not developed seeds.

The main idea of the first paragraph is ______.