Đề luyện đọc Tiếng Anh trình độ B1 - Đề số 3

Because different tree species adapted to different climates and soil types have evolved over millennia, many kinds of forests occupy the earth today. The primitive forests of several hundred million years ago consisted of fewer kinds of trees. In fact, the earliest “trees,” which grew nearly 500 million years ago, were like giant club mosses. They lacked true roots and consisted of a tangle of specialized branches that clambered over rocky ground. Fifty million years later came the dense forests of tree ferns that prevailed in tropical climates of that era. The forerunners of modern conifers were on the scene 300 million years ago, when plant life abundantly colonized marshy land, building the tremendous coal and oil reserves so important today. By the time the dinosaurs roamed the earth some 180 million years ago, during the Cretaceous period, seed-bearing trees had evolved that shed their leaves in winter; from these have sprung the angiosperms and our present deciduous forests.
1. What is the main idea of the passage?