Throughout much of the Tertiary period (most of the past 65 million years), the Arctic supported continuous forests. Only toward the end of that period does the fossil evidence show that certain present-day Arctic plants were established and widely distributed throughout the Arctic. Many Arctic plants are thought to have originated in the high mountain ranges of central Asia and North America, to have spread northward to the Arctic as global temperatures fell in the late Tertiary, and to have achieved a circumpolar distribution by the end of the Tertiary (about 2 million years ago). However, fossil evidence to support these proposals is either lacking or fragmentary. Consequently, the routes by which these plants expanded their ranges during their colonization of the Arctic remain unknown.