Since the origin of life some 3.5 billion years ago, a series of significant evolutionary developments may be noted. The independent movement of organisms, the rise of animals with backbones, sexual reproduction, the emergence of organs for sight, flowering plants, the development of flight and the ability of animals to maintain a constant body temperature all mark important evolutionary milestones. While these developments did not imply progress in any directional sense — they were not cumulative, sequential or confined to a single lineage of animals — they were nonetheless biological revolutions. These innovations vastly increased the potentiality of life forms by opening and diversifying a range of niches.
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