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Oldest rocks on Earth Origin of Earth

Oldest rocks on Earth Origin of Earth

Figure 8.1 Key events in the history of life associated with the evolution of the genome. Source: Adapted mainly from R. L. Carroll (2002) and S. B. Carroll (2001).

by about 3.5 billion years ago, and the first aerobic photoautotrophic bacteria and the nitrate-reducing and sulphate-reducing bacteria by 2.5 billion years ago. Eukaryotes had evolved by around 1.9 billion years ago, and metazoans by 600 million years ago. During the Phanerozoic aeon, key events in the biosphere were the appearance of the following: calcareous and siliceous skeletons (570 million years ago); the origin of vertebrates (510 million years ago); land plants (458 million years ago); wingless insects (420 million years ago); winged insects (310 million years ago); mammals (220 million years ago); birds (150 million years ago); flowering plants (137 million years ago); and humans (3 million years ago).

Three trends evident in the fossil record require scrutiny. First, multicellular organisms evolved independently many times from unicellular organisms in all three domains of life. Second, following the evolution of multicellular organisms from different unicellular ancestors, macroscopic forms with new body plans or physiologies and representing higher grades of morphological complexity - for example, multicellular protists, animals, and land plants - arose. And third, the emergence of new forms was often followed (after sometimes considerable delays) by periods of rapid diversification - for example, the Cambrian explosion of animals, the rise of insects in the Devonian and Carboniferous, the radiation of flowering plants in the Late Cretaceous, and the mammalian radiation in the early Tertiary.

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