key concept 24.1 Events in Earth’s History Can Be Dated

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Some evolutionary changes happen rapidly enough to be studied directly and manipulated experimentally. Plant and animal breeding by agriculturalists and the evolution of surface proteins in influenza viruses are examples of rapid, short-term evolution that we have discussed in previous chapters. Such changes can take place in months, years, decades, or centuries. Other evolutionary changes, such as the appearance of new species and major evolutionary lineages, usually take place over a geological time scale (Table 24.1).

table 24.1 Earth’s Geological History
Eon Era Period Onset Major physical changes on Earth Major events in the history of life
Cenozoic Quaternary (Q) 2.6 mya Cold/dry climate; repeated glaciations Humans evolve; many large mammals become extinct
Tertiary (T) 65.5 mya Continents near current positions; climate cools Diversification of birds, mammals, flowering plants, and insects
Mesozoic Cretaceous (K) 145.5 mya Laurasian continents attached to one another; Gondwana begins to drift apart; meteorite strikes near current Yucatán Peninsula at end of period Dinosaurs continue to diversify; mass extinction at end of period (~76% of species lost)
Jurassic (J) 201.6 mya Two large continents form: Laurasia (north) and Gondwana (south); climate warm Diverse dinosaurs; radiation of ray-finned fishes; first fossils of flowering plants
Triassic (Tr) 251.0 mya Pangaea begins to drift apart; hot/humid climate Early dinosaurs; first mammals; marine invertebrates diversify; mass extinction at end of period (~65% of species lost)
Phanerozoic (~0.5 billion years long) Paleozoic Permian (P) 299 mya Extensive lowland swamps; O2 levels 50% higher than present; by end of period continents aggregate to form Pangaea, and O2 levels drop rapidly Reptiles diversify; giant amphibians and flying insects present; mass extinction at end of period (~96% of species lost)
Carboniferous (C) 359 mya Climate cools; marked latitudinal climate gradients Extensive fern/horsetail/giant club moss forests; first reptiles; insects diversify
Devonian (D) 416 mya Continents collide at end of period; giant meteorite probably strikes Earth Jawed fishes diversify; first insects and amphibians; mass extinction at end of period (~75% of marine species lost)
Silurian (S) 444 mya Sea levels rise; two large land masses emerge; hot/humid climate Jawless fishes diversify; first ray-finned fishes; plants and animals colonize land
Ordovician (O) 488 mya Massive glaciation; sea level drops 50 meters Mass extinction at end of period (~75% of species lost)
Cambrian (C) 542 mya Atmospheric O2 levels approach current levels Rapid diversification of multicellular animals; diverse photosynthetic protists
Proterozoic Collectively called the Precambrian (~4 billion years long) 2.5 bya Atmospheric O2 levels increase from negligible to about 18%; “snowball Earth” from about 750 to 580 mya Origin of photosynthesis, multicellular organisms, and eukaryotes
Archean 3.8 bya Earth accumulates more atmosphere (still almost no O2); meteorite impacts greatly reduced Origin of life; prokaryotes flourish
Hadean 4.5–4.6 bya Formation of Earth; cooling of Earth’s surface; atmosphere contains almost no free O2; oceans form; Earth under almost continuous bombardment from meteorites Life not yet present
Note: mya, million years ago; bya, billion years ago.

Media Clip 24.1 The Age of the Earth

www.life11e.com/mc24.1

focus your learning

  • Geologists use several methods to date ancient events.

  • Scientists have developed a geological time scale.

  • Geologists construct geological maps based on the age of rocks.

To understand long-term patterns of evolutionary change, we must not only think in time scales spanning many millions of years, but also consider events and conditions very different from those we observe today. Earth of the distant past was so unlike our present Earth that it would seem like a foreign planet inhabited by strange organisms. The continents were not where they are now, and climates were sometimes dramatically different from those of today. We know this because much of Earth’s history is recorded in its rocks.

We cannot tell the ages of rocks just by looking at them, but we can visually determine the ages of rocks relative to one another. The first person to formally recognize this fact was the seventeenth-century Danish physician Nicolaus Steno. Steno realized that in undisturbed sedimentary rocks (rocks formed by the accumulation of sediments), the oldest layers of rock, or strata (singular stratum), lie at the bottom, and successively higher strata are progressively younger.

Geologists subsequently combined Steno’s insight with their observations of fossils contained in sedimentary rocks. They developed the following principles of stratigraphy:

These patterns revealed much about the relative ages of sedimentary rocks and the fossils they contained, as well as patterns in the evolution of life. But the geologists still could not determine the age of particular rocks. A method for absolute dating of rocks—that is, determining their actual age rather than just their age relative to one another—did not become available until after radioactivity was discovered at the beginning of the twentieth century.