CHAPTER 17 Summary

• The age of Earth and its rock layers can be determined by measuring the amount of radioactive isotopes present in rocks, a method known as radiometric dating.
• Life on Earth may have emerged in stages, as inorganic molecules combined to form organic ones in the primordial soup, and as these were incorporated into lipid bubbles to form cells.
• From geological evidence and the fossil record, paleontologists have been able to construct a geologic timeline of life on Earth.
• Earth’s history can be divided into important eras and periods. Dinosaurs, for example, lived primarily from 250 to 65 million years ago, during the Mesozoic Era, from the Triassic through the Cretaceous periods.
• The history of life on Earth is marked by repeated extinctions and adaptive radiations, a phenomenon of intermittent rather than steady change known as punctuated equilibrium.
• Ancient movement of Earth’s major landmasses affected the eventual distribution of species around the globe, the study of which is known as biogeography.
• Convergent evolution is the evolution of similar adaptations in response to similar environmental challenges in groups of organisms that are not closely related.
• Life is astoundingly diverse. Current estimates of the total number of species on Earth range anywhere from 5 to 30 million, of which 1.5 million have been formally described.
• Biologists sort organisms into a series of nested categories based on shared anatomical and genetic features: domain, kingdom, phylum, class, order, family, genus, species.
• The scientific name of an organism is given by its genus and species names (the scientific name of humans is Homo sapiens).
• Both physical evidence and genetic evidence are used to understand evolutionary history, or phylogeny. Branching trees of common ancestry are used to represent that history visually.
• On the basis of genetic evidence, all living organisms can be classified into one of three domains: Bacteria, Archaea, or Eukarya.

• MORE TO EXPLORE

  • Newman, W. L. (1997) “Age of the Earth.” In Geologic Time. U.S. Geological Service. http://pubs.usgs.gov/gip/geotime/age.html
  • Gould, S. J. (1989) Wonderful Life: Burgess Shale and the Nature of History. New York: W. W. Norton.
  • Mora, C., et al. (2011) How many species are there on Earth and in the ocean? PLOS Biology 9(8):e1001127.doi:10.1371/journal.pbio.1001127. http://www.plosbiology.org/article/info:doi/10.1371/journal.pbio.1001127
  • TimeTree, The Timescale of Life http://www.timetree.org/index.php
  • Woese, C.R., et al. (1990) Towards a natural system of organisms: proposal for the domains Archaea, Bacteria, and Eucarya. Proceedings of the National Academy of Sciences 87:4576–4579