The History of Life on Earth

1. 18 The History of Life on Earth

2. Chapter 18 The History of Life on Earth • Key Concepts • 18.1 Events in Earth’s History Can Be Dated • 18.2 Changes in Earth’s Physical Environment Have Affected the Evolution of Life • 18.3 Major Events in the Evolution of Life Can Be Read in the Fossil Record

3. Chapter 18 Opening Question Can modern experiments test hypotheses about the evolutionary impact of ancient environmental changes?

4. Concept 18.1 Events in Earth’s History Can Be Dated • To understand long-term patterns of evolutionary change, we must think in time scales spanning many millions of years and consider conditions very different from today’s. • Much of Earth’s history is recorded in rocks.

5. Concept 18.1 Events in Earth’s History Can Be Dated Ages of rocks relative to one another can be determined by stratigraphy: In sedimentary rock layers (strata), the oldest layers are at the bottom, and successively higher strata are progressively younger. Certain fossils are always found in younger rocks, others are found in older rocks. Fossils in more recent strata are more similar to modern organisms.

6. Concept 18.1 Events in Earth’s History Can Be Dated Actual ages of rocks can be determined using radioisotopes—isotopes that decay in a predictable pattern. Half-life is the time in which one half of the remaining radioisotope decays, changing into another element. This is the basis for radiometric dating.

7. Figure 18.1 Radioactive Isotopes Allow Us to Date Ancient Rocks (Part 1)

8. Figure 18.1 Radioactive Isotopes Allow Us to Date Ancient Rocks (Part 2)

9. Concept 18.1 Events in Earth’s History Can Be Dated To date an event, the original concentration of the isotope must be known or estimated, and the half-life of the isotope must be known. The amount of isotope remaining is indicative of how much time has passed since the event.

10. Concept 18.1 Events in Earth’s History Can Be Dated Sedimentary rocks can not be dated accurately; the materials that form the rocks existed for varying lengths of time before being transported and converted to rock. Dating rocks older than 50,000 years requires estimating isotope concentrations in igneous rocks (formed when molten material cools).

11. Concept 18.1 Events in Earth’s History Can Be Dated Paleomagnetic datingrelates ages of rocks to patterns in Earth’s magnetism. Earth’s magnetic poles move and occasionally reverse. Sedimentary and igneous rocks preserve a record of Earth’s magnetic field at the time they were formed.

12. Concept 18.1 Events in Earth’s History Can Be Dated Using the various dating methods and fossil stratigraphy, a geological time scale was developed. The history of life is divided into eras and subdivided into periods. Boundaries between the divisions are based on abrupt changes in fossil organisms.

13. Table 18.1 Earth’s Geological History (Part 1)

14. Table 18.1 Earth’s Geological History (Part 2)

15. Concept 18.2 Changes in Earth’s Physical EnvironmentHave Affected the Evolution of Life Physical changes in the Earth and its atmosphere have influenced the evolution of life. And life has also influenced Earth’s physical environment.

16. Concept 18.2 Changes in Earth’s Physical EnvironmentHave Affected the Evolution of Life The continents have moved. The idea that land masses have moved over time was first suggested by Alfred Wegener in 1912. By the 1960s, evidence of plate tectonics (geophysics of the movement of land masses) convinced geologists that he was right.

17. Concept 18.2 Changes in Earth’s Physical EnvironmentHave Affected the Evolution of Life Earth’s crust consists of solid plates about 40 km thick—the lithosphere. The plates float on a fluid layer of liquid rock, or magma. Heat from radioactive decay in Earth’s core causes the magma to circulate in convection currents. This exerts pressure on the plates and causes them to move.

18. Figure 18.2 Plate Tectonics and Continental Drift

19. Concept 18.2 Changes in Earth’s Physical EnvironmentHave Affected the Evolution of Life Movement of the lithospheric plates is continental drift. Position and size of the continents influences oceanic circulation patterns, global climates, and sea levels. Dramatic physical changes resulted in mass extinctions, during which a large proportion of species living at the time disappeared.

20. Concept 18.2 Changes in Earth’s Physical EnvironmentHave Affected the Evolution of Life Earth’s climate has changed through time; sometimes much warmer than today, sometimes much colder. Drops in sea level were related to glaciation and often resulted in mass extinctions. Earth’s cold periods were separated by long periods of milder climates. The Quaternary period has been marked by a series of glacial advances interspersed with warmer interglacial intervals.

21. Figure 18.3 Sea Levels Have Changed Repeatedly

22. Concept 18.2 Changes in Earth’s Physical EnvironmentHave Affected the Evolution of Life Some major climatic shifts are rapid (5,000 to 10,000 yrs), as a result of changes in Earth’s orbit around the sun.

23. Concept 18.2 Changes in Earth’s Physical EnvironmentHave Affected the Evolution of Life Today’s rapid climate change is due to increasing CO2 concentrations, mostly from burning fossil fuels. We are reversing the process of organic burial that occurred in the Carboniferous and Permian. It took millions of years for these deposits to accumulate.

24. Concept 18.2 Changes in Earth’s Physical EnvironmentHave Affected the Evolution of Life The current rate of increase of atmospheric CO2 is unprecedented in Earth’s history. If CO2 concentration doubles, average Earth temperature will increase, causing droughts, rising sea level, melting ice caps, and other major changes.

25. Concept 18.2 Changes in Earth’s Physical EnvironmentHave Affected the Evolution of Life Large volcanic eruptions can have major impacts. Ash and SO2 are injected into the atmosphere, which blocks sunlight and results in cooling. Collision of continents during the Permian formed a single land mass and caused massive volcanic eruptions, leading to the greatest mass extinction in Earth’s history.

26. Figure 18.4 Volcanic Eruptions Can Cool Global Temperatures

27. Concept 18.2 Changes in Earth’s Physical EnvironmentHave Affected the Evolution of Life Collision of meteorites and comets may have also caused mass extinctions. Evidence includes impact craters and disfigured rocks with isotope ratios characteristic of meteorites.

28. Concept 18.2 Changes in Earth’s Physical EnvironmentHave Affected the Evolution of Life A meteorite probably caused the mass extinction at the end of the Cretaceous (about 65 mya). Evidence: A thin rock layer with high iridium content (common in meteorites); a huge impact crater beneath the northern coast of the Yucatán Peninsula. The resulting tsunamis, debris plumes that blocked the sun, and massive fires had a devastating effect on biodiversity.

29. Figure 18.5 Evidence of a Meteorite Impact

30. Concept 18.2 Changes in Earth’s Physical EnvironmentHave Affected the Evolution of Life Atmospheric oxygen concentration has also changed over time. The early atmosphere probably had little or no O2. O2 first increased when certain bacteria evolved photosynthesis (about 2.4 bya). Cyanobacteria formed stromatolites which are abundant in the fossil record.

31. Figure 18.6 Stromatolites (Part 1)

32. Figure 18.6 Stromatolites (Part 2)

33. Concept 18.2 Changes in Earth’s Physical EnvironmentHave Affected the Evolution of Life O2 released by Cyanobacteria allowed evolution of oxidation reactions as the energy source for ATP synthesis. Later, eukaryote cells with chloroplasts evolved, and O2 increased again. Organisms with aerobic metabolism replaced anaerobes in most of Earth’s environments.

34. Concept 18.2 Changes in Earth’s Physical EnvironmentHave Affected the Evolution of Life O2 also allowed larger and more complex organisms to evolve. Larger cells have lower surface area-to-volume ratios and require higher O2 concentrations. Further increases in O2 in the late Precambrian enabled evolution of multicellular organisms.

35. Figure 18.7 Larger Cells and Organisms Need More Oxygen

36. Concept 18.2 Changes in Earth’s Physical EnvironmentHave Affected the Evolution of Life O2 concentrations increased again during the Carboniferous and Permian as large vascular plants evolved. Plant debris was not oxidized but buried in swamps (forming coal deposits). Living plants were producing large quantities of O2. High concentrations of O2 allowed evolution of giant flying insects and amphibians.

37. Concept 18.2 Changes in Earth’s Physical EnvironmentHave Affected the Evolution of Life In experiments with hyperoxic conditions (high O2), Drosophila evolve larger body sizes over just a few generations. The stabilizing selection on body size at present O2 concentrations can quickly switch to directional selection for increased body size in response to higher O2.

38. Figure 18.8 Atmospheric Oxygen Concentrations and Body Size in Insects (Part 1)

39. Figure 18.8 Atmospheric Oxygen Concentrations and Body Size in Insects (Part 2)

40. Figure 18.8 Atmospheric Oxygen Concentrations and Body Size in Insects (Part 3)

41. Concept 18.3 Major Events in the Evolution of LifeCan Be Read in the Fossil Record The fossil record is used to reconstruct life’s history. A biota—all organisms of all kinds living at a particular time or place. All plants living at a particular time or place are its flora; all animals are its fauna.

42. Concept 18.3 Major Events in the Evolution of LifeCan Be Read in the Fossil Record The number of fossil species that have been found are only a tiny fraction of the species that have ever lived. Only a tiny fraction of organisms become fossils, and only a fraction of those are found by paleontologists.

43. Concept 18.3 Major Events in the Evolution of LifeCan Be Read in the Fossil Record Most organisms are decomposed quickly after death. If they are transported to sites with no oxygen, where decomposition is very slow, fossilization could occur. Many geologic processes transform rocks and destroy the fossils they contain or bury them too deeply to be accessible.

44. Concept 18.3 Major Events in the Evolution of LifeCan Be Read in the Fossil Record The fossil record is most complete for marine animals with hard skeletons or shells. Insects and spiders are also well represented. Although the fossil record is incomplete, it is enough to document the history of the evolution of life.

45. Figure 18.9 Insect Fossils

46. Concept 18.3 Major Events in the Evolution of LifeCan Be Read in the Fossil Record Precambrian era For most of this era, life consisted of microscopic prokaryotes living in oceans. Life first appeared about 3.8 bya. Eukaryotes evolved about 1.5 bya.

47. Figure 18.10 A Sense of Life’s Time

48. Concept 18.3 Major Events in the Evolution of LifeCan Be Read in the Fossil Record By the late Precambrian, many kinds of multicellular soft-bodied animals had evolved. Some were very different from any animals living today and may have no living descendants.

49. Figure 18.11 Precambrian Life

50. Concept 18.3 Major Events in the Evolution of LifeCan Be Read in the Fossil Record Cambrian period:Beginning of the Paleozoic era. O2 concentration was near modern levels. The Cambrian explosion was a rapid diversification of life. Such periods of rapid diversification are known as evolutionary radiations.