The History of Life on Earth

1. The History of Life on Earth

2. 21 The History of Life on Earth • 21.1 How Do Scientists Date Ancient Events? • 21.2 How Have Earth’s Continents and Climates Changed over Time? • 21.3 What Are the Major Events in Life’s History? • 21.4 Why Do Evolutionary Rates Differ among Groups of Organisms?

3. 21.1 How Do Scientists Date Ancient Events? Many evolutionary changes take place over long periods of time. To study long-term evolutionary change, we must think in time frames spanning millions of years; and imagine conditions very different from today’s.

4. 21.1 How Do Scientists Date Ancient Events? Fossils are preserved remains of ancient organisms, they tell us about body form or morphology, and where and how they lived. Earth’s history is recorded in rocks. Layers of rocks are called strata.

5. 21.1 How Do Scientists Date Ancient Events? Relative ages of rocks can be determined by looking at strata of undisturbed sedimentary rock. The oldest layers are at the bottom, youngest at the top. First observed in the 17th century by Nicolaus Steno.

6. Chapter Opener 2 Younger Rocks Lie on Top of Older Rocks

7. 21.1 How Do Scientists Date Ancient Events? In the eighteenth century, geologists realized that fossils could also be used to age rocks. Certain fossils were always found in younger rocks, others were found in older rocks. Fossils in more recent strata were more similar to modern organisms.

8. 21.1 How Do Scientists Date Ancient Events? Radioisotopes can be used to determine the actual age of rocks. Radioisotopes decay in a predictable pattern. Half-life is the time interval over which one half of the remaining radioisotope decays, changing into another element.

9. Figure 21.1 Radioactive Isotopes Allow Us to Date Ancient Rocks

10. Table 21.1 Each radioisotope has a characteristic half-life.

11. 21.1 How Do Scientists Date Ancient Events? To date an event, we must know (or be able to estimate) the concentration of the radioisotope at the start of the event. For 14C, production in the upper atmosphere is about equal to its natural decay. In an organism, the ratio of 14C to 12C stays constant during its lifetime.

12. 21.1 How Do Scientists Date Ancient Events? When an organism dies, it is no longer incorporating 14C from the environment. The 14C that was present in the body decays with no replacement and the ratio of 14C to 12C decreases. This ratio can then be used to date fossils, up to about 50,000 years old.

13. 21.1 How Do Scientists Date Ancient Events? 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. But igneous rocks (e.g., lava or volcanic ash), that have intruded into layers of sedimentary rock can be dated.

14. 21.1 How Do Scientists Date Ancient Events? Other radioisotopes are used to date older rocks. Decay of potassium-40 to argon-40 is used for the most ancient rocks. Radioisotope dating is combined with fossil analysis.

15. 21.1 How Do Scientists Date Ancient Events? Other dating methods include paleomagnetic dating: Movement and reversals of Earth’s magnetic poles are recorded in igneous and sedimentary rocks at the time they were formed, by alignment of mineral grains and other characteristics.

16. 21.1 How Do Scientists Date Ancient Events? The history of life is divided into geologic eras, which are subdivided into periods. Boundaries are based on changes in fossils. The eras were established before actual ages of rocks were known.

17. Table 21.2 (Part 1)

18. Table 21.2 (Part 2)

19. 21.2 How Have Earth’s Continents and ClimatesChanged over Time? The idea that land masses have moved over time was first suggested by Alfred Wegener in 1912. By the 1960s, evidence of plate tectonics convinced geologists that he was right.

20. 21.2 How Have Earth’s Continents and ClimatesChanged over Time? Earth’s crust is divided into solid plates about 40 km thick—collectively, 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, setting up convection currents.

21. 21.2 How Have Earth’s Continents and ClimatesChanged over Time? The movement of plates is called continental drift. Where plates are pushed together, they move sideways past one another, or one is pushed underneath the other. Mountain ranges are pushed up, and deep rift valleys or trenches are formed. Where plates are pushed apart, ocean basins form.

22. Figure 21.2 Plate Tectonics and Continental Drift

23. 21.2 How Have Earth’s Continents and ClimatesChanged over Time? Position of the continents has changed dramatically over time. Influences of ocean circulation patterns, sea level, and global climate Mass extinctions of marine animals have occurred when sea level dropped, exposing the continental shelves.

24. Figure 21.3 Sea Levels Have Changed Repeatedly

25. 21.2 How Have Earth’s Continents and ClimatesChanged over Time? Earth’s atmosphere has also changed. Early atmosphere probably contained little or no free oxygen (O2). O2 began to increase when certain bacteria evolved the ability to use H2O as a source of H+ ions in photosynthesis. O2 was a waste product.

26. 21.2 How Have Earth’s Continents and ClimatesChanged over Time? Cyanobacteria formed rock-like structures called stromatolites which are abundant in the fossil record. Enough O2 was liberated to allow evolution of oxidation reactions to synthesize ATP.

27. Figure 21.4 Stromatolites (Part 1)

28. Figure 21.4 Stromatolites (Part 2)

29. 21.2 How Have Earth’s Continents and ClimatesChanged over Time? The evolution of life changed the physical nature of Earth. These changes in turn influenced the evolution of life. When O2 first appeared in the atmosphere it was poisonous to the anaerobic prokaryotes.

30. 21.2 How Have Earth’s Continents and ClimatesChanged over Time? Some evolved the ability to metabolize the O2. Advantages: aerobic metabolism is faster and more energy is harvested. Aerobes replaced anaerobes in most environments.

31. 21.2 How Have Earth’s Continents and ClimatesChanged over Time? Atmospheric O2 also made possible larger and more complex cells. About 1 billion years ago, eukaryote cells appeared.

32. Figure 21.5 Larger Cells Need More Oxygen

33. 21.2 How Have Earth’s Continents and ClimatesChanged over Time? Change in atmospheric O2 concentrations was unidirectional. Most physical conditions have oscillated over time in response to drifting continents, volcanic activity, and even extraterrestrial events such as meteorite impacts. Sometimes these events caused mass extinctions.

34. 21.2 How Have Earth’s Continents and ClimatesChanged over Time? Earth’s climate has changed over time. Sometimes Earth was considerably hotter than today; sometimes colder, with extensive glaciation.

35. Figure 21.6 Hot/Humid and Cold/Dry Conditions Have Alternated over Earth’s History

36. 21.2 How Have Earth’s Continents and ClimatesChanged over Time? For Earth to be cold and dry, atmospheric CO2 must have been much lower, but it is unclear what would cause low concentrations. Some climate changes have been very rapid. Extinctions caused by them appear to be “instantaneous” in the fossil record.

37. 21.2 How Have Earth’s Continents and ClimatesChanged over Time? Today we are in a period of rapid climate change due to increasing CO2 concentrations, mostly from burning fossil fuels. Current CO2 concentration is greater than it has been for several thousand years. If CO2 concentration doubles, average Earth temperature will increase, causing droughts, sea increase, melting ice caps, and other major changes.

38. 21.2 How Have Earth’s Continents and ClimatesChanged over Time? Volcanic eruptions can trigger major climate change. When continents came together to form Pangaea in the Permian period, many volcanic eruptions reduced sunlight penetration and thus photosynthesis. Massive glaciation resulted.

39. 21.2 How Have Earth’s Continents and ClimatesChanged over Time? Collisions with large meteorites are probably the cause of several mass extinctions. Evidence of impacts include large craters and disfigured rocks; molecules with helium and argon isotope ratios characteristic of meteorites.

40. 21.2 How Have Earth’s Continents and ClimatesChanged over Time? A meteorite is thought to have caused or contributed to the mass extinction at the end of the Cretaceous period, 65 million years ago. First evidence was from a thin layer containing the element iridium. This element is very rare on Earth but abundant in some meteorites.

41. Figure 21.7 Evidence of a Meteorite Impact

42. 21.2 How Have Earth’s Continents and ClimatesChanged over Time? A large crater has been located beneath the northern coast of the Yucatán Peninsula, Mexico. A massive plume of debris from the impact heated the atmosphere, ignited fires, and blocked the sunlight. Settling debris formed the iridium-rich layer.

43. An Artist’s Conception of the Presumed Meteorite Impact of 65 Million Years Ago

44. Skim through the remaining sections!

45. Photo 21.1 Lava dome on Krakatoa Island, Indonesia, has formed since the eruption in 1883.

46. Photo 21.2 A forest is becoming re-established on Krakatoa Island.

47. Photo 21.3 Barringer Crater, a meteor impact crater east of Flagstaff, AZ.

48. Photo 21.4 Fossil sea urchin from Cretaceous vs. test of recent sea urchin.

49. Photo 21.5 Fly in Baltic amber from the Eocene (50 million years old).

50. Photo 21.6 50-million-year-old flower preserved in Baltic amber.