21 The History of Life on Earth

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?

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.

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.

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 17 th century by Nicolaus Steno.

Chapter Opener 2 Younger Rocks Lie on Top of Older Rocks

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.

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.

Figure 21.1 Radioactive Isotopes Allow Us to Date Ancient Rocks

Table 21.1 Each radioisotope has a characteristic half-life.

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.

Table 21.2 (Part 1)

Table 21.2 (Part 2)

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

21.2 How Have Earth’s Continents and Climates Changed 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.

21.2 How Have Earth’s Continents and Climates Changed 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.

Figure 21.2 Plate Tectonics and Continental Drift

21.2 How Have Earth’s Continents and Climates Changed 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.

Figure 21.3 Sea Levels Have Changed Repeatedly

21.2 How Have Earth’s Continents and Climates Changed over Time? Earth’s atmosphere has also changed. Early atmosphere probably contained little or no free oxygen (O 2 ). O 2 began to increase when certain bacteria evolved the ability to use H 2 O as a source of H + ions in photosynthesis. O 2 was a waste product.

21.2 How Have Earth’s Continents and Climates Changed over Time? Cyanobacteria formed rock-like structures called stromatolites which are abundant in the fossil record. Enough O 2 was liberated to allow evolution of oxidation reactions to synthesize ATP.

Figure 21.4 Stromatolites (Part 1)

Figure 21.4 Stromatolites (Part 2)

21.2 How Have Earth’s Continents and Climates Changed over Time? The evolution of life changed the physical nature of Earth. These changes in turn influenced the evolution of life. When O 2 first appeared in the atmosphere it was poisonous to the anaerobic prokaryotes.

21.2 How Have Earth’s Continents and Climates Changed over Time? Some evolved the ability to metabolize the O 2. Advantages: aerobic metabolism is faster and more energy is harvested. Aerobes replaced anaerobes in most environments.

21.2 How Have Earth’s Continents and Climates Changed over Time? Atmospheric O 2 also made possible larger and more complex cells. About 1 billion years ago, eukaryote cells appeared.

Figure 21.5 Larger Cells Need More Oxygen

21.2 How Have Earth’s Continents and Climates Changed over Time? Change in atmospheric O 2 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.

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

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

21.2 How Have Earth’s Continents and Climates Changed over Time? For Earth to be cold and dry, atmospheric CO 2 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.

21.2 How Have Earth’s Continents and Climates Changed over Time? Today we are in a period of rapid climate change due to increasing CO 2 concentrations, mostly from burning fossil fuels. Current CO 2 concentration is greater than it has been for several thousand years. If CO 2 concentration doubles, average Earth temperature will increase, causing droughts, sea increase, melting ice caps, and other major changes.

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

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

21.3 What Are the Major Events in Life’s History? Rapid diversification of life took place— called the Cambrian explosion. Most of the major groups of animals living today appeared in the Cambrian. Three different Cambrian fossil beds have preserved the soft parts of many animals—the Burgess Shale, Sirius Passet, and Chengjiang site.

21.4 Why Do Evolutionary Rates Differ among Groups of Organisms? The rate of evolutionary change has varied greatly at different times and in different lineages. Changes in the physical and biological environment are likely to stimulate evolutionary change. Climate change can shift ranges of organisms, bringing them into contact with previously unknown competitors or predators.

21.4 Why Do Evolutionary Rates Differ among Groups of Organisms? An organism’s diet can also affect extinction rates. Animals with specialized diets are more vulnerable to loss of their food supply. Large, specialized carnivores may be more likely to go extinct than small carnivores with generalized diets. This hypothesis has been tested using canid fossils.

21.4 Why Do Evolutionary Rates Differ among Groups of Organisms? Humans move thousands of species around the globe, deliberately and accidentally changing the ranges of species. Humans practice artificial selection and biotechnology that influences the evolution of some species. Humans have become a dominant agent of evolutionary change.