The History of Life

1. The History of Life Chapter 17 – Miller · Levine

2. Fossils • Paleontologists – scientists who study fossils • Infer what past life forms were like • Classify fossil organisms • Fossil record – information about past life • Provides evidence about the history of life on Earth • Shows how different groups of organisms have changed over time

3. How Fossils Form • Either the remains of the organism or some trace of its presence must be preserved • Most fossils form in sedimentary rock • An imprint of soft parts can be made • The hard parts can become mineralized • Can be preserved in rock, ice, amber, tar, etc.

4. Relative Dating • The age of a fossil is determined by comparing its placement with that of fossils in other layers of rock • Older fossils will be in the bottom layers • More recent fossils will be in the upper layers • Index fossils – fossils of species that existed for a short period but had a wide range • Allows paleontologists to estimate a fossil’s age compared with that of other fossils

5. Relative Dating

6. Radioactive Dating • Scientists use radioactive decay to assign absolute ages to rocks • Calculate the age of a sample based on the amount of remaining radioactive isotopes it contains • Half-life – the length of time required for half of the radioactive atoms in a sample to decay

7. Geologic Time Scale • Precambrian Time – covers about 88% of Earth’s history • Paleozoic Era – many vertebrates and invertebrates • Mesozoic Era – “Age of the Dinosaurs” • Mammals evolved • Cenozoic Era – “Age of Mammals” • Eras are subdivided into periods

8. Geologic Time Scale

9. Formation of Earth • Pieces of cosmic debris were probably attracted to one another • The early atmosphere probably contained hydrogen cyanide, carbon dioxide, carbon monoxide, nitrogen, hydrogen sulfide, and water • Violent volcanic activity, comets and asteroids bombarded the surface, oceans did not exist • About 3.8 billion years ago, Earth’s surface cooled enough for oceans to form

10. How Did Life Begin? • The first organic molecules were able to form because of the atmosphere being bombarded by electricity (lightning) • RNA probably evolved before DNA

11. The Rise of Oxygen… • The first life forms must have evolved in the absence of oxygen (anaerobic prokaryotes) • Over time, photosynthetic bacteria evolved, adding oxygen to the atmosphere • The ozone layer formed • The rise of oxygen caused some life forms to go extinct while other organisms evolved

12. Endosymbiotic Theory • Prokaryotic organisms entered the “ancestral eukaryote” • Formed a symbiotic relationship • Some had the ability to perform respiration – became modern mitochondria • Others could perform photosynthesis – became the modern chloroplasts

13. Endosymbiotic Theory

14. Macroevolution • Large scale evolutionary changes that take place over long periods of time • Mass extinctions • Adaptive radiation • Convergent evolution • Coevolution • Punctuated equilibrium

15. Mass Extinctions • Extinction occurs all the time • More than 99% of all species that have ever lived are now extinct • Mass extinctions occur when large numbers of species go extinct in a relatively short period of time • Most mass extinctions were probably caused by multiple factors • Usually followed by mass speciation

16. Adaptive Radiation • A single species or a small group of species has evolved into several different forms that live in different ways • Also known as divergent evolution • Often caused by geographic isolation – geological change (river, canyon, or mountain) that isolates segments of a population

17. Adaptive Radiation

18. Convergent Evolution • Adaptive radiation in groups of different organisms in different places or at different times, but in similar environments • Unrelated organisms resemble one another • Analogous structures – look and function similarly but do not share a common ancestry • Fins of dolphins, seals, penguins, sharks

19. Coevolution • Sometimes organisms that are closely connected to one another evolve together • An evolutionary change in one organism may be followed by a corresponding change in another organism • Snails developed thicker shells, so crabs evolved stronger claws • Plants produce poisonous chemicals, herbivores evolved the ability to detoxify these chemicals

20. Gradualism vs. Punctuated Equilibrium • Gradualism – biological change is a slow and steady process • Punctuated equilibrium – long, stable periods interrupted by brief periods of more rapid change