The History of LifeChapter 17 – Miller · Levine

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

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.

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

Relative Dating

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

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

Geologic Time Scale

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

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

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

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

Endosymbiotic Theory

Macroevolution

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

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

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

Adaptive Radiation

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

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

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