Chapter 17 Notes- The History of Life pages 423-428, 435-440

• 17–2 Earth’s Early HistoryA. Formation of EarthB. The First Organic MoleculesC. How Did Life Begin?

1. Formation of Microspheres2. Evolution of RNA and DNA

D. Free OxygenE. Origin of Eukaryotic CellsF. Sexual Reproduction and

Multicellularity

• 17–4 Patterns of EvolutionA. ExtinctionB. Adaptive RadiationC. Convergent EvolutionD. CoevolutionE. Punctuated EquilibriumF. Developmental Genes and Body Plans

Origins: The Early Idea

• In the past, the ideas that decaying meat produced maggots, mud produced fishes, and grain produced mice were reasonable explanations for what people observed occurring in their environment.

• Such observations led people to believe in spontaneous generation—the idea that nonliving material can produce life.

Spontaneous generation is disproved• In 1668, an Italian physician, Francesco Redi, disproved a

commonly held belief at the time—the idea that decaying meat produced maggots, which are immature flies.

• However, during Redi’s time, scientists began to use the latest tool in biology—the microscope.

• Although Redi had disproved the spontaneous generation of large organisms, many scientists thought that microorganisms were so numerous and widespread that they must arise spontaneously-probably from a vital force in the air.

Control groupTime

Time

Experimental group

Spontaneous generation is disproved

• In the mid-1800s, Louis Pasteur designed an experiment that disproved the spontaneous generation of microorganisms.

• Pasteur set up an experiment in which air, but no microorganisms, was allowed to contact a broth that contained nutrients.

• Pasteur’s experiment showed that microorganisms do not simply arise in broth, even in the presence of air.

• From that time on, biogenesis, the idea that living organisms come only from other living organisms, became a cornerstone of biology.

• The cell theory states that cells can arise from preexisting cells.

Broth is boiled. Broth is free of

microorganismsfor a year.

Curved neckis removed.

Broth is teeming with

microorganisms.

The Origin of Life• Francesco Redi and Louis Pasteur designed controlled

experiments to disprove spontaneous generation. • Their experiments and others like them convinced scientists to

accept biogenesis.• But if life comes only from life, then how did life on Earth first

begin? • No one has yet proven scientifically how life on Earth began.

• However, scientists have developed theories about the origin of life on Earth from testing scientific hypotheses about conditions on early Earth.

Origins: The Modern Ideas

1. Early Earth was hot; atmosphere contained poisonous gases.

– Some scientists suggest that it was probably very hot. The energy from colliding meteorites could have heated its surface, while both the compression of minerals and the decay of radioactive materials heated its interior.

– Volcanoes might have frequently spewed lava and gases, relieving some of the pressure in Earth’s hot interior. These gases helped form Earth’s early atmosphere. Earth’s early atmosphere probably contained hydrogen cyanide, carbon dioxide, carbon monoxide, nitrogen, hydrogen sulfide, and water.

Origins: The Modern Ideas2. Earth cooled and oceans condensed.

• About 4.4 billion years ago, Earth might have cooled enough for the water in its atmosphere to condense. This might have led to millions of years of rainstorms with lightning—enough rain to fill depressions that became Earth’s oceans.

Origins: The Modern Ideas3. Simple organic molecules may

have formed in the oceans.

– In the 1930s, a Russian scientist, Alexander Oparin, hypothesized that life began in the oceans that formed on early Earth.

– He suggested that energy from the sun, lightning, and Earth’s heat triggered chemical reactions to produce small organic molecules from the substances present in the atmosphere

– Then, rain probably washed the molecules into the oceans to form what is often called a primordial soup.

– In 1953, two American scientists, Stanley Miller and Harold Urey, tested Oparin’s hypothesis by simulating the conditions of early Earth in the laboratory

Origins: The Modern Ideas

4. Small sequences of RNA may have formed and replicated.

• Scientists hypothesize that two developments must have preceded the appearance of life on Earth.

– First, simple organic molecules, or molecules that contain carbon, must have formed.

– Then these molecules must have become organized into complex organic molecules such as proteins, carbohydrates, and nucleic acids that are essential to life.

5. First prokaryotes may have formed when RNA or DNA was enclosed in microspheres.

• The first forms of life may have been prokaryotic forms that evolved from a protocell. A protocell is a large, ordered structure, enclosed by a membrane, that carries out some life activities, such as growth and division.– Because Earth’s atmosphere lacked oxygen, these organisms were most

likely anaerobic.

– For food, the first prokaryotes probably used some of the organic molecules in oceans.

– Over time, these heterotrophs would have used up the food supply.

• However, organisms that could make food had probably evolved by the time the food was gone.– These first autotrophs were probably similar to present-day

archaebacteria.

– Archaebacteria are prokaryotic and live in harsh environments, such as deep-sea vents and hot springs.

– The earliest autotrophs probably made glucose by chemosynthesis rather than by photosynthesis.

– In chemosynthesis, autotrophs release the energy of inorganic compounds, such as sulfur compounds, in their environment to make their food.

• 6. Later prokaryotes were photosynthetic and produced oxygen.

– Photosynthesizing prokaryotes might have been the next type of organism to evolve.

– As the first photosynthetic organisms increased in number, the concentration of oxygen in Earth’s atmosphere began to increase.

– Organisms that could respire aerobically would have evolved and thrived.

– The presence of oxygen in Earth’s atmosphere probably affected life on Earth in another important way.

• 7. An oxygenated atmosphere capped by the ozone layer protected Earth.

– The sun’s rays would have converted much of the oxygen into ozone molecules that would then have formed a layer that contained more ozone than the rest of the atmosphere.

• 8. First eukaryotes may have been communities of prokaryotes.

– Complex eukaryotic cells probably evolved from prokaryotic cells.

– The endosymbiont theory, proposed by American biologist Lynn Margulis explains how eukaryotic cells may have arisen. The theory proposes that eukaryotes evolved through a symbiotic relationship between ancient prokaryotes.

• New evidence from scientific research supports this theory and has shown that chloroplasts and mitochondria have their own ribosomes that are similar to the ribosomes in prokaryotes.

• In addition, both chloroplasts and mitochondria reproduce independently of the cells that contain them.

• The fact that some modern prokaryotes live in close association with eukaryotes also supports the theory.

Aerobic bacteria

Ancient Prokaryotes

Ancient Anaerobic Prokaryote

Primitive Aerobic Eukaryote

Primitive Photosynthetic Eukaryote

Chloroplast

Photosynthetic bacteria

Nuclear envelope evolving Mitochondrion

Plants and plantlike protists

Animals, fungi, and non-plantlike protists

Origins: The Modern Ideas

9. Sexual reproduction increased genetic variability, hastening evolution.

• increase in genetic variation greatly increases the chances of evolutionary change in a species due to natural selection

10. Multicellular eukaryotes evolved.

• cells increased in diversity more rapidly

Evolution of Life Review

Evolution of Life

Sexual reproduction increased genetic variability, hastening evolution.

Early Earth was hot; atmosphere contained poisonous gases.

Earth cooled and oceans condensed.

Simple organic molecules may have formed in the oceans..

Small sequences of RNA may have formed and replicated.

First prokaryotes may have formed when RNA or DNA was enclosed in microspheres.

Later prokaryotes were photosynthetic and produced oxygen.

An oxygenated atmosphere capped by the ozone layer protected Earth.

First eukaryotes may have been communities of prokaryotes.

Multicellular eukaryotes evolved.

Patterns of Evolution

• The history of life is the story of increasing complexity and diversity, but what does this history reveal about the process of evolution?

• Biologists have observed different patterns of evolution that occur throughout the world in different natural environments.

• Macroevolution refers to the large-scale evolutionary changes that take place over long periods of time.

• Six important patterns of macroevolution are – mass extinctions– adaptive radiation– convergent evolution– coevolution– punctuated equilibrium– changes in developmental genes

• These patterns support the idea that natural selection is an important agent for evolution.

Mass Extinctions• The term used to describe a species that has died out is extinct.• More than 99 percent of all species that ever lived are now extinct.• Huge numbers of species have disappeared in mass extinctions. • The disappearance of so many species left many habitats open. • For the survivors, there was a new world of ecological opportunity. • Often, the result was a burst of evolution that produced an abundance of new

species.

Divergent Evolution• Divergent evolution occurs when populations change as they

adapt to different environmental conditions, eventually resulting in new species. Species that were once similar to an ancestral species diverge, or become increasingly distinct.

• One type of divergent evolution is adaptive radiation.

• Ex: Darwin’s Finches

Amakihi Extinct mamo

Crestedhoneycreeper

Akialoa

Akepa

Akiapolaau LiwiMaui parrotbill

Apapane

Ou

Grosbeak finch

PalilaAkikiki

Niihau

Kauai

Oahu

Lanai

Molokai

Maui

KahoolaweHawaii

Possible AncestralLasan finch

Adaptive Radiation

• When an ancestral species evolves into an array of species to fit a number of diverse habitats, the result is called adaptive radiation.

• Adaptive radiation in both plants and animals has occurred and continues to occur throughout the world and is common on islands.

Coevolution• The process by which two species evolve in response to

changes in each other over time is called coevolution.

• The trees have evolved in response to their seed predators, we can observe geographic differences in pinecones. – Where there are squirrels, the pinecones are

heavier with fewer seeds, but have thinner scales, like the pinecone on the left.

– Where there are only crossbills, pinecones are lighter with more seeds, but have thick scales, like the one on the right.

• The crossbills have evolved in response to the pine trees, we can observe geographic differences in birds. – Where the pinecones have thick scales, birds should

have deeper, less curved bills (below right) than where the pinecones have thin scales (below left).

Convergent Evolution• A pattern of evolution in which distantly related organisms evolve

similar traits is called convergent evolution. • Convergent evolution occurs when unrelated species occupy

similar environments in different parts of the world. • Each of these animals has a streamlined body and various

appendages that enable it to move rapidly through water. Yet, the shark is a fish, the penguin is a bird, and the dolphin is a mammal.

• They have evolved similar traits because they share similar environmental pressures.

Gradualism vs. Punctuated Equilibrium• Scientists once argued that evolution occurs at a slow, steady rate, with small,

adaptive changes gradually accumulating over time in populations. • In 1972, scientists proposed a different hypothesis known as punctuated

equilibrium, which argues that speciation occurs relatively quickly, in rapid bursts, with long periods of genetic equilibrium in between.

• Environmental changes, such as higher temperatures or the introduction of a competitive species, lead to rapid changes in a small population’s gene pool that is reproductively isolated from the main population.

• Speciation happens quickly—in about 10,000 years or less.

Loxodonta africana

Elephas maximus

Mammuthusprimigenius

Mammuthus

Elephas

Loxodonta

Primelephas

about 55 million years agoAncestral species

6

5

4

3

2

1

0

Mil

lion

s of

Yea

rs A

go

Gradualism vs. Punctuated Equilibrium

• Gradualism involves a slow, steady change in a particular line of descent.

• Punctuated equilibrium involves stable periods interrupted by rapid changes involving many different lines of descent.

• Biologists generally agree that both gradualism and punctuated equilibrium can result in speciation, depending on the circumstances.

Developmental Genes and Body Plans

• Hox genes are called “master control genes,” because they control growth as an embryo develops.

• Some hox genes determine which parts become front, rear, top, and bottom.

• Others control the growth of body parts such as arms, legs, and wings.

• The timing of genetic control during embryonic development can make the difference between various traits.

Patterns of Evolution Flow Chart

that are

can undergo can undergo can undergo can undergo can undergo

in underunderform inin

Species

Unrelated Related

Inter-relationships Similar environments

Intense environmental

pressure

Small populations

Different environments

Co evolution Convergent evolution

ExtinctionPunctuated equilibrium

Adaptive radiation