History of LifeOrigins and Extinctions: Chapter 17, 16.4

DAILY

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• Many ecologists are concerned about species extinctions, especially in groups that we know little about.

• What are some human-related causes of extinction?

• What are some natural causes of extinction?

Extinction

• Over 90% of the species that have lived on Earth have gone extinct.

• Past extinction events have had natural causes.

• The present extinction event is human-caused. How is this different?

Adaptive radiation led to the many varieties of silversword plant on the Hawai’an Islands.

Why are these species threatened today?

Natural Causes

• Species arose and went extinct long before there were humans on the planet.

• Global disasters and global climate change drove mass extinction events in the past.

• Individual species may go extinct for other natural reasons.

Species with a very small geographic range are vulnerable to extinction if their habitat is threatened or eliminated. This Devil’s Hole

pupfish lives in only one waterhole in Nevada.

Species with very specialized habits

are more vulnerable to

extinction. The Fender’s Blue butterfly feeds

only on the Kincaid’s Lupine as

both larva and adult. The butterfly was thought to be

extinct until recently

rediscovered near Corvallis.

DAILY

WORK

• Some coral reef fish are highly specialized in their feeding or nesting habits. How does this affect:

• their short-term survivability as populations? (Think of what we learned about competition.)

• their long-term survivability as a species?

Human Effects

• Humans have “changed the game” when it comes to extinctions. Never before has one species had such widespread effects on the planet.

• Human effects can be summed up by the HIPPO acronym: Habitat destruction, Invasive species, Pollution, Population, and Overharvest.

History of Earth’s Diversity

• Studying the history of life’s diversity in the past helps us understand ecosystems today.

• By understanding which ecosystems were most vulnerable in the past, we can better predict effects of human intervention and natural disasters.

Life’s Origins

• Evolution studies changes to living things over time.

• Cell Theory states that all living cells come from other living cells.

• The question remains: Where did the first living cells come from?

Early Earth was very different from what is is today: no free oxygen in the atmosphere,

much hotter surface and atmosphere, no ozone to shield the surface from UV

radiation – and no life.

• Stanley Miller and Harold Urey set up an apparatus to test Oparin’s hypothesis. They recreated the best understanding of early Earth’s atmosphere, with sparks to simulate lightning, and got organic molecules.

•Miller’s studies showed that organic molecules could form on a watery surface, given energy.

•Other, similar experiments, showed that deep ocean thermal vents could also be a source of organic molecules.

What about cells?

• Experiments have shown that lipids (fats and oils, which are organic molecules) spontaneously form cell-sized bubbles when agitated in water.

Cell membranes in all organisms today are made

up largely of lipids.

• Lipid bubbles can collect other organic molecules within. If these formed compartments where chemical reactions took place, could this be the origin of some of the chemical reactions that led to metabolism.

“Metabolism First” theory

“Self-replication first” Theory

•The “RNA World” hypothesis suggests that RNA was the first self-replicating molecule.

•Unlike DNA, RNA does not need enzymes to replicate itself. It can also act as an enzyme to catalyze some chemical reactions.

• Of the two theories the RNA world hypothesis has the most supporting evidence at present.

• However, both processes could contribute to the formation of cells.

Simple to Complex Cells• The earliest cells in the fossil record

are prokaryotic organisms, such as bacteria.

• The development of eukaryotic cells appears to have involved symbiosis between prokaryotic organisms.

• Chloroplasts and Mitochondria have their own DNA and operate like symbionts in the cells of plants and animals.

1 Anaerobic, predatoryprokaryotic cell engulfs anaerobic bacterium.

2 Descendants ofengulfed bacterium evolveinto mitochondria.

3 Mitochondria-containingcell engulfs a photosyntheticbacterium.

4 Descendants ofphotosynthetic bacteriumevolve into chloroplasts.

Endosymbiosis Theory

Changing Communities

• As the Earth changed over time, living organisms changed in response.

• Selection favored traits that gave organisms an advantage in changing ecosystems.

• Rapid change resulted in extinction of many organisms.

DAILY

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• We’ve seen that communities can change over time, which we call succession. In succession, communities change as known species move in and replace other, pioneering species. Can communities also evolve?

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The history of life on Earth has been marked by numerous extinction events, followed by

adaptive radiation.

Prokaryotes, early Eukaryotic single-celled

organisms, and the earliest multicellular

prokaryotic organisms arose during the

Precambrian. These soft-bodied organisms left

few fossil traces.

The “Cambrian Explosion” was a period of rapid adaptive radiation. Many of the major groups of organisms alive today arose in the

Paleozoic Era.

The Silurian period in the Paleozoic Era saw an enormous increase in the diversity of

marine organisms. Many of these organisms left abundant fossils behind.

DAILY

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• What were the first organisms to live on land?

Invading the Land

• Water provides support, nutrients, and a means for sperm to reach egg during sexual reproduction.

• Land-based living is challenging. Organisms must resist drying out, must be able to find food, and must reproduce without relying on external water.

The very first organisms to exist on land were probably bacteria, like these

mat-forming bacteria at Yellowstone. Many of these prokaryotic organisms can form biofilms and would have been able to

cling to rocks at the edges of oceans and

lakes.

The very first Eukaryotic organisms to colonize the land

were the green algaes. These first

land-based organisms most likely gave rises to the mosses, which

were the first terrestrial plants.

These provided food and habitat needed for the first animals to colonize the land.

The first land-dwelling animals were the arthropods. Like the modern land crab and isopod, early arthropods still needed damp

habitats to keep their gills moist, but had an external skeleton that resisted drying out and legs that were an advantage for locomotion on

land.

The first land vertebrates were the amphibians.

They may descend from

fish with strong pectoral fins.

These fish may have fed on

arthropods living on mudflats or other intertidal areas, where

strong forelimbs gave them an

advantage.

Ancient Communities

• As is true today, ancient organisms lived in complex communities.

• Communities changed as the Earth’s climate changed. Organisms went extinct because of global changes; others arose by adaptive radiation following major extinctions.

During the Carboniferous period of the Paleozoic Era, tree ferns and thick mosses

formed forests where insects and amphibians thrived. What would the climate have been

like?

340 million years ago

NorthAmerica

Eurasia

SouthAmerica

AfricaAustralia

India

Antarctica

At the time of the Carboniferous period, most of the major land masses formed one large

continent. Most of the land mass was in the southern hemisphere. During this time, large

amounts of carbon were stored in “coal swamps.”

Coral reefs arose and vanished many times during the history of the earth. While coral reefs are highly diverse ecosystems, the narrow specialization of many coral reef

organisms make them vulnerable to major climate changes.

A major extinction event (possibly caused by a meteorite or volcanism) marks the boundary between the Permian and the Mesozoic Era. Changes in climate may

have been caused by the extinction event trigger, or may have contributed to

extinctions. The event was followed by rapid adaptive radiation in marine and

terrestrial habitats.

NorthAmerica

PANGAEA

Eurasia

SouthAmerica

Africa

AustraliaIndia

Antarctica225 million years ago

Movement of land masses also changed local climates over time. The large land mass at

the south pole was breaking up. Larger land masses moved north. Coal swamps still

existed in tropical areas.

EASTGONDWANA

LAURASIA

NorthAmerica

Eurasia

SouthAmerica Africa

AustraliaIndia

Antarctica

WEST GONDWANA

135 million years ago

Later in the Mesozoic, the continents broke up further into the land masses that would form

today’s continents.

DAILY

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• The size of the land mass at the South Pole changed over time. Did this affect climate? Why or why not?

The first seed plants and

later the first flowering

plants developed at

this time. These plants

dominated the forests,

driving many tree ferns into

extinction. Conifers were more efficient at reproduction than ferns and better adapted to dry climates

Among the first flowering plants were the grasses. Like the conifers, flowering plants

were more efficient at reproduction and better able to cope with dry climates. Grasses have coarse cell walls, making them hard to

digest. As grasses invaded the land, herbivorous dinosaurs adapted to softer food

were at a disadvantage.

The end of the Mesozoic and the beginning of the Cenozoic Era is marked by a major

extinction event that was most likely caused by an asteroid or large meteor

striking the earth. About 70% of life on the planet went extinct, including the

dinosaurs. Mammals, arthropods, and many seed-bearing plants survived and underwent adaptive radiation, claiming

vacant niches.

Mammals and birds experienced rapid

diversification early in the Cenozoic. Many

modern species arose early during this era.

Primates arose and diversified during the Tertiary period. These

included Australopithecus, the first hominids, which

appeared about 4 million years ago in Africa.

H. neanderthalensis

millions of years ago

A. africanus

Australopithecus afarensisSahelanthropus tchadensis

Ardipithecus ramidus

Orrorin tugenensis A.anamensis

A. boisei

A. robustus

H. erectus

H.habilis

Homo ergaster

H. heidelbergensis

H. sapiens

Human evolution is still a puzzle. It is fairly certain that humans originated in Africa and

radiated from there.

The spread of Homoerectus began at least1.8 million years ago.

African Replacement Hypothesis

Homo sapiens’ expansionbegan around 100,000years ago.

It is possible that all modern humans descend directly from African ancestors. Later

migrations displaced descendants of earlier migrations.

Multiregional Hypothesis

Regional populationsof Homo erectusevolved intoHomo sapiens whileintermingling withone another.

Some evidence suggests one initial migration, then local adaptation and intermingling of

groups.

Recap

• The history of life on earth shows that communities change and evolve over time.

• Extinction is a part of earth’s history. Until recently, extinction has been caused by natural events.