Chapter 12:

History of Life on Earth

Biology II

Radiometric Dating Scientists estimate that Earth formed

~4.5 bya using a technique called

radiometric dating

Estimation of the age of an object by

measuring its content of certain radioactive

isotopes

Radioisotopes An isotope is a form of an element

whose atomic mass differs from that of

other atoms of the same element

Radioisotope – unstable isotope that

breaks down and gives off energy in the

form of charged particles (radiation)

This radioactive decay

results in isotopes that

are smaller and more

stable

Half-Lives By measuring the proportions of certain

radioactive isotopes and their products of decay, scientists can compute how many half-lives have passed since a rock was formed

Half-life – time it takes for one-half of a given amount of radioisotope to decay

Formation of Life Though present hypotheses differ on

the origin of life on Earth, most scientists agree that it developed through natural chemical and physical processes

Thought to have began when molecules of nonliving matter reacted chemically during the 1st billion years of Earth’s history These chemical reactions produced many

different simple organic molecules that, energized by sun and volcanic heat, eventually formed more complex molecules that became building blocks of the 1st cells

Primordial Soup Model In 1920s, Oparin and Haldane suggested that

Earth’s early oceans contained large amounts

of organic molecules

Oceans believed to have formed when the Earth’s

surface cooled and water vapor in atmosphere

condensed

Became known as primordial soup model,

since oceans were thought to be filled with

many different organic molecules, like a soup

filled with many different vegetables and meat

Formation of Organic

Molecules

Oparin and Haldane hypothesized that

these organic molecules formed

spontaneously in chemical reactions

activated by energy from solar radiation,

volcanic eruptions, and lightning

Formation of Organic

Molecules It is believed that Earth’s early atmosphere

lacked O2, and was instead rich in nitrogen

gas (N2), hydrogen gas (H2), and hydrogen-

containing gases like water vapor, ammonia

(NH3), and methane (CH4)

It was reasoned that electrons in these gases

would have been frequently pushed to higher

energy levels by light particles from sun or

electrical energy in lightning

Without O2, these electrons would be free to react with

hydrogen-rich molecules, forming variety of organic

compounds

The Miller-Urey Experiment Primordial soup model was firs tested

by Miller and Urey in 1953

Placed gases proposed to have existed on early Earth into a device like the one below Provided electrical sparks to simulate lightning

After a few days, they found complex collection of organic molecules in apparatus, including some of life’s basic building blocks

– Amino acids, fatty acids, and other hydrocarbons

Results support hypothesis that some basic chemicals of life could have formed spontaneously under these conditions

The Miller-Urey Experiment

Reevaluating the Miller-Urey

Model Recent discoveries have shown that

molecules used in Miller’s experiment could

not have existed in abundance on early Earth

Ozone layer did not exist, meaning that UV

radiation would have destroyed any ammonia and

methane present in atmosphere

In absence of these gases, key biological

molecules are not made in experiment

If these chemicals needed to form life were not

in atmosphere, where did they come from???

The Bubble Model In 1986, Louis Lerman suggested that

key processes that formed the chemicals needed for life took place within bubbles on ocean’s surface

Step 1: Ammonia, methane, and other gases resulting from numerous eruptions of undersea volcanoes were trapped in underwater bubbles

Step 2: Bubbles protected gases from UV radiation Chemical reactions would also take place much

faster in bubbles, where reactants would be more concentrated

The Bubble Model

Step 3: Bubbles rose to surface and burst,

releasing simple organic molecules into air

Step 4: Organic molecules were carried

upward by winds and exposed to UV

radiation and lightning, providing energy for

further reactions

Step 5: More complex organic molecules that

formed by further reactions fell into ocean

with rain, starting another cycle

Lerman’s Bubble Model

RNA as the Precursor to

Macromolecules Scientists have not been able to make

proteins or DNA spontaneously under

laboratory conditions

Short chains of RNA,

however, have been made to

form on their own in water

It is hypothesized that RNA was

the 1st self-replicating

information-storage molecule,

and it catalyzed the assembly

of the 1st proteins

Such a molecule would have been

able to change from one generation

to the next

Microspheres and

Coacervates Lab experiments have shown that, in water,

short chains of amino acids can gather

together in tiny drops called microspheres

Scientists think that microspheres

might have been 1st step toward

cellular organization, though they

could not be considered true cells

without heredity

Another type of droplet, called a coacervate,

is composed of molecules of different types,

including linked amino acids and sugars

Origin of Heredity

Most scientist agree that DNA evolved

after RNA, and that RNA “enzymes”

catalyzed the assembly of the earliest

proteins

Some scientists also think that

microspheres that contained RNA

developed a means of transferring their

characteristics to offspring

Not yet understood how DNA, RNA, and

hereditary mechanisms first developed

The First Prokaryotes Fossil – preserved or

mineralized remains or imprint of organism that lived long ago

Oldest known fossils are microscopic fossils of prokaryotes that come from a 2.5 by old rock Marine cyanobacteria, or photosynthetic

prokaryotic organisms, were one of first prokaryotes to evolve

– After 100s of millions of years, the oxygen gas produced by cyanobacteria began to escape into the air

2 Groups of Prokaryotes 2 different groups of prokaryotes

evolved early in Earth’s history

Eubacteria – prokaryotes that contain a chemical called peptidoglycan in their cell walls Include many bacteria that cause disease and

decay, including E.coli

Commonly called bacteria

Archaebacteria – prokaryotes that lack peptidoglycan in their cell walls and have unique lipids in their cell membranes Chemical evidence shows that archaebacteria

and eubacteria diverged very early

The 1st Eukaryotes

1st eukaryotes appeared 1.5 bya

Much larger than prokaryotic cell

Contain nucleus and

other membrane-

bound organelles,

including

mitochondria and

sometimes

chloroplasts, both which contain their own

DNA

Endosymbiosis Most scientists think that mitochondria

and chloroplasts originated through endosymbiosis

Theory proposed in 1966 by Lynn Margulis stating that mitochondria are the descendents of symbiotic, aerobic eubacteria and chloroplasts are the descendents of symbiotic photosynthetic eubacteria

Endosymbiosis

According to the theory, bacteria

entered large cells either as parasites or

undigested prey

Rather than being digested, bacteria began

to live inside host cell, where they

performed either cellular respiration

(mitochondria) or photosynthesis

(chloroplasts)

Support for Endosymbiosis Both mitochondria and chloroplasts have

characteristics similar to those of bacteria Size and structure: mitochondria are about same

size as most eubacteria and chloroplasts are same size as some cyanobacteria Both have 2 membranes

Both chloroplasts and cyanobacteria contain thylakoids

Genetic material: both have unique and circular DNA similar to chromosomes found in bacteria

Ribosomes: both have ribosomes similar in size and structure to bacterial ribosomes

Reproduction: both reproduce by simple fission, which takes place independently of cell cycle of host cell

Multicellularity Unicellular body plan has been very

successful, with unicellular organisms today

making about half of Earth’s biomass

Single cell must carry out all activities of organisms,

unlike in multicellular organisms, where distinct

types of cells have specialized function

1st multicellular fossils found in 700 my old rock

The 1st eukaryotic kingdom was kingdom

Protista

Protists - make up large, varied group that includes

both multicellular and unicellular organisms

Other 3 kingdoms evolved later and also consist of

eukaryotes

Origin of Modern Organisms Most animal phyla that exist today probably

originated during late Precambrian and

Cambrian periods (10-100 mya)

Rapid diversification of animals is sometimes known

as “Cambrian explosion”

Very rich collection of Cambrian fossils uncovered in

1909 in geological formation in Canada called

Burgess Shale

Ordovician period (505-438 mya) followed

Cambrian, and many different animals,

including trilobites, marine arthropods that went

extinct 250 mya, continued to inhabit the seas

Mass Extinctions Fossil record indicates that sudden change

occurred at end of Ordivician, ~440 mya

Large percentage of organisms on Earth suddenly

became extinct

Extinction – death of all members of a species

This was 1st of 5 major mass extinctions that have

occurred on Earth

Mass extinction – episode during which large numbers

of species become extinct

Another mass extinction of about same size

occurred 360 mya, followed by the 3rd, most

devastating mass extinction ~245 mya, at end of

Permian (96% of all species went extinct)

Mass Extinctions 4th mass extinction took place 35 my later,

followed by 5th mass extinction, ~65 mya, when 2/3 of land species, including dinosaurs, went extinct

Some scientists think that another mass extinction is occurring today Earth’s ecosystems, including rainforests, are

being destroyed by human activity

½ of all tropical rain forests have already been destroyed

At this rate, 22-47% of Earth’s plant species will be lost, along with 2,000 of 9,000 species of birds, and countless insect species

The Ozone Layer About 2.5 bya, photosynthesis by

cyanobacteria began adding oxygen to Earth’s atmosphere

As oxygen reached upper atmosphere, it reacted chemically with sunlight, forming molecules of ozone, which blocks the sun’s harmful UV rays After

millions of years, enough ozone layer had accumulated to make Earth’s land a safe place to live

Plants and Fungi on Land 1st multicellular organisms to live on

land may have been fungi living

together with plants or algae, about 430

mya

Early plants and fungi formed biological

partnerships called mycorrhizae, which

enabled them to live on harsh habitat of

bare rock

Fungus provides minerals to plant, and plant

provides nutrients to fungus

– Mutualism – relationship between 2 species in

which both species benefit

Arthropods 1st animals to successfully invade land were

arthropods

Arthropod – kind of animal with a hard outer

skeleton, segmented body, and paired, jointed

limbs

Include lobsters, crabs, insects, and spiders

The insect evolved from the 1st land dwellers

and were the first animals to have wings

Allowed insects to efficiently search

for food, mates, and nesting sites

Also led to partnership between

insects and flowering plants

Vertebrates

Vertebrate – animal with a backbone

1st vertebrates were jawless fish that

evolved in oceans ~530 mya

Jawed fish 1st appeared ~430 mya

– Jaws enabled fish to bite and chew food,

making them more efficient predators

Fish are most successful

living vertebrate, making

up more than half of all

modern vertebrate

species

Amphibians 1st land vertebrates were early

amphibians that evolved 370 mya

Amphibians are smooth-skinned, 4-legged animals that include frogs, toads, and salamanders

Several structural changes occurred as adaptations to life on land Lungs to absorb oxygen

from air

Strong, flexible internal skeleton

Limbs derived from bones of fish fins

Reptiles

Reptiles evolved from amphibian

ancestors ~340 mya

Include snakes, lizards, turtles, and

crocodiles

Better suited to

dry land than

amphibians

Watertight skin

Watertight eggs

Mammals and Birds Birds are thought to have evolved from

feathered dinosaurs during or after Jurassic

Therapsids, reptiles with complex teeth and legs positioned beneath their body, gave rise to mammals about the same time that dinosaurs evolved, during the Triassic After 5th mass extinction 65 mya, birds and

mammals became dominant vertebrates on land, since dinosaurs went extinct

Continental drift – movement of Earth’s land masses over Earth’s surface through geological time Both extinction and continental drift played large

role in evolution