NOTES: CH 25 - The History of

Life on Earth

Origin of Earth4500

History

of Life

Boundaries

between units in

the Geologic

Time Scale are

marked by

dramatic biotic

change

Eras

Overview: Lost Worlds

● Past organisms were very different from those

now alive

● The fossil record shows macroevolutionary

changes over large time scales, for example:

– The emergence of terrestrial vertebrates

– The impact of mass extinctions

– The origin of flight in birds

Prebiotic Chemical Evolution:

● Earth’s ancient environment was different

from today:

-very little atmospheric oxygen

-lightning, volcanic activity, meteorite,

bombardment, UV radiation were all more

intense

● Chemical evolution may have

occurred in four stages:

1) abiotic synthesis of monomers

2) joining of monomers into polymers

(e.g. proteins, nucleic acids)

3) formation of protocells (droplets

formed from clusters of molecules)

4) origin of self-replicating molecules

that eventually made inheritance

possible (likely that RNA was first)

Oparin / Haldane hypothesis (1920s):the reducing atmosphere and greater UV

radiation on primitive Earth favored reactions

that built complex organic molecules from

simple monomers as building blocks

Miller / Urey experiment:

Simulated conditions on early Earth by

constructing an apparatus containing H2O, H2,

CH4, and NH3.

Results:

● They produced amino acids and other organic molecules.

● Additional follow-up experiments have produced all 20 amino acids, ATP, some sugars, lipids and purine and pyrimidine bases of RNA and DNA.

Ma

ss

of

am

ino

ac

ids

(m

g)

Nu

mb

er

of

am

ino

ac

ids

20

10

0

1953 2008

200

100

0

1953 2008

● Protocells: collections of abiotically

produced molecules able to maintain an

internal environment different from their

surroundings and exhibiting some life

properties such as metabolism,

semipermeable membranes, and

excitability

(experimental evidence

suggests spontaneous

formation of

protocells)

Abiotic genetic replication

possible

formation

of protocells;

self-repliating

RNA

as early

“genes”

Origin of Life - Different Theories:

*Experiments indicate key steps that could have occurred.

● Panspermia: some organic compounds may have reached Earth by way of meteorites and comets

meteorite

● Sea floor / Deep-sea vents: hot water and minerals emitted from deep sea vents may have provided energy and chemicals needed for early protobionts

● Simpler hereditary systems (self-replicating molecules) may have preceded nucleic acid genes.

Phylogeny: the evolutionary history of a

species● Systematics:

the study of biological diversity in an evolutionary context

● The fossil record: the ordered array of fossils, within layers, or strata, of sedimentary rock

● Paleontologists: collect and interpret fossils

● A FOSSIL is the remains or evidence of a living thing

-bone of an organism or the print of a shell in a rock

-burrow or tunnel left by an ancient worm

-most common fossils: bones, shells, pollen grains, seeds.

Dimetrodon

Stromatolites

Fossilizedstromatolite

Coccosteuscuspidatus

4.5 cm

0.5 m

2.5

cm

Present

Rhomaleosaurus victor

Tiktaalik

Hallucigenia

Dickinsonia costata

Tappania

1 cm

1 m

100 mya

175

200

300

375400

500525

565

600

1,500

3,500

270

Figure 25.4

PETRIFICATION is the process by which plant or animal remains are turned into stone over time. The remains are buried, partially dissolved, and filled in with stone or other mineral deposits.

A MOLD is an empty space that has the shape of the organism that was once there. A CASTcan be thought of as a filled in mold. Mineral deposits can often form casts.

Thin objects, such as leaves and feathers, leave IMPRINTS, or impressions, in soft sediments such as mud. When the sediments harden into rock, the imprints are preserved as fossils.

Examples of different kinds of fossils

PRESERVATION OF ENTIRE ORGANISMS:It is quite rare for an entire organism to be preserved because the soft parts decay easily. However, there are a few special situations that allow organisms to be preserved whole.

FREEZING: This prevents substances from decaying. On rare occasions, extinct species have been found frozen in ice.

AMBER: When the resin (sap) from certain evergreen trees hardens, it forms a hard substance called amber. Flies and other insects are sometimes trapped in the sticky resin that flows from trees. When the resin hardens, the insects are preserved perfectly.

TAR PITS: These are large pools of tar. Animals could get trapped in the sticky tar when they went to drink the water that often covered the pits. Other animals came to feed on these animals and then also became trapped.

TRACE FOSSILS: These fossils reveal much about an animal’s appearance without showing any part of the animal. They are marks or evidence of animal activities, such as tracks, burrows, wormholes, etc.

The fossil record● Sedimentary rock: rock

formed from sand and mud that once settled on the bottom of seas, lakes, and marshes

Methods for Dating Fossils:

● RELATIVE DATING: used to establish the geologic time scale; sequence of species

● ABSOLUTE DATING: radiometric dating; determine exact age using half-lives of radioactive isotopes

Where would you expect to

find older fossils and where

are the younger fossils?

Why?

Relative Dating:

● What is an INDEX FOSSIL?

fossil used to help determine the relative age of the fossils around it

must be easily recognized and must have existed for a short period BUT over wide geographical area.

Radiometric Dating:

● Calculating the ABSOLUTE age of fossils

based on the amount of remaining

radioactive isotopes it contains.

Isotope = atom of an element that has a

number of neutrons different from that of

other atoms of the same element

Radiometric Dating:

● Certain naturally occurring elements / isotopes are radioactive, and they decay (break down) at predictable rates

● An isotope (the “parent”) loses particles from its nucleus to form a isotope of the new element (the “daughter”)

● The rate of decay is expressed in a “half-life”

0

50

100

150

200

250

0 5 10 15 20

Amount

Tim

e

Parent

Daughter

Accumulating

“daughter”

isotope

Fra

cti

on

of

pare

nt

iso

top

e r

em

ain

ing

Remaining

“parent” isotope

Time (half-lives)

1 2 3 4

12

14

18

116

Figure 25.5

Half life= the amount of time it

takes for ½ of a radioactive

element to decay.

To determine the age of a fossil:

1) compare the amount of the “parent” isotope to the amount of the “daughter” element

2) knowing the half-life, do the math to calculate the age!

Parent Isotope Daughter Half-Life

Uranium-238 Lead-206 4.5 billion years

Uranium-235 Lead-207 704 million years

Thorium-232 Lead-208 14.0 billion years

Rubidium-87 Strontium-87 48.8 billion years

Potassium-40 Argon-40 1.25 billion years

Samarium-147 Neodymium-143 106 billion years

Radioactive Dating:

Example: Carbon 14

● Used to date material that was once alive

● C-14 is in all plants and animals

(C-12 is too, but it does NOT decay!)

● When an organism dies, the amount of C-14

decreases because it is being converted back

to N-14 by radioactive decay

Example: Carbon 14

● By measuring the amount of C-14 compared to N-14, the time of death can be calculated

● C-14 has a half life of 5,730 years

● Since the half life is considered short, it can only date organisms that have died within the past 70,000 years

Radioactive Decay of Potassium-40

0

20

40

60

80

100

120

140

160

180

200

220

0 0.5 1 1.5 2 2.5 3 3.5 4 4.5 5 5.5 6 6.5 7 7.5 8 8.5 9 9.5 10 10.5 11

Time (billions of years)

Am

ou

nt

of

Po

tas

siu

m-4

0 (

g)

What is the half-life of Potassium-40?

How many half-lives will it take for Potassium-40 to decay to 50 g?

How long will it take for Potassium-40 to decay to 50 g?

Radioactive Decay of Potassium-40

0

20

40

60

80

100

120

140

160

180

200

220

0 0.5 1 1.5 2 2.5 3 3.5 4 4.5 5 5.5 6 6.5 7 7.5 8 8.5 9 9.5 10 10.5 11

Time (billions of years)

Am

ou

nt

of

Po

tas

siu

m-4

0 (

g)

What is the half-life of Potassium-40? 1.2 billion years

How many half-lives will it take for Potassium-40 to decay to 50 g?

2 half-lives

How long will it take for Potassium-40 to decay to 50g? 2.6 billion yrs.

How is the decay rate of a

radioactive substance expressed?

Equation: A = Ao x (1/2)n

A = amount remaining

Ao = initial amount

n = # of half-lives

(**to find n, calculate t/T, where t = time, and T =

half-life, in the same time units as t), so you can

rewrite the above equation as:

A = Ao x (1/2)t/T

½ Life Example #1:

● Nitrogen-13 decays to carbon-13 with t1/2

= 10 minutes. Assume a starting mass of 2.00 g of N-13.

A) How long is three half-lives?

B) How many grams of the isotope will still be present at the end of three half-lives?

½ Life Example #1:

● Nitrogen-13 decays to carbon-13 with t1/2

= 10 minutes. Assume a starting mass of 2.00 g of N-13.

A) How long is three half-lives?

(3 half-lives) x (10 min. / h.l.) =

30 minutes

½ Life Example #1:

● Nitrogen-13 decays to carbon-13 with t1/2

= 10 minutes. Assume a starting mass of

2.00 g of N-13.

B) How many grams of the isotope will still

be present at the end of three half-lives?

2.00 g x ½ x ½ x ½ = 0.25 g

½ Life Example #1:

● Nitrogen-13 decays to carbon-13 with t1/2

= 10 minutes. Assume a starting mass of 2.00 g of N-13.

B) How many grams of the isotope will still be present at the end of three half-lives?

A = Ao x (1/2)n

A = (2.00 g) x (1/2)3

A = 0.25 g

½ Life Example #2:

● Mn-56 has a half-life of 2.6 hr. What is the

mass of Mn-56 in a 1.0 mg sample of the

isotope at the end of 10.4 hr?

½ Life Example #2:

● Mn-56 has a half-life of 2.6 hr. What is the

mass of Mn-56 in a 1.0 mg sample of the

isotope at the end of 10.4 hr?

A = ? n = t / T = 10.4 hr / 2.6 hr

A0 = 1.0 mg n = 4 half-lives

A = (1.0 mg) x (1/2)4 = 0.0625 mg

½ Life Example #3:

● Strontium-90 has a half-life of 29 years.

What is the mass of strontium-90 in a 5.0

g sample of the isotope at the end of 87

years?

½ Life Example #3:

● Strontium-90 has a half-life of 29 years.

What is the mass of strontium-90 in a 5.0

g sample of the isotope at the end of 87

years?

A = ? n = t / T = 87 yrs / 29 yrs

A0 = 5.0 g n = 3 half-lives

A = (5.0 g) x (1/2)3

A = 0.625 g

*The history of living organisms and the

history of Earth are inextricably linked:

● Formation and subsequent breakup of

Pangaea affected biotic diversity

BIOGEOGRAPHY: the study of the past and

present distribution of species

● Formation of Pangaea - 250

m.y.a.

(Permian extinction)

● Break-up of Pangaea – 180

m.y.a.

(led to extreme cases of

geographic isolation!)

EX: Australian marsupials!

Apparent

continental

drift results

from PLATE

TECTONICS

Plate Tectonics:

● At three points in time, the land masses of Earth have formed a supercontinent: 1.1 billion, 600 million, and 250 million years ago

● According to the theory of plate tectonics, Earth’s crust is composed of plates floating on Earth’s mantle

Crust

Mantle

Outer

core

Inner

core

● Tectonic plates move slowly through the process of continental drift

● Oceanic and continental plates can collide, separate, or slide past each other

● Interactions between plates cause the formation of mountains and islands, and earthquakes

Plate Tectonics:

Juan de Fuca

Plate

North

American

Plate

Caribbean

Plate

Cocos Plate

Pacific

PlateNazca

Plate

South

American

Plate

Eurasian Plate

Philippine

Plate

Indian

Plate

African

Plate

Antarctic

Plate

Australian

Plate

Scotia Plate

Arabian

Plate

Plate Tectonics:

Consequences of Continental

Drift:

● Formation of the supercontinent Pangaea about

250 million years ago had many effects

– A deepening of ocean basins

– A reduction in shallow water habitat

– A colder and drier climate inland

Figure 25.14

65.5

135

251

Pre

se

nt

Ce

no

zo

ic

Eurasia

Africa

South

America

India

Antarctica

Madagascar

Meso

zo

icP

ale

ozo

ic

Millio

ns o

f ye

ars

ag

oLaurasia

● The first photosynthetic organisms released oxygen into the air and altered Earth’s atmosphere

● Members of Homo sapiens have changed the land, water, and air on a scale and at a rate unprecedented for a single species!

(CE)

Figure 2. Sea level is changing.

Observing stations from around the

world report year-to-year changes

in sea level. The reports are

combined to produce a global

average time series. The year 1976

is arbitrarily chosen as zero for

display purpose.

Figure 1. Global warming revealed.

Air temperature measured at

weather stations on continents and

sea temperature measured along

ship tracks on the oceans are

combined to produce a global mean

temperature each year. This 150-

year time series constitutes the

direct, instrumental record of global

warming.

History of Life on Earth:● Life on Earth originated between 3.5 and 4.0 billion

years ago

● Because of the relatively simple structure of prokaryotes, it is assumed that the earliest organisms were prokaryotes

*this is supported by

fossil evidence

(spherical & filamentous

prokaryotes recovered

from 3.5 billion year

old stromatolites in

Australia and Africa)

The First Single-Celled

Organisms

● The oldest known fossils are stromatolites, rocks

formed by the accumulation of sedimentary layers

on bacterial mats

● Stromatolites date back 3.5 billion years ago

● Prokaryotes were Earth’s sole inhabitants from 3.5

to about 2.1 billion years ago

Major Episodes in the History of Life:

● first prokaryotes: 3.5 to 4.0 billion years ago

● photosynthetic bacteria: 2.5-2.7 billion

years ago

Photosynthesis and the

Oxygen Revolution

● Most atmospheric oxygen (O2) is of biological

origin

● This “oxygen revolution” from 2.7 to 2.3 billion

years ago caused the extinction of many

prokaryotic groups

● Some groups survived and adapted using cellular

respiration to harvest energy

Figure 25.8

“Oxygen

revolution”

Time (billions of years ago)

4 3 2 1 0

1,000

100

10

1

0.1

0.01

0.0001

Atm

os

ph

eri

c O

2

(pe

rce

nt

of

pre

se

nt-

da

y le

ve

ls;

log

sc

ale

)

0.001

● first eukaryotes: 2 billion years ago~The oldest

unequivocal remains

of a diversity of

microorganisms

occur in the 2.0 BYO

Gunflint Chert of the

Canadian Shield

~This fauna includes

not only bacteria and

cyanobacteria but

also ammonia

consuming

Kakabekia and some

things that

ressemble green

algae and fungus-

like organisms

The First Eukaryotes

● The oldest fossils of eukaryotic cells date back 2.1 billion years

● Eukaryotic cells have a nuclear envelope, mitochondria, endoplasmic reticulum, and a cytoskeleton

● The endosymbiont theory proposes that mitochondria and plastids (chloroplasts and related organelles) were formerly small prokaryotes living within larger host cells

● An endosymbiont is a cell that lives within a host cell

Endosymbiont Theory:

● The prokaryotic ancestors of mitochondria and

plastids probably gained entry to the host cell as

undigested prey or internal parasites

● In the process of becoming more

interdependent, the host and endosymbionts

would have become a single organism

● Serial endosymbiosis supposes that

mitochondria evolved before plastids through a

sequence of endosymbiotic events

Figure 25.9-1

Plasma membrane

DNA

Cytoplasm

Ancestral

prokaryote

Nuclear envelope

Nucleus Endoplasmic

reticulum

Figure 25.9-2

Plasma membrane

DNA

Cytoplasm

Ancestral

prokaryote

Nuclear envelope

Nucleus Endoplasmic

reticulum

Aerobic heterotrophic

prokaryote

Mitochondrion

Ancestral

heterotrophic eukaryote

Figure 25.9-3

Plasma membrane

DNA

Cytoplasm

Ancestral

prokaryote

Nuclear envelope

Nucleus Endoplasmic

reticulum

Aerobic heterotrophic

prokaryote

Mitochondrion

Ancestral

heterotrophic eukaryote

Photosynthetic

prokaryote

Mitochondrion

Plastid

Ancestral photosynthetic

eukaryote

● Key evidence supporting an endosymbiotic origin

of mitochondria and plastids:

– Inner membranes are similar to plasma

membranes of prokaryotes

– Division is similar in these organelles and some

prokaryotes

– These organelles transcribe and translate their

own DNA

– Their ribosomes are more similar to prokaryotic

than eukaryotic ribosomes

Endosymbiont Theory:

The Origin of Multicellularity

● The evolution of eukaryotic cells allowed for a

greater range of unicellular forms

● A second wave of diversification occurred when

multicellularity evolved and gave rise to algae,

plants, fungi, and animals

● plants evolved from green algae

● fungi and animals arose from different

groups of heterotrophic unicellular

organisms

● first animals (soft-bodied invertebrates): 550-700 million years ago

● first terrestrial colonization by plants and fungi:

475-500 million years ago

plants transformed the landscape and created new opportunities for all forms of life

The Cambrian Explosion

● The Cambrian explosion refers to the sudden appearance of fossils resembling modern animal phyla in the Cambrian period (535 to 525 million years ago)

● A few animal phyla appear even earlier: sponges, cnidarians, and molluscs

● The Cambrian explosion provides the first evidence of predator-prey interactions

Sponges

Cnidarians

Echinoderms

Chordates

Brachiopods

Annelids

Molluscs

Arthropods

Ediacaran Cambrian

PROTEROZOIC PALEOZOIC

Time (millions of years ago)

635 605 575 545 515 485 0

The “Big Five” Mass Extinction

Events

● In each of the five mass extinction events, more

than 50% of Earth’s species became extinct

Permian mass

extinction

Extinction of >90% of species

Macroevolution

& Phylogeny

Cretaceous mass

extinction

Asteroid impacts may

have caused mass

extinction events

Mass extinctions:

● Permian (250 m.y.a.): 90% of marine

animals; Pangaea merges

● Cretaceous (65 m.y.a.): death of

dinosaurs, 50% of marine species; low

angle comet

NORTH

AMERICA

Yucatán

Peninsula

Chicxulub

crater

Consequences of Mass

Extinctions

● Mass extinction can alter ecological communities and the niches available to organisms

● It can take from 5 to 100 million years for diversity to recover following a mass extinction

● The percentage of marine organisms that were predators increased after the Permian and Cretaceous mass extinctions

● Mass extinction can pave the way for adaptive radiations

Pre

dato

r g

en

era

(perc

en

tag

e o

f m

ari

ne g

en

era

) 50

40

30

20

10

0Era

Period

542 488 444 416

E O S D

359 299

C

251

P Tr

200 65.5

J C

Mesozoic

P N

Cenozoic

0

Paleozoic

145 Q

Cretaceous mass

extinction

Permian mass

extinction

Time (millions of years ago)

Adaptive Radiations

● Adaptive radiation is the evolution of diversely adapted species from a common ancestor

● Adaptive radiations may follow

– Mass extinctions

– The evolution of novel characteristics

– The colonization of new regions

Worldwide Adaptive Radiations

● Mammals underwent an adaptive radiation after

the extinction of terrestrial dinosaurs

● The disappearance of dinosaurs (except birds)

allowed for the expansion of mammals in diversity

and size

● Other notable radiations include photosynthetic

prokaryotes, large predators in the Cambrian, land

plants, insects, and tetrapods

Ancestral

mammal

ANCESTRAL

CYNODONT

250 200 150 100 50 0

Time (millions of years ago)

Monotremes

(5 species)

Marsupials

(324 species)

Eutherians

(5,010

species)

Regional Adaptive Radiations

● Adaptive radiations can occur when organisms

colonize new environments with little competition

● The Hawaiian Islands are one of the world’s great

showcases of adaptive radiation

Close North American relative,

the tarweed Carlquistia muirii

KAUAI5.1

million years OAHU

3.7million years

1.3millionyears

MOLOKAI

LANAI MAUI

HAWAII

0.4millionyears

N

Argyroxiphium

sandwicense

Dubautia laxa

Dubautia scabra

Dubautia linearis

Dubautia waialealae

Figure 25.20

Evolution is not goal oriented:

● Evolution is like tinkering — it is a process in

which new forms arise by the slight

modification of existing forms

Evolutionary Novelties

● Most novel biological structures evolve in many stages from previously existing structures

● Complex eyes have evolved from simple photosensitive cells independently many times

● Natural selection can only improve a structure in the context of its current utility

(a) Patch of pigmented cells (b) Eyecup

Pigmented cells

(photoreceptors)Pigmented

cells

Nerve fibersNerve fibers

Epithelium

CorneaCornea

Lens

Retina

Optic nerveOptic nerveOptic

nerve

(c) Pinhole camera-type eye (d) Eye with primitive lens (e) Complex camera lens-type eye

EpitheliumFluid-filled

cavity

Cellular

mass

(lens)

Pigmented

layer

(retina)