The History of Life on Earth Chapter 25. Overview: Lost Worlds Past organisms were very different from those alive Fossil record shows macroevolutionary.

The History of Life on Earth

Chapter 25

Overview: Lost Worlds

• Past organisms were very different from those alive

• Fossil record shows macroevolutionary changes over large time scales – For example:

– Emergence of terrestrial vertebrates – Impact of mass extinctions– Origin of flight in birds

Where were these fossils found?

Cryolophosaurus skull

500 Mya the waters surroundingAntarctica were warm and fullof tropical invertebrates.

The continent was covered withForests.

These Cryolophosaurus, inhabitedThese forests.

Conditions on early Earth made the origin of life possible

• Chemical and physical processes on early Earth produced simple cells through these stages:

1. Abiotic synthesis of small organic molecules

2. Joining of small molecules into macromolecules

3. Packaging of molecules into protocells

4. Origin of self-replicating molecules

Synthesis of Organic Compounds

• Earth and solar system formed +/- 4.6 Bya

• Bombardment of Earth by rocks and ice, generating heat, vaporized water

• Earth’s early atmosphere contained water vapor and chemicals released by volcanic eruptions (nitrogen, nitrogen oxides, carbon dioxide, methane, ammonia, hydrogen, hydrogen sulfide)

Early Research• 1920s – Oparin and Haldane hypothesized that early atmosphere

was reducing (electron adding)

• 1953 – Miller & Urey showed that abiotic synthesis of organic molecules in reducing atmosphere is possible

• Evidence not yet convincing that early atmosphere was reducing

• 1st organic compounds may have been synthesized near volcanoes or deep-sea vents

• Amino acids have also been found in meteorites

Figure 25.2

Mas

s of

am

ino

acid

s (m

g)

Num

ber o

f am

ino

acid

s

20

10

01953 2008

200

100

01953 2008

Miller & Urey, 1953 and 2008

Amino acid synthesis in a simulated volcaniceruption

Abiotic Synthesis of Macromolecules

• RNA monomers have been produced spontaneously from simple molecules

• Small organic molecules polymerize when concentrated on hot sand, clay, or rock

Protocells

• Replication and metabolism are key properties of life and may have appeared together

• Protocells may have been fluid-filled vesicles with membrane-like structure

• In water, lipids and other organic molecules spontaneously form vesicles with lipid bilayer

• Adding clay can increase rate of vesicle formation• Vesicles exhibit simple reproduction & metabolism

and maintain internal chemical environment

(a) Self-assembly

Time (minutes)

Precursor molecules plusmontmorillonite clay

Precursor molecules onlyRe

lativ

e tu

rbid

ity,

an in

dex

of v

esic

le n

umbe

r

0

20 m

(b) Reproduction (c) Absorption of RNA

Vesicle boundary

1 m

0

0.2

0.4

4020 60

Presence of clayincreases rateof vesicleformation

Can divide ontheir own

This vesiclehas absorbed clay coated with RNA

Self-Replicating RNA & Dawn of Natural Selection

• First genetic material was probably RNA, not DNA• RNA molecules called ribozymes have been found to catalyze

many different reactions– Ribozymes can make complementary copies of short

stretches of RNA• Natural selection has produced self-replicating RNA• RNA molecules that were more stable or replicated more

quickly would have left most descendent RNA molecules• Early genetic material might have formed an “RNA world”

Protocells

• Vesicles with RNA capable of replication would have been protocells

• RNA could have provided template for DNA, a more stable genetic material

The fossil record documents the history of life

• Fossil record reveals changes in history of life on Earth

• Sedimentary rocks deposited into layers called strata; richest source of fossils

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

175200

300

375400

500525

565600

1,500

3,500

270

• Few individuals fossilized; even fewer discovered

• Fossil record is biased in favor of species that– Existed for a long time– Were abundant and widespread– Had hard parts

• Fossil discoveries can be a matter of chance or prediction

– Paleontologists found Tiktaalik (early terrestrial vertebrate) by targeting sedimentary rock from specific time and environment

Animation: The Geologic Record Right-click slide / select “Play”

How Rocks and Fossils Are Dated • Sedimentary strata reveal relative ages of fossils• Absolute ages determined by radiometric dating• “Parent” isotope decays to “daughter” isotope at constant

rate

• Each isotope has a known half-life – time required for half of parent isotope to decay

• Radiocarbon dating of Carbon can be used to date fossils up to 75,000 years old

• For older fossils, some isotopes (ie. Uranium) can be used to date sedimentary rock layers above and below fossil

Accumulating “daughter”

isotope

Frac

tion

of p

aren

t is

otop

e re

mai

ning

Remaining “parent” isotope

Time (half-lives)1 2 3 4

1 2

1 41 8 1 16

Origin of New Groups of Organisms

• Mammals belong to group of animals called tetrapods

• Evolution of unique mammalian features can be traced through gradual changes over time

OTHERTETRA-PODS

Temporal fenestra

Hinge

†Dimetrodon

†Very late (non-mammalian) cynodonts

Mammals

Synapsids

Therapsids

Cynodonts

Reptiles (including dinosaurs and birds)

Key to skull bones

Articular

Quadrate Squamosal

Dentary

Temporal fenestra

HingeHinge

Hinge

Hinges

Temporal fenestra(partial view)

Early cynodont (260 mya)

Very late cynodont (195 mya)

Synapsid (300 mya)

Therapsid (280 mya)

Later cynodont (220 mya)

Figure 25.6

Key events in life’s history

• Geologic record divided into Archaean, Proterozoic, and Phanerozoic eons

• Phanerozoic encompasses multicellular eukaryotic life– Phanerozoic divided into three eras: Paleozoic, Mesozoic,

and Cenozoic– Major boundaries between geological divisions correspond

to extinction events in fossil record

Origin of solar system and Earth

Prokaryotes

Atmospheric oxygen

Archaean

4

3

Billions of years

ago

Origin of solar system and Earth

Prokaryotes

Atmospheric oxygen

Archaean

4

3

Proterozoic

2

Animals

Multicellular eukaryotes

Single-celled eukaryotes

1

Billions of years

ago

Origin of solar system and Earth

Prokaryotes

Atmospheric oxygen

Archaean

4

3

Proterozoic

2

Animals

Multicellular eukaryotes

Single-celled eukaryotes

Colonization of land

Humans

CenozoicMeso-

zoic

Paleozoic

1

Billions of years

ago

First Single-Celled Organisms

• Oldest known fossils are stromatolites, rocks formed by accumulation of sedimentary layers on bacterial mats

• Date back 3.5 Byz

• Prokaryotes were Earth’s sole inhabitants from 3.5 to 2.1 Bya

Prokaryotes

1

2 3

4

i

y arag

oi

o

ll o

s

B

e

sn

f

Photosynthesis & Oxygen Revolution

• Most atmospheric oxygen (O2) is of biological origin

• O2 produced by oxygenic photosynthesis reacted with dissolved iron & precipitated out to form banded formations

• By ~ 2.7 Bya., O2 began accumulating in atmosphere and rusting iron-rich terrestrial rocks

• “Oxygen revolution” from 2.7 to 2.3 Bya caused extinction of many prokaryotic groups

• Some groups survived and adapted using cellular respiration to harvest energy

“Oxygen revolution”

Time (billions of years ago)

4 3 2 1 0

1,000

100

10

1

0.1

0.01

0.0001

Atm

osph

eric

O2

(per

cent

of p

rese

nt-d

ay le

vels

; log

sca

le)

0.001

Atmospheric oxygen

1

2 3

4

i

y ar

agoi

o

ll o

s

B

e

snf

Cyanobacteria

• Early rise in O2 likely caused by ancient cyanobacteria

• Later increase in O2 may have been caused by evolution of eukaryotic cells containing chloroplasts

First Eukaryotes

• Oldest fossils of eukaryotic cells date back 2.1 billion years

• Endosymbiont theory: mitochondria and plastids were formerly small prokaryotes living within larger host cells

• Prokaryotic ancestors of mitochondria and plastids probably gained entry to host cell as undigested prey or internal parasites

• In process of becoming more interdependent, host and endosymbionts became single organism

• Serial endosymbiosis: mitochondria evolved before plastids through sequence of endosymbiotic events

Single-celledeukaryotes

1

2 3

4

i

y ar

agoi

o

ll o

sB

e

sn

f

Plasma membrane

DNA

Cytoplasm

Ancestralprokaryote

Nuclear envelope

Nucleus Endoplasmic reticulum

Serial Endosymbiosis

Plasma membrane

DNA

Cytoplasm

Ancestralprokaryote

Nuclear envelope

Nucleus Endoplasmic reticulum

Aerobic heterotrophicprokaryote

Mitochondrion

Ancestralheterotrophic eukaryote

Plasma membrane

DNA

Cytoplasm

Ancestralprokaryote

Nuclear envelope

Nucleus Endoplasmic reticulum

Aerobic heterotrophicprokaryote

Mitochondrion

Ancestralheterotrophic eukaryote

Photosyntheticprokaryote

Mitochondrion

Plastid

Ancestral photosyntheticeukaryote

Key evidence supporting endosymbiotic origin of mitochondria and plastids:

• Inner membranes similar to plasma membranes of prokaryotes

• Division similar in these organelles and some prokaryotes

• Organelles transcribe and translate their own DNA

• Ribosomes are more similar to prokaryotic than eukaryotic ribosomes

© 2011 Pearson Education, Inc.

Origin of Multicellularity

• Evolution of eukaryotic cells allowed for greater range of unicellular forms

• Second wave of diversification occurred when multicellularity evolved and gave rise to algae, plants, fungi, and animals

• Comparisons of DNA sequences date common ancestor of multicellular eukaryotes to 1.5 Bya

• Oldest known fossils of multicellular eukaryotes are small algae that lived about 1.2 Bya

© 2011 Pearson Education, Inc.

Multicellulareukaryotes

1

2 3

4

i

y ar

agoi

o

ll o

sB

e

sn

f

• “Snowball Earth” hypothesis: periods of extreme glaciation confined life to equatorial region or deep-sea vents 750 - 580 Mya.

• Ediacaran biota = assemblage of larger and more diverse soft-bodied organisms that lived 575 - 535 Mya.

Cambrian Explosion• Sudden appearance of fossils resembling modern animal

phyla in Cambrian period (535 - 525 Mya)• Sponges, cnidarians, molluscs appear even earlier• Cambrian explosion provides 1st evidence of predator-prey

interactions• DNA analyses – many animal phyla diverged before Cambrian

explosion, perhaps 700 Mya. to 1 Bya.• Fossils in China provide evidence of modern animal phyla tens

of millions of years before Cambrian explosion• Chinese fossils suggest that Cambrian explosion lasted 40

million years

Sponges

Cnidarians

Echinoderms

Chordates

Brachiopods

Annelids

Molluscs

Arthropods

Ediacaran CambrianPROTEROZOIC PALEOZOIC

Time (millions of years ago)635 605 575 545 515 485 0

Animals

1

2 3

4

i

y ar

agoi

o

ll o

sB

e

sn

f

150 m (b) Later stage 200 m(a) Two-cell stage

Proterozoic fossils that may be animal embryos (SEM).

The Colonization of Land

• Fungi, plants, & animals began to colonize land about 500 Mya

• Vascular tissue in plants appeared ~ 420 Mya• Plants and fungi today form mutually beneficial

associations and likely colonized land together• Arthropods and tetrapods – most widespread and

diverse land animals• Tetrapods evolved from lobe-finned fishes ~ 365 Mya

Colonization of land

1

2 3

4

i

y ar

agoi

o

llo

sB

e

snf

The rise and fall of groups of organisms reflect differences in speciation and

extinction rates

• History of life on Earth has seen rise and fall of many groups of organisms

• Rise and fall of groups depends on speciation and extinction rates

Plate Tectonics

• At three points in time, land masses of Earth have formed a supercontinent: 1.1 Bya., 600 Mya., & 250 Mya.

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

• Tectonic plates move slowly through process of continental drift

• Oceanic & continental plates can collide, separate, or slide past each other

• Interactions between plates cause formation of mountains and islands, and earthquakes

Crust

Mantle

Outercore

Innercore

Juan de FucaPlate

NorthAmerican Plate

CaribbeanPlate

Cocos Plate

PacificPlate

NazcaPlate

SouthAmericanPlate

Eurasian Plate

Philippine Plate

Indian Plate

African Plate

Antarctic Plate

Australian Plate

Scotia Plate

Arabian Plate

Earth’s major tectonic plates.

Consequences of Continental Drift

• Formation of Pangaea ~ 250 Mya had many effects– Deepening of ocean basins– Reduction in shallow water habitat– Colder and drier climate inland

• Continental drift has many effects on living organisms– Continent’s climate can change as it moves– Separation of masses can lead to allopatric speciation

• Distribution of fossils and living groups reflects historic movement of continents

– Similarity of fossils in South America and Africa consistent with idea that continents were attached

65.5

135

251

Pres

ent

Ceno

zoic

North America

Eurasia

Africa

SouthAmerica

India

Antarctica

Madagascar

Australia

Mes

ozoi

cPa

leoz

oic

Mill

ions

of y

ears

ago

Laurasia

Gondwana

Pangaea

Mass Extinctions

• Fossil record shows that most species that have ever lived are now extinct

• Extinction can be caused by changes to a species’ environment

• At times, rate of extinction has increased dramatically and caused mass extinction– Mass extinction is result of disruptive global environmental

changes

• In each of five mass extinction events, more than 50% of Earth’s species became extinct

25

20

15

10

5

0

542 488 444

EraPeriod

416

E O S D

359 299

C

251

P Tr

200 65.5

J CMesozoic

P NCenozoic

0

0

Q

100

200

300

400

500

600

700

800

900

1,000

1,100To

tal e

xtinc

tion

rate

(fam

ilies

per

mill

ion

year

s):

Num

ber o

f fam

ilies

:

Paleozoic

145

Permian Mass Extinction

• Defines boundary between Paleozoic and Mesozoic eras

• 251 Mya

• Mass extinction occurred in less than 5 million years and caused extinction of about 96% of marine animal species

• Number of factors might have contributed:– Intense volcanism in what is now Siberia

– Global warming resulting from emission of large amounts of CO2 from volcanoes

– Reduced temperature gradient from equator to poles

– Oceanic anoxia from reduced mixing of ocean waters

Cretaceous Mass Extinction

• 65.5 Mya

• Separates Mesozoic from Cenozoic

• Organisms that went extinct include about half of all marine species and many terrestrial plants and animals, including most dinosaurs

• Presence of iridium in sedimentary rocks suggests a meteorite impact about 65 Mya.

• Dust clouds caused by impact would have blocked sunlight and disturbed global climate

• Chicxulub crater off coast of Mexico is evidence of a meteorite that dates to same time

NORTH AMERICA

YucatánPeninsula

Chicxulubcrater

Is a Sixth Mass Extinction Under Way?

• Scientists estimate that current rate of extinction is 100 to 1,000 times typical background rate

• Extinction rates tend to increase when global temperatures increase

• Data suggest that sixth, human-caused mass extinction is likely to occur unless dramatic action is taken

Mass extinctions

Cooler Warmer

Rela

tive

extin

ction

rate

of m

arin

e an

imal

gen

era

3

2

1

0

1

23 2 1 0 1 2 3 4

Relative temperature

Consequences of Mass Extinctions

• Mass extinction can alter ecological communities and niches available to organisms

• It can take from 5 - 100 million years for diversity to recover following mass extinction

• Percentage of marine organisms that were predators increased after Permian & Cretaceous mass extinctions

• Mass extinction can pave way for adaptive radiations

Pred

ator

gen

era

(per

cent

age

of m

arin

e ge

nera

) 50

40

30

20

10

0EraPeriod

542 488 444 416

E O S D

359 299

C

251

P Tr

200 65.5

J C

MesozoicP N

Cenozoic

0

Paleozoic

145 Q

Cretaceous massextinction

Permian massextinction

Time (millions of years ago)

Adaptive Radiations

• Adaptive radiation = evolution of diversely adapted species from common ancestor

• Adaptive radiations may follow– Mass extinctions– Evolution of novel characteristics– Colonization of new regions

Worldwide Adaptive Radiations

• Mammals underwent adaptive radiation after extinction of terrestrial dinosaurs

• Disappearance of dinosaurs (except birds) allowed for expansion of mammals in diversity and size

• Other notable radiations include photosynthetic prokaryotes, large predators in Cambrian, land plants, insects, and tetrapods

Ancestralmammal

ANCESTRALCYNODONT

250 200 150 100 50 0Time (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

• Hawaiian Islands are one of 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

HAWAII0.4

millionyears

N

Argyroxiphium sandwicense

Dubautia laxa

Dubautia scabraDubautia linearis

Dubautia waialealae

Major changes in body form can result from changes in developmental genes

• Studying genetic mechanisms of change can provide insight into large-scale evolutionary change

• Genes that program development control rate, timing, & spatial pattern of changes in organism’s form as it develops into adult

Changes in Rate and Timing

• Heterochrony: evolutionary change in rate or timing of developmental events

• Can have significant impact on body shape

• Contrasting shapes of human & chimpanzee skulls are result of small changes in relative growth rates

© 2011 Pearson Education, Inc.

Animation: Allometric Growth Right-click slide / select “Play”

Chimpanzee infant Chimpanzee adult

Chimpanzee adult

Human adultHuman fetus

Chimpanzee fetus

• Heterochrony can alter timing of reproductive development relative to development of nonreproductive organs

• Paedomorphosis – rate of reproductive development accelerates compared with somatic development

• Sexually mature species may retain body features that were juvenile structures in ancestral species

Figure 25.22

Gills

Changes in Spatial Pattern

• Substantial evolutionary change can result from alterations in genes that control placement and organization of body parts

• Homeotic genes determine such basic features as where wings and legs will develop or how flower parts are arranged

• Hox genes = class of homeotic genes that provide positional information

• If Hox genes are expressed in wrong location, body parts can be produced in wrong location

• In crustaceans, swimming appendage can be produced instead of feeding appendage

Evolution of Development

• Tremendous increase in diversity during Cambrian explosion is puzzling

• Developmental genes may play especially important role

• Changes in developmental genes can result in new morphological forms

Changes in Genes

• New morphological forms likely come from gene duplication events that produce new developmental genes

• Possible mechanism for evolution of six-legged insects from many-legged crustacean ancestor has been demonstrated in lab experiments

• Specific changes in Ubx gene have been identified that can “turn off” leg development

Hox gene 6 Hox gene 7 Hox gene 8

Ubx

About 400 mya

Drosophila Artemia

Changes in Gene Regulation

• Changes in morphology likely result from changes in regulation of developmental genes rather than changes in sequence of developmental genes

– Threespine sticklebacks in lakes have fewer spines than their marine relatives

– Gene sequence remains same, but regulation of gene expression different in the two groups

Threespine stickleback(Gasterosteus aculeatus)

Ventral spines

Test of Hypothesis A:Differences in the codingsequence of the Pitx1 gene?

Marine stickleback embryo

Close-up of mouth

Close-up of ventral surface

Lake stickleback embryo

Test of Hypothesis B:Differences in the regulation of expression of Pitx1?

Result:No

Result:Yes

The 283 amino acids of the Pitx1 protein are identical.

Pitx1 is expressed in the ventral spine and mouth regions of developing marine sticklebacks but only in the mouth region of developing lake sticklebacks.

RESULTS

Evolution is not goal-oriented

• Evolution is like tinkering— new forms arise by slight modification of existing forms

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

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

• Exaptations are structures that evolve in one context but become co-opted for different function

• Natural selection can only improve structures in context of its current utility

Figure 25.26

(a) Patch of pigmented cells (b) Eyecup

Pigmented cells(photoreceptors)

Pigmented cells

Nerve fibersNerve fibers

Epithelium

CorneaCornea

Lens

RetinaOptic nerve

Optic nerveOptic nerve

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

EpitheliumFluid-filled cavity

Cellularmass(lens)

Pigmented layer (retina)

Evolutionary Trends

• Extracting single evolutionary progression from fossil record can be misleading

• Apparent trends should be examined in broader context

• Species selection model suggests that differential speciation success may determine evolutionary trends

• Evolutionary trends do not imply intrinsic drive toward particular phenotype

Holocene

Pleistocene

Pliocene

0

5

10

Anchitherium

Mio

cene

15

20

25

30 Olig

ocen

e

Mill

ions

of y

ears

ago

35

40

50

45

55

Eoce

ne

Equus

Pliohippus

Merychippus

Sino

hipp

us

Meg

ahip

pus

Hyp

ohip

pus

Arch

aeoh

ippu

s

Para

hipp

us

Mio

hipp

us

Mesohippus

Prop

alae

othe

rium

Pach

ynol

ophu

s

Pala

eoth

eriu

m

Hap

lohi

ppus

Epih

ippu

s

Oro

hipp

us

Hyracotherium relatives

Hyracotherium

KeyGrazersBrowsers

Hip

pario

n

Neo

hipp

ario

n

Nan

nipp

us

Calli

ppus

Hip

pidi

on a

nd

clos

e re

lativ

es