Chapter 22

The Origin and History of LifeEarly Evolution

2

• The universe began with the Big Bang about 13.7 bya (billion years ago)

• Our solar system began about 4.6 bya• The Earth is 4.55 billion years old• By 4 bya the Earth had cooled enough for

outer layers to solidify and oceans to form• By 4-3.5 bya life emerged

bya

3

Proposed Stages of Life’s Origins4. Polymers enclosed in

membranes acquired cellular properties

1. Nucleotides and amino acids were produced prior to the existence of cells

2. Nucleotides polymerized to form DNA and RNA, amino acids polymerized to form proteins

3. Polymers became enclosed in membranes protobionts

4

Stage 1: Origin of organic molecules

• Conditions on primitive Earth may have been more conducive to spontaneous formation of organic molecules

• Prebiotic or abiotic synthesis– Little free oxygen gas– Dilute solution called “prebiotic soup”

• Several hypotheses on where and how organic molecules originated

5

ElectrodesElectrical discharge

GasesH2OH2CH4NH3

To vacuum Cold water

Condenser

TrapBoiling water

Sample containingorganic moleculessuch as amino acids

Precipitatingdroplets

Miller-Urey experiment

Products included amino acids, purines, pyrimidines

Reducing atmosphere thought to be like early earth

6

• Reducing atmosphere hypothesis– Based on geological data– Atmosphere rich in water vapor, H2, CH4, NH3

(and little if any O2)– Miller and Urey Chamber simulates atmosphere

and bolts of lightning• Formed precursors, amino acids, sugars and

nitrogenous bases• First attempt to apply scientific experiments to

understand origin of life– Since 1950s, ideas about early Earth

atmosphere have changed, may have been a neutral environment• Still similar results for abiotic synthesis

7

• Extraterrestrial hypothesis– Meteorites brought organic carbon to Earth

• Including amino acids and nucleic acid bases– Opponents argue that most of this would be destroyed

in the intense heating and collision

•Deep-sea vent hypothesis–Biologically important molecules may have been formed in the temperature gradient between extremely hot vent water and cold ocean water–Supported by experiments–Complex biological communities found here that derive energy from chemicals in the vent (not the sun)

8

Polymerization

Reaction: Condensation

or Dehydration Synthesis

Monomers are joined to form long chains. Sugars form carbohydrates, amino acids form proteins,

and nucleotides form nucleic acids.

9

Stage 2: Organic polymers

• Experimentally, prebiotic synthesis of polymers is usually not possible in aqueous solutions– Hydrolysis competes with polymerization

• Experiments have shown formation of nucleic acid polymers and polypeptides (proteins) on clay surfaces

10

Stage 3: Formation of boundaries

• Protobionts are cell-like collections of polymers– 4 characteristics

1. Boundary separated external environment from internal contents

2. Polymers inside the protobiont contained information

3. Polymers inside the protobiont had enzymatic function

4. Protobionts capable of self-replication

11

Types of Protobionts

• Coacervates– Droplets that form

spontaneously from the association of charged polymers

– Enzymes trapped inside can perform primitive metabolic functions

• Liposomes– Vesicles surrounded by a

lipid layer– Clay can catalyze

formation of liposomes that grow and divide

– Can enclose RNA11

57 µm

200 nm

12

Stage 4: RNA world

• Most scientists favor RNA as the first macromolecule of protobionts

• Important RNA functions1. Ability to store information2. Capacity for self-replication3. Enzymatic function – ribozymes

• DNA and proteins do not have all 3 functions

13

• Information storage– DNA would have relieved RNA of

informational role and allowed RNA to do other functions

– DNA is less likely to suffer mutations

• Metabolism and other cellular functions– Proteins have a greater catalytic potential and

efficiency– Proteins can fulfill other functions such as

transport and stabilizing cell structure

Advantages of DNA/RNA/protein world

14

How did metabolism develop?

• Reactions occurred by chance in protobionts

• Useful reactions could be retained by selection for successful protobionts

• Metabolic pathways evolved backward• Use of ATP and breakdown of glucose by

glycolysis represent early pathways now shared by all living organisms

15

From Protobionts to Living Cells

• In contrast to protobionts, cells contain– specific and reproducible reaction sequences

to maintain metabolism– specific macromolecules to maintain cell

structure– ability to control internal processes– ability to reproduce

• The transition from protobionts to living cells has not been demonstrated in the laboratory

16

1.8144

354 Devonian

Silurian

Ordovician

Cambrian

4,550

3,800

3,400

3,000

2,500

1,600

543

900

248

443

0

Permian

Triassic

Jurassic

Cretaceous

Quaternary

Tertiary

CarboniferousH

ad

ean

Arc

ha

ean

Pro

tero

zoic

PR

EC

AM

BR

IAN

Ph

ane

rozo

ic

MYA

Late

Middle

Early

Late

Middle

Early

• Geological time scale– From 4.55 bya to

present• 4 eons

– Hadean, Archaean, Proterozoic, Phanerozoic

– 1st three are called Precambrian

• Each eon is further divided into eras

History of life on Earth

Eons

17

Earth’s History Condensed Into a One Year Timeline

Earth’s scientists estimate the age of the earth to be ~4.6 billion years old. To make this time span a bit more tangible, I'll be marking events in earth’s history on a one year timeline using these scales:

One month represents ~375 million yearsOne day represents ~12.3 million years

Assuming that the earth formed on January 1st….

18

Prokaryotic cells arose during Archaeon Eon

• Archaeon Eon- when diverse microbial life flourished in primordial oceans

• First cells were prokaryotic– Includes Bacteria and

Archaea– Organisms were anaerobic

due to scarcity of free oxygen– First cells were heterotrophs

• Prokaryotic autotrophs evolved as supply of organic molecules

dwindled

19

Stromatolites• First known fossils from 3.5 bya• Autotrophic cyanobacteria were

preserved while heterotrophic ancestors were not– Stromatolite formation involves

layers of calcium carbonate

• Cyanobacteria produce O2 as a by-product of photosynthesis

• Release of O2 spelled doom for many prokaryotic groups that were anaerobic

• Allowed the evolution of aerobic species

March 18th in the evolutionary year

20

Photosynthesis and the Oxygen Revolution

• Oxygen began accumulating in the atmosphere about 2.7 billion years ago.

• Banded iron formations are evidence of the age of oxygenic photosynthesis – approximately 2 bya in photo

21

• During the Proterozoic Eon evidence of fossils of eukaryotic cells appears ~2.1 bya– Note the presence of

a nucleus– Both bacteria and archaea

contributed substantially to nuclear genome

• Endosymbiosis hypothesis– mitochondria and plastids (chloroplast precursors)

were formerly small prokaryotes living within larger host cells

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

The First Eukaryotes July 10th in the evolutionary year

22

1.5 BYA 1.0 BYA2 BYA2.5 BYA3 BYA3.5 BYA

Aerobicprokaryote

Photosyntheticprokaryote

Infolding ofplasmamembrane

Endosymbioticorigin of

mitochondria Endosymbioticorigin of

chloroplastsOrigin ofProkaryotes

Origin ofEukaryotes

Endosymbiosis Hypothesis

"Serial endosymbiosis"

23

• Key evidence supporting an endosymbiotic origin of mitochondria and plastids:– Similarities in inner membrane structures and

functions

– Division is similar in these organelles and some prokaryotes

– These organelles transcribe their own DNA into RNA and produce proteins from this RNA

– Their ribosomes are more similar to prokaryotic ribosomes than to eukaryotic ribosomes

24

• Also during the Proterzoic Eon a wave of diversification occurred when multicellularity evolved at ~1.5 bya

• Comparisons of DNA sequences give evidence that multicellular ancestors gave rise to algae, plants, fungi, and animals

The Origin of Multicellularity

unicellular alga 8 identical cells 64+ cells, two types 1,000+ cells, 2 types

August 30th in the evolutionary year

25

Phanerzoic Eon

• Proliferation of multicellular eukaryotic life was extensive (from 543 mya to today)

• Includes the Cambrian explosion – The Cambrian explosion refers to the

sudden appearance of fossils resembling modern phyla in the Cambrian period(533 to 525 mya)

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

November 17th in the evolutionary year

26

• Arthropods are the most abundant land animals• Tetrapods arrived ~365 mya

– Our species arrived ~170,000 years ago

The Colonization of Land ~500 mya

• Adaptations developed for organisms to live on land

–Plants produced waterproof coating and a vascular system for internal transport –Fungi followed plants

November 20th in the evolutionary year

December 1st in the evolutionary year

Last 23 minutes of December 31st!

27

Relative Dating of rock layers and fossils/environments: Which rocks and fossils

are the oldest? Why?

A

B

C

DEF

28

Distribution of Fossils

• The lowest stratum, or layer, in a cross section of Earth is oldest, while the top stratum is the most recent.

• Fossils found within a single stratum are of the same approximate age.

• Relative age of a fossil says that a given fossil is younger or older than another based on what stratum it is found

• Absolute age could be estimated from radioisotope dating

29

Radioisotope dating

• Fossils can be dated using elemental isotopes in accompanying rock

• Half-life – length of time required for exactly one-half of original isotope to decay

• Measure amount of a given isotope as well as the amount of the decay product

• As paleontologists are unlikely to find the first member of a species, expect fossil record to underestimate actual date species came into existence

30

31

How Rocks and Fossils Are Dated

• After every half-life, the amount of parent material decreases by one-half.

• C-14 has a ½ life of ~5,730 years

32

Major environmental changes• Climate/temperature• Atmosphere• Land masses• Continental drift• Flood• Glaciation• Volcanic eruptions• Meteoric impacts

These environmental changes • Can allow for new types of

organisms• Responsible for many

extinctions

33

Five Mass Extinctions so far

34

Is a Sixth Mass Extinction Under Way?

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

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

35

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

• Mass extinction can pave the way for adaptive radiations

36

Adaptive radiation• An organism’s movement into a variety of

different environments or exploitation of a variety of different food sources leads to adaptive radiation.

• Adaptive radiation produces a wide array of descendant species from one type of ancestor.

• The mass extinction of dinosaurs gave way to adaptive radiation of mammals 65 million years ago.

• Hawaii is an excellent laboratory to study adaptive radiation.

37

KAUAI5.1

millionyears OAHU

3.7millionyears

HAWAII0.4

millionyears

1.3millionyears

MAUIMOLOKAI

LANAI Argyroxiphium sandwicense

Dubautia linearisDubautia scabra

Dubautia waialealae

Dubautia laxa

N

Descendants of ancestral tarweed that arrived 5 mya from

North America

38

Mammalian adaptive radiation

39

Evolution is not goal oriented

40

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

• Leads to species that are adapted to a specific environment