CHAPTER 25 ORIGIN OF LIFE ON EARTH Past organisms were very different from those now alive The fossil record shows macroevolutionary changes over large

CHAPTER 25 ORIGIN OF LIFE ON EARTH

• 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

© 2011 Pearson Education, Inc.

Figure 25.1

So where did the first organic molecules come from?

• Miller-Urey and others demonstrated that organic materials can be produced from inorganic molecules under certain circumstances.

MILLER-UREY EXPERIMENT

•Conditions of Early Atmosphere of Earth Before Life

Products of Experiment

Flask with H, methane,

ammonia, H20 (no free

oxygen! O2)Spark (to represent lightning)

Some amino acids, and cytosine and uracil

http://www.google.com/url?q=http://www.coolhunting.com/style/ashland.php&sa=U&ei=W1-vUv_LJsS6yQGsjoGwBQ&ved=0CCwQ9QEwATgU&usg=AFQjCNHV4dUhFEXEO-6dj9K94WHoZ1p4NQ

Figure 25.2a

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

Under Volcanic Conditions (green) the results were even more notable than the original experiment (orange)

Abiotic Synthesis of Macromolecules

• RNA most likely first genetic (replicating) material.• RNA monomers have been produced spontaneously from

simple molecules• Small organic molecules polymerize when they are

concentrated on hot sand, clay, or rock


“Bubble Theory”

• RNA monomers have been produced spontaneously from simple molecules

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

• This can form a type of “bubble”– AKA : Protocell, vesicle, micelle– Provides an enclosed protected area for reactions to take

place


Protocells

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

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

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


Which came first?

• DNA, RNA, or protein?DNA first- probably not.• DNA first- Probably not. What does DNA

require in order to replicate?• RNA first

• Not only can RNA replicate, but it can sometimes act like an enzyme.

• Protein first• Not only can proteins perform countless functions, but

they can also replicate (prions).

Self-Replicating RNA and the Dawn of Natural Selection

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

catalyze many different reactions– For example, ribozymes can make complementary

copies of short stretches of RNA


Concept 25.2: The fossil record documents the history of life

• The fossil record reveals changes in the history of life on Earth

• Sedimentary rocks are deposited into layers called strata and are the 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

Figure 25.4

Stromatolite fossils are the remains of prokaryotes

How Rocks and Fossils Are Dated

• Sedimentary strata reveal the relative ages of fossils• The absolute ages of fossils can be determined by

radiometric dating• A “parent” isotope decays to a “daughter” isotope at a

constant rate• Each isotope has a known half-life, the time required

for half the parent isotope to decay


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

Figure 25.5

• The geologic record is divided into the Archaean, the Proterozoic, and the Phanerozoic eons

• The Phanerozoic encompasses multicellular eukaryotic life

• The Phanerozoic is divided into three eras: the Paleozoic, Mesozoic, and Cenozoic

Concept 25.3: Key events in life’s history include the origins of single-celled and multicelled organisms and the colonization

of land


Table 25.1

Geologic Time Scale Page 515

Cambrian Explosion

• Concurrent events that may have lead to an overwhelming increase in the number of species– Hox Genes– Predator / Prey Relationships (defined by

cephalization- having a head)– Sequestration of carbon dioxide into algae and

fossilization (what is happening to that carbon today?)


Figure 25.10

Sponges

Cnidarians

Echinoderms

Chordates

Brachiopods

Annelids

Molluscs

Arthropods

Ediacaran CambrianPROTEROZOIC PALEOZOIC

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

EXTINCTION

• Major boundaries between geological divisions correspond to extinction events in the fossil record


Mass Extinctions

• The 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, the rate of extinction has increased dramatically and caused a mass extinction

• Mass extinction is the result of disruptive global environmental changes


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

Figure 25.15

• A number of factors might have contributed to these extinctions

– Intense volcanism in what is now Siberia– Global warming resulting from the emission of large

amounts of CO2 from the volcanoes– Reduced temperature gradient from equator to poles– Oceanic anoxia (low oxygen) from reduced mixing of

ocean waters


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

• Extinction rates tend to increase when global temperatures increase

• Data suggest that a sixth, human-caused mass extinction is likely to occur unless dramatic action is taken (by 2050)– Did you have any plans for the second half of this

century?© 2011 Pearson Education, Inc.

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


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

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

• Natural selection can only improve a structure in the 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)

A; Limpit (Patella)B: Some mollusksC: NautilusD: Marine snailE: Squid (Similar to Mammal)

Figure 25.27

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

Page 531

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CHAPTER 25 ORIGIN OF LIFE ON EARTH Past organisms were very different from those now alive The fossil record shows macroevolutionary changes over large