Chapter 25 The History of Life on

Earth

Antarctica many millions of years ago

Antarctica now… WOW!!

• Past organisms were very different from today’s.

• The fossil record shows macroevolutionary changes over large time scales including– The origin of photosynthesis– The emergence of terrestrial vertebrates – Long-term impacts of mass extinctions

Prebiotic Chemical Evolution & the Origin of Life

Hypothesis: First cells originated by chemical evolution

- non living materials became organized into molecules; molecules were able to replicate & metabolize.- possible because atmosphere was really different; no

O2, volcanoes, UV, lightning, etc.

Four Main Stages of Cell Emergence:

1. small organic molecules are made abiotically

2. monomers polymers (macromolecules)

3. protocells (droplets of aggregated molecules)

4. Origin of self replicating molecules/ beginning to heredity

Stage 1: Synthesis of Organic Compounds on Early Earth• Earth formed about 4.6 bya• Earth’s early atmosphere likely contained water

vapor and chemicals released by volcanic eruptions (nitrogen, nitrogen oxides, carbon dioxide, methane, ammonia, hydrogen, hydrogen sulfide)

TED Talk: The Line Between Life and Non-life

• A. I. Oparin & J. B. S. Haldane hypothesized that the early atmosphere was a reducing environment (no oxygen)

• Stanley Miller and Harold Urey conducted lab experiments that showed that the abiotic synthesis of organic molecules in a reducing atmosphere is possible

= Primeval Soup Hypothesis

Video: Hydrothermal Vent

OR…Organic compounds were created near hydrothermal vents OR…They rained down from outer space

• What came first, the amino acid or the enzyme?– How would macromolecules

form without enzymes/dehydration synthesis?

• Dilute solutions containing monomers dripped onto hot sand, clay, or rock vaporizes water– “Proteinoids” (proteins

formed abiotically) were made this way

• Maybe waves splashed monomers onto hot lava?

Stage 2: Abiotic Synthesis of Macromolecules

Stage 3: Protocells

• Replication & metabolism are key properties of life

• Protocellss are aggregates of abiotically produced molecules surrounded by a membrane or membrane-like structure

• Exhibit – simple reproduction– metabolism– maintain an internal chemical

environment

(a) Simple reproduction by liposomes (aggregates of lipids)

(b) Simple metabolism Possible to contain enzyme within; catalyze RXNs, give off product

Phosphate

Maltose

Phosphatase

Maltose

Amylase

Starch

Glucose-phosphate

Glucose-phosphate

20 µm

Protocells can behave similarly to a cell (osmotic swelling, membrane potential like nerve cell)

Stage 4: Self-Replicating RNA and the Dawn of Natural Selection

• RNA = probably the first genetic material, then DNA

• Ribozymes can make complementary copies of short stretches of their own sequence or other short pieces of RNA

• Base sequences provide blueprints for amino acid sequence (polypeptides)

• Early protocells with self-replicating, catalytic RNA would have been more effective at using resources (“fitness”) & would have increased in # due to natural selection.

• RNA could have provided template for DNA (more stable, better at replicating)

The stage has now been set for life!

Fig. 25-7

Animals

Colonizationof land

Paleozoic

Meso-

zoic

Humans

Ceno-zoic

Origin of solarsystem andEarth

ProkaryotesProterozoic Archaean

Billions of years ago

1 4

32

Multicellulareukaryotes

Single-celledeukaryotes

Atmosphericoxygen

Fig. 25-4Present

Dimetrodon

Coccosteus cuspidatus

Fossilizedstromatolite

Stromatolites Tappania, aunicellulareukaryote

Dickinsoniacostata

Hallucigenia

Casts ofammonites

Rhomaleosaurus victor, a plesiosaur

10

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27

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55

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65

60

03

, 500

1, 5

0 0

2.5 cm4.5 cm

1 cm

Table 25-1

Table 25-1a

Table 25-1b

Animation: The Animation: The Geologic Record

Fig 25-UN2

Prokaryotes

Billions of year

s ag

o

4

32

1

The First Single-Celled Organisms

• Oldest known fossils are stromatolites– rock-like structures composed

of many layers of bacteria and sediment

– Dated 3.5 billion years ago• Prokaryotes were Earth’s sole

inhabitants from 3.5 to about 2.1 billion years ago

Fig. 25-4i

Stromatolites

3.5 BYA

Fossilized stromatolite

Fig 25-UN3

Atmosphericoxygen

Billions of year

s ag

o4

32

1

Photosynthesis & the Oxygen Revolution• By about 2.7 bya, O2 began accumulating in the atmosphere rusting iron-rich terrestrial rocks– O2 produced by oxygenic photosynthesis reacted with dissolved iron and

precipitated out to form banded iron formations

• “Oxygen revolution” = rapid increase in O2 around 2.2 bya – Posed a challenge for life; some microbes hid out in anaerobic

environments – Provided opportunity to gain energy from light– Allowed organisms to exploit new ecosystems as old ones died, opening

up new niches

• Source of O2 was likely bacteria similar to modern cyanobacteria– Later rapid increase attributed to evolution of eukaryotes

Fig. 25-8

Fig 25-UN4

Single-celledeukaryotes

Billions of year

s ag

o

4

32

1

Ancestral photosyntheticeukaryote

Photosyntheticprokaryote

Mitochondrion

Plastid

Nucleus

Cytoplasm

DNA

Plasma membrane

Endoplasmic reticulum

Nuclear envelope

Ancestralprokaryote

Aerobicheterotrophicprokaryote

Mitochondrion

Ancestralheterotrophiceukaryote

The First Eukaryotes• Oldest fossils of eukaryotes go

back 2.1 bya

• Endosymbiosis – mitochondria & plastids

(chloroplasts & related organelles) were formerly small prokaryotes living within larger host cells

– At first, undigested prey or internal parasites?

– 2 became interdependent; host + endosymbionts became a single organism

Evidence supporting endosymbiosis:– Similarities in inner

membrane structures and functions between chloroplasts/mitochondria and prokaryotes

– Organelle division is similar to prokaryotes

– Organelles transcribe & translate their own DNA

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

Fig. 25-4h

Tappania, a unicellular eukaryote

1.5 BYA

Multicellulareukaryotes

Billions of

year

s ag

o

4

32

1

The Origin of Multicellularity• eukaryotic cells allowed for a

greater range of unicellular forms

• Once multicellularity evolved then… algae, plants, fungi, and animals

• Ancestor appeared rougly 1.5 bya, though oldest fossil is algae dated to 1.2 bya

Ediacaran biota (Proterozoic Eon)– large & more diverse soft-bodied organisms that

lived from 565 to 535 mya after snowball Earth – Thaw opened up niches that allowed for speciation

Fig. 25-4g

Dickinsonia costata 2.5 cm

565 MYA

Fig 25-UN6

Animals

Billions of year

s ag

o

4

32

1

The Cambrian Explosion• sudden appearance of fossils resembling modern

phyla in the Cambrian period (Phanerozoic Eon, 535 to 525 mya)

• first evidence of predator-prey interactions; claws, hard-shells, spikes, etc.

Burgess Shale

Fig. 25-4f

Hallucigenia

1 cm

525 MYA

Fig. 25-4e

Coccosteus cuspidatus

4.5 cm

400 MYA

Fig. 25-10

Sp

on

ge

s

LateProterozoiceon

EarlyPaleozoicera(Cambrianperiod)

Cn

idar

ian

s

An

nel

ids

Bra

ch

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od

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hin

od

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Ch

ord

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Mill

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hro

po

ds

Mo

llus

cs

Fig. 25-11

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

Fig 25-UN7

Colonization of land

Billions of year

s ag

o4

32

1

The Colonization of Land• Fungi, plants, and animals began to

move to land 500 mya• Plants & fungi 420 mya: adaptations

to reproduce on land • Arthropods & tetrapods are the most

widespread and diverse land animals– Tetrapods evolved from lobe-finned

fishes around 365 million years ago– Amphibians, reptiles, then birds and

mammals

Fig 25-UN8

Millions of years ago (mya)

1.2 bya:First multicellular eukaryotes

2.1 bya:First eukaryotes (single-celled)

3.5 billion years ago (bya):First prokaryotes (single-celled)

535–525 mya:Cambrian explosion(great increasein diversity ofanimal forms)

500 mya:Colonizationof land byfungi, plantsand animals

Pre

sen

t

500

2,00

0

1,50

0

1,00

0

3,00

0

2,50

0

3,50

0

4,00

0

Major Influences on Life on Earth• Continental Drift: 3 occasions of formation, then

separation of supercontinents; next one will occur in roughly 250 million years.– Collision and separation of oceanic and terrestrial plates shape

mountains, cause earthquakes– Pangaea (250 mya) caused drastic changes in habitats = evolution!

• Mass extinctions: 5 major ones in Earth’s history– Opens up niches for future species– Usually takes 5-10 million years to return diversity to its pre-extinction

levels

• Adaptive Radiation: Periods of evolutionary change in which groups of organisms form many new species whose adaptations allow them to fill different niches (with little competition)

Adaptive Radiation– Occur after mass extinctions

• Rise of mammals after Cretaceous extinction

– Colonized regions (i.e. new islands)• Hawaiian Islands

How can evolutionary novelties/major changes in form

come about?

• Evolutionary developmental biology, or evo-devo, is the study of the evolution of developmental processes in multicellular organisms

• Genomic information shows that minor differences in gene sequence or regulation can result in major differences in form…think fruit flies with legs instead of antennae

Evo-devo• Changes in rate and timing

(regulation) of developmental genes is called heterochrony – Accelerated growth in bone

structures (finger bones to wings in bats) or slowed growth (reduction in leg bones in whale ancestors)

– Paedomorphosis: fast development of reproductive system compared to other development; leads to maintenance of juvenile features though sexually mature (phenotypic variation)

(a) Differential growth rates in a human

(b) Comparison of chimpanzee and human skull growth

NewbornAge (years)

Adult1552

Chimpanzee fetus Chimpanzee adult

Human fetus Human adult

•allometric growth

Gills

• Changes in spatial pattern of developmental genes (homeotic genes = master regulatory genes)– determine where, when,

and how body segments develop

– Small changes in regulatory sequences of certain genes lead to major changes in body form

More Evo-devoFig. 21-17

Adultfruit fly

Fruit fly embryo(10 hours)

Flychromosome

Mousechromosomes

Mouse embryo(12 days)

Adult mouse

Hox genes of the fruit fly and mouse show the same linear sequence on the chromosomes

Vertebrates (with jaws)with four Hox clusters

Hypothetical earlyvertebrates (jawless)with two Hox clusters

Hypothetical vertebrateancestor (invertebrate)with a single Hox cluster

Second Hox duplication

First Hox duplication

• Change in location of two Hox genes in Crustaceans led to the conversion of swimming appendage to feeding appendage

• Duplications of Hox genes in vertebrates may have influenced the evolution of vertebrates from invertebrates

• Homeobox/Hox genes code for transcription factors that turn on developmental genes in embryos

The expression of 2 Hox genes in snakes suppresses the development of legs…the same genes are expressed in chickens in the area between their limbs

Even more Evo-devo • Changes in genes and

where they are expressed– Differing patterns of Hox

gene expression = variation in segmentation

– Suppression of leg formation in insects vs. crustaceans

– Change in expression, not gene, can cause differences in form

Fig. 25-22

Hox gene 6 Hox gene 7 Hox gene 8

About 400 mya

Drosophila Artemia

Ubx

Ubx gene expressed in Abdomen – supressing leg formation

Ubx gene expressedIn main trunk – doesn’t supress legs

insectcrustaceans

Fig. 21-18

ThoraxGenitalsegments

Thorax Abdomen

Abdomen

Brine shrimp Artemia in comparison to grasshopper Hox expression

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

Result:No

Marine stickleback embryo

Close-up of ventral surface

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

Pitx1 is expressed in the ventral spineand mouth regions of developing marinesticklebacks but only in the mouth regionof developing lake stickbacks.

The 283 amino acids of the Pitx1 proteinare identical.

Result:Yes

Lake stickleback embryo

Close-upof mouth

RESULTSFig. 25-23

Fig. 25-24

(a) Patch of pigmented cells

Opticnerve Pigmented

layer (retina)

Pigmented cells(photoreceptors)

Fluid-filled cavity

Epithelium

Epithelium

(c) Pinhole camera-type eye

Optic nerve

Cornea

Retina

Lens

(e) Complex camera-type eye

(d) Eye with primitive lens

Optic nerve

CorneaCellularmass(lens)

(b) Eyecup

Pigmentedcells

Nerve fibers Nerve fibers

Eye Evolution Video

Evolutionary “Novelties” are actually just new forms arising by slight modifications of existing forms