z chapter 25 ~ early earth and the origin of life
TRANSCRIPT
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Chapter 25 ~ Early Earth and The Origin of Life
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Conditions on early Earth made the origin of life possible
Most biologists now think that it is at least a credible hypothesis That chemical and physical processes on
early Earth produced very simple cells through a sequence of stages
According to one hypothetical scenario There were four main stages in this
process
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The Origin of Living CellsThe 4-stage Origin of
life Hypothesis: 1- Abiotic synthesis of
organic monomers 2- Polymer formation 3- Origin of Self-
replicating moleculesRNA world
4- Molecule packaging (“protocells”)
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How Did Life Begin
Big Bang~ 13.7 billion years ago (bya)4.6 billion years ago the Earth was formed
as a fiery ball of molten rock. H2, N2, CO, CO2 present (no oxygen)
The Earth’s interior (the core) was melting rock and lighter materials floated to the surface (the crust and mantle).
Eventually earth’s surface cooled, and water condensed to make the seas.
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The First Organic Molecules
Miller preformed an experiment (1953) to simulate early Earth conditions.
Proposed gases were combined with water vapor and electricity and amino acids, nitrogen bases, & ATP formed
Organic monomers dripped on hot sand, clay or rock may have assembled into complex organic molecules through condensation
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Proteinoid Microspheres
Instead of forming in the atmosphere The first organic
compounds on Earth may have been synthesized near submerged volcanoes and deep-sea vents
Proteinoids: amino acids arrangements that may have metabolic abilities (Proposed by Sydney Fox)
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Extraterrestrial Sources of Organic Compounds
Some of the organic compounds from which the first life on Earth arose May have come from space
Carbon compounds Have been found in some of the
meteorites that have landed on EarthThe possibility that life is not restricted
to Earth Is becoming more accessible to scientific
testing Video Clip
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The Origin of Living CellsThe 4-stage Origin of
life Hypothesis: 1- Abiotic synthesis of
organic monomers 2- Polymer formation 3- Origin of Self-
replicating moleculesRNA world
4- Molecule packaging (“protocells”)
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Abiotic Synthesis of Polymers
Small organic molecules Polymerize when they are
concentrated on hot sand, clay, or rockProtocells
Are aggregates of abiotically produced molecules surrounded by a membrane or membrane-like structure
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The “RNA World” and the Dawn of Natural Selection
The first genetic material Was probably RNA, not DNA
Article on RNA nucleotide synthesis http://www.eurekalert.org/pub_releases/2008-12/asfb-asf121808.php
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RNA molecules called ribozymes have been found to catalyze many different reactions, including Self-splicing Making
complementary copies of short stretches of their own sequence or other short pieces of RNA
Ribozyme(RNA molecule)
Template
Nucleotides
Complementary RNA copy
3
5 5
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Laboratory experiments demonstrate that protocells Could have formed spontaneously from
abiotically produced organic compoundsFor example, small membrane-
bounded droplets called liposomes (proposed by David Deamer) Can form when lipids or other organic
molecules are added to water
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Liposomes (David Deamer)
20 m
(a)Simple reproduction. This liposome is “giving birth” to smaller liposomes (LM).
(b)Simple metabolism. If enzymes—in this case, phosphorylase and amylase—are included in the solution from which the droplets self-assemble, some liposomes can carry out simple metabolic reactions and export the products.
Glucose-phosphate
Glucose-phosphatePhosphorylase
Starch
Amylase
Maltose
Maltose
Phosphate
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Early protocells with self-replicating, catalytic RNA Would have been more effective at
using resources and would have increased in number through natural selection
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Evidence for early life
The geologic record is divided into Three eons: the Archaean, the
Proterozoic, and the Phanerozoic Many eras and periods
Many of these time periods Mark major changes in the composition
of fossil species
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Geologic Time Scale
http://www.ucmp.berkeley.edu/education/explorations/tours/geotime/gtiframe.html
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The fossil record
Sedimentary rock: rock formed from sand and mud that once settled on the bottom of seas, lakes, and marshes
Dating: 1- Relative~ geologic time
scale; sequence of species 2- Absolute~ radiometric
dating; age using half-lives of radioactive isotopes
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Though sedimentary fossils are the most common
Paleontologists study a wide variety of fossils
(a) Dinosaur bones being excavated from sandstone
(g) Tusks of a 23,000-year-old mammoth, frozen whole in Siberian ice
(e) Boy standing in a 150-million-year-old dinosaur track in Colorado
(d) Casts of ammonites, about 375 million years old
(f) Insects preserved whole in amber
(b) Petrified tree in Arizona, about 190 million years old
(c) Leaf fossil, about 40 million years old
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Radioisotopes: unstable elements that decay into more stable elements over a known period of time.
Half-life: the time it takes for ½ of a radioisotope to decay
Radiometric dating: calculating the age of something using radioisotopes
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The analogy of a clock
Can be used to place major events in the Earth’s history in the context of the geological record
Land plants
Animals
Multicellulareukaryotes
Single-celledeukaryotes
Atmosphericoxygen
Prokaryotes
Origin of solarsystem andEarth
Humans
Ceno-zoicMeso-
zoic
Paleozoic
ArchaeanEon
Billions of years ago
ProterozoicEon
1
2 3
4
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• As prokaryotes evolved, they exploited and changed young Earth
• The oldest known fossils are stromatolites– Rocklike structures
composed of many layers of bacteria and sediment
– Which date back 3.5 billion years ago
Some bacterial mats form rocklike structures called stromatolites, such as these in Shark Bay, Western Australia. The Shark Bay stromatolites began forming about 3,000 years ago. The inse shows a section through a fossilized stromatolite that is about 3.5 billion years old.
Lynn Margulis, of UMASS, and Kenneth Nealson, of the USC, are shown collecting bacterial mats in a Baja California lagoon. The mats are produced by colonies of bacteria that live in environments inhospitable to most other life. A section through a mat (inset) shows layers of sediment that adhere to the sticky bacteria as the bacteria migrate upward.
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The First Prokaryotes
Prokaryotes were Earth’s sole inhabitants From 3.5 to about 2 billion years ago
Electron transport systems of a variety of types Were essential to early life Have: some aspects that possibly precede
life itself (cytochrome molecules)The earliest types of photosynthesis
Did not produce oxygen
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Oxygenic photosynthesis
Probably evolved about 2.7 billion years ago in cyanobacteria
Reddish bands indicated in this sedimentary rock suggest when the presence of iron oxide first occurred on earth.
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When oxygen began to accumulate in the atmosphere about 2.5 billion years ago It posed a challenge for life It provided an opportunity to gain
abundant energy from light It provided organisms an opportunity to
exploit new ecosystems
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Eukaryotic cells arose from symbioses and genetic exchanges between prokaryotes
Among the most fundamental questions in biology Is how complex eukaryotic cells evolved
from much simpler prokaryotic cellsThe oldest fossils of eukaryotic cells
Date back 2.1 billion years
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Endosymbiotic Origin of Mitochondria and Chloroplasts
The theory of endosymbiosis (Lynn Margulis) Proposes that mitochondria and
chloroplasts were formerly small prokaryotes living within larger host cells
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The prokaryotic ancestors of mitochondria and chloroplasts
Probably gained entry to the host cell as undigested prey or internal parasitesCytoplasm DNA
Plasmamembrane
Ancestralprokaryote
Infolding ofplasma membrane
Engulfing of aerobicHeterotrophic prokaryote
Endoplasmicreticulum
Nuclear envelope
Nucleus
Cell with nucleus and endomembrane system
Mitochondrion
Ancestralheterotrophiceukaryote Chloroplast
Mitochondrion
Engulfing of photosynthetic prokaryote in some cells
Ancestral Photosynthetic eukaryote
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In the process of becoming more interdependent The host and endosymbionts would
have become a single organism
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The evidence supporting an endosymbiotic origin of mitochondria and chloroplasts includes Similarities in inner membrane
structures and functions Both have their own circular DNA
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Larger organisms do not appear in the fossil record Until several hundred million years later
Chinese paleontologists recently described 570-million-year-old fossils That are probably animal embryos
150 m 200 m(a) Two-cell stage (b) Later stage
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Early history of life
Life~ 3.5 to 4.0 byaProkaryotes~ 3.5 to 2.0 bya stromatolitesOxygen accumulation~ 2.7 bya
photosynthetic cyanobacteriaEukaryotic life~ 2.1 byaMulticelluar eukaryotes~ 1.2 bya
(endosymbiosis)Animal diversity~ 543 myaLand colonization~ 500 mya
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Prokaryotic Domains
Eubacteria: bacteria that have the same type of lipids in their cell membrane. Many cause disease and decay.
Archaebacteria: bacteria that have unique lipids in the cell membrane. Closely related to the first bacteria that lived on Earth. Evolved into 1st eukaryotic cells.
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Eukaryotes Emerge
Protists: members of the kingdom Protista. 1st eukaryotic kingdom.
First appeared ~2.1 BYAUnicellular body plan is very
successful.Multicellular protists evolved around
1.2 BYA (algae)
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The “Cambrian Explosion”
Most of the major phyla of animals Appear suddenly in the fossil record
that was laid down during the first 20 million years of the Cambrian period
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Cambrian Period: 570 – 505 MYA (Paleozoic Era)
Most large bodied organism types that exist today originated during this time period
Many fossils from this period were found in a place called the Burgess Shale in Canada.
C = An onycophoran, Aysheaia, which is intermediate between segmented worms and arthropods; D and E = Arthropods
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Animal Diversity
Cnidaria (Jellyfish) and Porifera (Sponges) have a longer history than other animals Are somewhat
older, dating from the late Proterozoic
Ediacaran ancestors
EarlyPaleozoicera(Cambrianperiod)
Mill
ions
of y
ears
ago
500
570
LateProterozoiceonS
pong
es
Cni
daria
ns
Ech
inod
erm
s
Cho
rdat
es
Bra
chio
pods
Ann
elid
s
Mol
lusc
s
Art
hrop
ods
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Molecular evidence Suggests that many animal phyla
originated and began to diverge much earlier, between 1 billion and 700 million years ago
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Colonization of Land by Plants, Fungi, and Animals
Plants, fungi, and animals Colonized land about 500 million years
ago
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Ordovician Period: 505 – 435 MYA (Paleozoic Era)
Many animals in the seas.
Trilobites were common until they went extinct 250 MYA
First jawless fishLand colonization
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Table 26.1
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Mass Extinctions
The fossil record chronicles a number of occasions When global
environmental changes were so rapid and disruptive that a majority of species were swept away
Cam
bria
n
Pro
tero
zoic
eon
Ord
ovic
ian
Silu
rian
Dev
onia
n
Car
boni
fero
us
Per
mia
n
Tria
ssic
Jura
ssic
Cre
tace
ous
Pal
eoge
ne
Neo
gene
Num
ber of families ( )
Number oftaxonomic
familiesExtinction rate
Cretaceous mass extinction
Permian mass extinction
Millions of years ago
Ext
inct
ion
rate
(
)
Paleozoic Mesozoic
0
20
60
40
80
100600 500 400 300 200 100 0
2,500
1,500
1,000
500
0
2,000
Ceno-zoic
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Continental Drift
Earth’s continents are not fixed They drift across our planet’s surface
on great plates of crust that float on the hot underlying mantle
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Often, these plates slide along the boundary of other plates Pulling apart or pushing against each
otherNorthAmericanPlate
CaribbeanPlate
Juan de FucaPlate
Cocos Plate
PacificPlate
NazcaPlate
SouthAmericanPlate African
Plate
Scotia Plate AntarcticPlate
ArabianPlate
Eurasian Plate
PhilippinePlate
IndianPlate
AustralianPlate
Continental Drift
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Continental Drift
Many important geological processes Occur at plate boundaries or at weak points
in the plates themselves
Volcanoes andvolcanic islands
TrenchOceanic ridge
Oceanic crust
Seafloor spreading
Subduction zone
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• The formation of the super-continent Pangaea during the late Paleozoic era– And its
breakup during the Mesozoic era explain many biogeographic puzzles
India collided with Eurasia just 10 millionyears ago, forming theHimalayas, the tallestand youngest of Earth’smajor mountainranges. The continentscontinue to drift.
By the end of theMesozoic, Laurasiaand Gondwanaseparated into thepresent-day continents.
By the mid-Mesozoic,Pangaea split intonorthern (Laurasia)and southern(Gondwana)landmasses.
Ce
no
zoic
North AmericaEurasia
AfricaSouthAmerica
Indiar
Antarctica Australia
Laurasia
Me
so
zoic Gondwana
At the end of thePaleozoic, all ofEarth’s landmasseswere joined in thesupercontinentPangaea.
Pangaea
Pa
leo
zoic
251
135
65.5
0
Mill
ion
s o
f ye
ars
ag
o
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Mass Extinctions
Two major mass extinctions, the Permian and the Cretaceous Have received the most attention
The Permian extinction Claimed about 96% of marine animal
species and 8 out of 27 orders of insects Is thought to have been caused by
enormous volcanic eruptions (Pangaea formation may have caused major continental plate rifts)
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The Cretaceous (K-T) extinction
Doomed many marine and terrestrial organisms, most notably the dinosaurs
Is thought to have been caused by the impact of a large meteor (global broiling hypothesis)
NORTHAMERICA
Chicxulubcrater
YucatánPeninsula
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5 Major Mass Extinctions
1. 435 MYA (end of Ordovician): first global ice age as Gondwana is near the South Pole
2. 370 MYA (end of Devonian): major shifts in sea level
3. 240 MYA (end of Permian) largest ME ever. 96% of all species went extinct
4. 205 MYA (end of Triassic): asteroid impact5. 65 MYA (end of Cretaceous) killed
dinosaurs & 2/3 of land animals
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Much remains to be learned about the causes of mass extinctions But it is clear that they provided life
with unparalleled opportunities for adaptive radiations into newly vacated ecological niches
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• Adaptive radiationAdaptive radiation– Is the evolution of diversely adapted Is the evolution of diversely adapted
species from a common ancestor upon species from a common ancestor upon introduction to new environmental introduction to new environmental opportunitiesopportunities
Pisonia seeds cling to the feathers of this black noddy tern which will soon migrate elsewhere
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• The Hawaiian archipelagoThe Hawaiian archipelago– Is one of the world’s great showcases of Is one of the world’s great showcases of
adaptive radiationadaptive radiation
Dubautia laxa
Dubautia waialealae
KAUA'I5.1
millionyears O'AHU
3.7millionyears
LANAI
MOLOKA'I
1.3 million years
MAUI
HAWAI'I0.4
millionyears
Argyroxiphium sandwicense
Dubautia scabraDubautia linearis
N
5 million years ago an ancestral tarweed arrived on Hawaii and quickly diverged into many different silversword species
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Evolution of the Genes That Evolution of the Genes That Control DevelopmentControl Development
• Genes that program developmentGenes that program development– Control the rate, timing, and spatial pattern Control the rate, timing, and spatial pattern
of changes in an organism’s form as it of changes in an organism’s form as it develops into an adultdevelops into an adult
– Ex. 2 species of larkspursEx. 2 species of larkspurs
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• Allometric growthAllometric growth– Is the proportioning that helps give a Is the proportioning that helps give a
body its specific formbody its specific form
Newborn 2 5 15 Adult
(a) Differential growth rates in a human. The arms and legs lengthen more during growth than the head and trunk, as can be seen in this conceptualization of an individual at different ages all rescaled to the same height.
Age (years)
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Different allometric patternsDifferent allometric patterns
• Contribute to the contrasting shapes Contribute to the contrasting shapes of human and chimpanzee skullsof human and chimpanzee skulls
Chimpanzee fetus Chimpanzee adult
Human fetus Human adult
(b) Comparison of chimpanzee and human skull growth. The fetal skulls of humans and chimpanzee are similar in shape. Allometric growth transforms the rounded skull and vertical face of a newborn chimpanzee into the elongated skull and sloping face characteristic of adult apes. The same allometric pattern of growth occurs in humans, but with a less accelerated elongation of the jaw relative to the rest of the skull.