z chapter 25 ~ early earth and the origin of life

Chapter 25 ~ Early Earth and The Origin of Life

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

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”)

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.

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

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)

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

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”)

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

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

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

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

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

Early protocells with self-replicating, catalytic RNA Would have been more effective at

using resources and would have increased in number through natural selection

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

Geologic Time Scale

http://www.ucmp.berkeley.edu/education/explorations/tours/geotime/gtiframe.html

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

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

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

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

• 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.

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

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.

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

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

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

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

In the process of becoming more interdependent The host and endosymbionts would

have become a single organism

The evidence supporting an endosymbiotic origin of mitochondria and chloroplasts includes Similarities in inner membrane

structures and functions Both have their own circular DNA

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

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

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.

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)

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

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

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

Molecular evidence Suggests that many animal phyla

originated and began to diverge much earlier, between 1 billion and 700 million years ago

Colonization of Land by Plants, Fungi, and Animals

Plants, fungi, and animals Colonized land about 500 million years

ago

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

Table 26.1

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

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

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

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

• 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

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)

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

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

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

• 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

• 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

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

• 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)

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.