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2007-2008
Life’s Origin & Early Evolution
(Ch. 20)
“…sparked by just the right combination of physical events & chemical processes…”
Bacteria Archae- bacteria
Animalia Fungi Protista Plantae
4500
4000
3500
3000
2500
2000
500
1500
0
1000
Formation of earth
Molten-hot surface of earth becomes cooler
Oldest definite fossils of prokaryotes
Appearance of oxygen in atmosphere
Oldest definite fossils of eukaryotes
First multicellular organisms
Appearance of animals and land plants
Colonization of land by animals Paleozoic
Mesozoic
Cenozoic
Millio
ns
of
ye
ars
ag
o
AR
CH
EA
N
PR
EC
AM
BR
IAN
PR
OT
ER
OZ
OIC
The evolutionary tree of life can be documented with evidence. The Origin of Life on Earth is another story…
What is Life?
• First we have to define LIFE… – organized as cells
– respond to stimuli
– regulate internal processes • homeostasis
– use energy to grow • metabolism
– develop • change & mature within lifetime
– reproduce • heredity
– DNA / RNA
• adaptation & evolution
Life comes from Non-Life
• Where is the line between living and non-living?
• “A vehicle (organism) built by design information (DNA) for the purpose of replicating that information”
• Good– but too inclusive??? (computer viruses, etc.)
• Terrestrial life is cellular (sorry viruses).
No worries— Alive or Not,
I exist!!!
The Origin of Life is a Hypothesis
• Special Creation – Life was created by a supernatural or
divine force
– not testable
• Extraterrestrial Origin – The original source of organic (carbon)
materials was comets & meteorites striking early Earth
– testable
• Spontaneous Abiotic Origin – Life evolved spontaneously from
inorganic molecules
– testable
The Big Bang Model
• The universe began in an instant, with a big bang about 13 to 15 billion years ago
• The universe is still expanding
• Earth formed from dust and debris orbiting the sun, about 4.6 billion years ago
Conditions on Early Earth
• Came from gas released by volcanoes
• Reducing atmosphere – water vapor (H2O), CO2, N2, NOx, H2, NH3, CH4,
H2S
– lots of available H & its electron
– no free oxygen
• Energy source – lightning, UV radiation,
volcanic
low O2 =
organic molecules
do not break down
as quickly
What’s missing from that
atmosphere?
Early Earth
5 Steps to go from Non-Life to Life
1. Formation (or presence) of biological molecules
2. Isolation of biological molecules from surroundings (aka “cells”)
3. Development of metabolism
4. Development of information molecules
5. Reproduction
Origin of Organic Molecules
• Abiotic synthesis – 1920
Oparin & Haldane propose reducing atmosphere hypothesis
– 1953 Miller & Urey test hypothesis • formed organic
compounds – amino acids
– adenine
Stanley Miller
• Produced – amino acids
– Hydrocarbons
– nitrogen bases
– other organics
University of Chicago
It’s ALIVE!
Possible Origins of Complex Organic Compounds
• Clay templates on tidal flats; iron-sulfide rocks at hydrothermal vents on the deep ocean floor
Origin of Cells (Protobionts)
• Bubbles separate inside from outside metabolism & reproduction
Bubbles… Tiny bubbles…
Origin of Genetics
• An RNA-based system of inheritance may have preceded DNA-based systems
• RNA world – A time when RNA both stored genetic information
and functioned like an enzyme in synthesis
• Ribozymes – Synthetic, self-replicating RNAs
A ribozyme capable of replicating RNA
Ribozyme
(RNA molecule)
Template
Nucleotides
Complementary RNA copy
3
5 5
Key Events in Origin of Life
• Key events in evolutionary history of life on Earth – life originated 3.5–
4.0 bya
– “Heterotroph Hypothesis”: cells eating other cells for ~700 million years.
Prokaryotes
• Prokaryotes dominated life on Earth from 3.5–2.0 bya
3.5 billion year old
fossil of bacteria
chains of one-celled cyanobacteria
modern bacteria
Stromatolites
Fossilized mats of prokaryotes resemble modern microbial colonies (cyanobacteria)
Lynn Margulis
Oxygen Atmosphere
• Oxygen begins to accumulate 2.7 bya – reducing oxidizing atmosphere
• evidence in banded iron in rocks = rusting
• makes aerobic respiration possible
– photosynthetic bacteria (blue-green algae)
The Rise of Eukaryotes
• Oldest eukaryotic fossils: 2.1 billion years
Origins of Nucleus, ER, and Golgi Body
• The nucleus and ER may have arisen through modification of infoldings of plasma membrane
Evolution of Mitochondria and Chloroplasts
• Mitochondria and chloroplasts descended from bacteria that were prey or parasites of early eukaryotic cells
• Endosymbiosis – The symbiont lives out its life inside a host
– Eventually, host and symbiont become incapable of living independently
internal membrane
system aerobic bacterium mitochondrion
Endosymbiosis
Evidence of Endosymbiosis
• Evidence – structural
• mitochondria & chloroplasts resemble bacterial structure
– genetic • mitochondria & chloroplasts
have their own circular DNA, like bacteria
– functional • mitochondria & chloroplasts
move freely within the cell • mitochondria & chloroplasts
reproduce independently from the cell
Theory of Endosymbiosis
Lynn Margulis
Cambrian Explosion
• Diversification of Animals – within 10–20 million years most of the major phyla of animals
appear in fossil record
543 mya
Ca
mb
ria
n
Pro
tero
zo
ic e
on
Ord
ovic
ian
Silu
ria
n
De
vo
nia
n
Ca
rbo
nifero
us
Pe
rmia
n
Tria
ssic
Ju
rassic
Cre
tace
ous
Pa
leo
ge
ne
Ne
og
en
e
Nu
mb
er o
f fam
ilies ( )
Number of
taxonomic
families Extinction rate
Cretaceous
mass extinction
Permian mass
extinction
Millions of years ago E
xtin
ctio
n r
ate
( )
Paleozoic Mesozoic
0
20
60
40
80
100 600 500 400 300 200 100 0
2,500
1,500
1,000
500
0
2,000
Ceno-
zoic
Diversity of life & periods of mass extinction
Cambrian
explosion
Cretaceous Extinction
• The Chicxulub impact crater in the Caribbean Sea near the Yucatan Peninsula of Mexico indicates an asteroid or comet struck the earth and changed conditions 65 million years ago
Early Mammal Evolution
• 125 mya mammals began to radiate out & fill niches
3 Domain system • Domains = “Super” Kingdoms
– Bacteria – Archaea
• extremophiles = live in extreme environments – methanogens – halogens – thermophiles
– Eukarya • eukaryotes
–protists –fungi –plants –animals
Kingdom
Protist
Kingdom
Fungi
Kingdom
Plant
Kingdom
Animal
Kingdom
Archaebacteria
Kingdom
Bacteria