Transcript The Origin and Evolution of Life on Earth

Homework #5
Due Tuesday, October 25, 6:00 pm
Exam #2 date
Wednesday, November 2
Exam #3 tentative date
Wednesday, November 30
Extremophiles
• Life that exists under “extreme”
conditions, conditions that until recently
were thought to be inhospitable to life.
Extremophiles
• Volcanic vents:
 Water temperature
reaches 400°C
(750°F), possible
because of the large
pressure
 Black smokers: mixed
with volcanic chemicals
Extremophiles
• Antarctic dry valleys:
 Microbes in small pockets of water in rocks
Extremophiles
• Lithophiles (rock lovers):
 Several kilometers below the surface
 Chemical energy from rocks
 Carbon from CO2 filtering down
Extremophiles
• Endospores (e.g., anthrax)
 Can lay dormant for long periods
 Can survive lack of water, extreme heat and
cold, and poisons
 Some can survive in vacuum
Implications for Extraterrestrial Life
 Sufficient atmospheric oxygen for eukarya has existed for
only ~10% of the Earth’s lifetime – complex life not
possible before this time
 What is the probability that eukarya-like organisms would
develop?
 We are more likely to find extremophiles elsewhere
 Extremophiles may be the norm, not the exception
The Origin and Evolution of
Life on Earth
When Did Life Begin?
• Stromatolites
– Living: colonies of bacteria living in outer layer of sedimentary rocks
– 3.5 Byr old rocks: almost identical layered structure
– Inconclusive evidence: sedimentation layering may mimic stromatolites
• Fossil evidence
–
–
–
–
3.5 Byr old Australian rock shows “cells”
Could this form naturally from minerals?
Younger sites: at least two more (3.2-3.5 byr old)
Older sites: sedimentary rock too altered to be useful
•
13C/12C
ratio
– Normal abundance ratio 1/89
– Living tissue and fossils show
less 13C
– Some rocks older than 3.85
byr show the low 13C
abundance
• Sterilization
– Last sterilization: 3.9-4.2 byr
ago (Late Heavy
Bombardment)
The evidence indicates life formed quickly
after the Earth became habitable.
Within a few
100 million
years,
Perhaps as
short as 100
million years
The Origin and Evolution of
Life on Earth
How Did Life Begin?
How did life get here?
No Life
?
few hundred million years
Life
• Simplest organisms today and those dated 3.5
billion years ago are remarkable advanced
• What are the natural chemical processes that
could have led to life?
• Assumptions
– Life began under chemical conditions of early Earth
– Life did not migrate to Earth
 Certain chemical processes are energetically
favored given specific circumstances, i.e., in the
presence of specific chemical elements, energy
sources & liquid(s)
 Can the presence of specific elements and
energy inevitably lead to the formation of life?
 We know it can lead to the building blocks
Simulated conditions
thought to exist on the
early Earth
 Tested for the
occurrence of chemical
evolution
 Considered to be the
classic experiment on
the origin of life
 After one week, Miller and Urey observed that 10–15%
of the carbon was now in the form of organic compounds.
 Recent re-analysis of Miller's archived solutions found 22
amino acids
• Different mixes of gases used for
Miller-Urey
Experiments
•
•
atmosphere
Different energy sources, like UV
(sunlight)
Results: With the exception of
when oxygen was present, ALL
VARIATIONS PRODUCE AMINO
ACIDS AND COMPLEX ORGANIC
MOLECULES
– Proper mix produces less than
original experiment
– THERE MUST be additional
sources of organic material –
the atmosphere is not
sufficient!
The Origin and Evolution of
Life on Earth
Where Did Life Begin?
Sources of Organic Molecules
• Chemical reactions in atmosphere
– Lab experiments show this is probably insufficient
• Organic material brought by impacts
– Chemical analysis of comets and carbonaceous
chondrites show that they have organic molecules
• Chemical reactions near deep-sea vents
– Heat from undersea volcanoes and vents can fuel
chemical reactions between water and minerals
Early Organic Chemistry
• No atmospheric oxygen
– Helps: Oxygen destroys many organic compounds
– Atmosphere is reducing, not oxidizing
• Miller-Urey experiments
– Can form amino-acid soup from methane (CH4) and ammonia (NH3) with
electric energy (mimicking lightning)
– Current thinking: early atmosphere was dominated by CO2
– Low yield
– Shallow ponds close to surface -> UV sterilization
• Alternative sources of amino acids
– Extraterrestrial: amino acids are abundant in meteorites
– Deep sea vents: abundant chemical energy & protected from
UV
Where Did Life Begin?
Unlikely on land
– Solar UV radiation: protection today by ozone (O3)
– But no atmospheric oxygen in the early Earth
– In water: no problem, UV absorbed effectively
Shallow ponds
– First evidence from Miller-Urey experiment
– Recent evidence: incorrect atmospheric content
Thermophiles
– DNA evidence shows early thermophiles
– Have advantage of more chemical energy
– Deeper sea vents better protected against
bombardment
Chemical reactions near deepsea vents: Considered the most
likely sites where life on Earth
originated
– “Black smokers”
The Origin and Evolution of
Life on Earth
Chemistry to life
Search for Self-Replicating Molecule
• Work backward from organisms that live today
• DNA is double-stranded = complicated
• RNA obvious candidate, simpler than DNA
– Hereditary information
– Can serve as template for replication
– Fewer steps to produce backbone structure
Search for Self-Replicating Molecule
• Problem: RNA and DNA require enzymes to
replicate (chicken and egg problem?)
• In 1980’s determined that RNA might catalyze
their own replication instead of other enzymes
• Conclusion: Early Earth-life probably used RNA
to encode its structure
• Early Earth: short strands of RNA-like molecules
produced spontaneously partially or completely
• RNA-like molecules that could replicate faster
with less errors soon dominated population
• Copying errors introduced mutations, ensuring
the production of many variations of successful
molecules
• Allowed molecular evolution to continue
• RNA-world gave way to DNA-world
– DNA less prone to copying errors
– DNA more flexible hereditary material
– RNA kept some of its original functions
Assembling Complex Organic Molecules
• Organic soup was too dilute to favor the creation
of complex organic molecules
• Lab experiment with possible solution: When
hot sand, clay or rock is placed in dilute organic
solution, complex molecules self-assemble
– Organic molecules stick to surface of clay
– Increases density and likelihood of reactions
– Strands of RNA up to 100 bases have been
spontaneously produced this way
• Other inorganic minerals may have also
had a similar role
• Iron pyrite (fool’s gold)
– Positive charges on surface which allows
organic molecules to adhere
– Formation of pyrite releases energy which
could be used as fuel for chemical reactions
Early Cell-like
Structures
• There are advantages
to enclosing enzymes
with RNA molecules
• Their close proximity
increases the rate of
reactions between
them
• Also, isolates contents
from outside world
• Lab experiments suggest that membrane
structures existed on early Earth
• These form spontaneously when…
– Cool down warm-water solution of amino
acids
– Mix lipids (fats) with water
Nonliving Pre-Cells have Lifelike Behavior
• Grow in size until unstable
then split to form a ‘daughter’
cell
• Selectively allow other types of
molecules to pass in/out of
membrane
• Store energy in the form of
electric voltage
Brief Summary
Synthesis of organic precursor molecules
Origin of self-replicating RNA
Origin of enclosed pre-cells
Origins of true cells with RNA genome
Evolution of modern cells with DNA genome
Migration of Life to Earth?
“Panspermia”
 Proposal: “Seeds of life”
exist everywhere around
the universe
 Life on earth started when
these ‘seeds’ came here,
probably by a meteor.
 It also suggests that these
seeds are taken to other
habitable places in the
universe.
Panspermia argument:
 Against:
– No atmosphere or water in space
– Solar and stellar radiation hazards in space
 For:
– organic material is everywhere, and some bacteria
can withstand large amounts of radiation and go
dormant under low atmospheric conditions
– Fact: amino acids are found in some meteorites
– Question is not “could” but “did” life originate
elsewhere
Panspermia: 2 schools of thought
• School 1: life did not evolve as easily as
imagined on early Earth in timescales
we’ve determined
• School 2: life evolved easily and was
everywhere with suitable conditions
School 1: life did not evolve as easily as
imagined on early Earth in timescales
we’ve determined
• Problem: entire solar system was under heavy
bombardment at the same time - hard to form
life quickly in another location in Solar System
• Other possibility: interstellar migration
• Problem: rock to be ejected out of its own
system, then fall into ours and hit the tiny planet
of Earth - very dificult
School 2: life evolved easily and was
everywhere with suitable conditions
• Earth was not first planet with suitable
conditions
• Migration of life from another planet (say
Mars) dominated before early life on Earth
could
– We’re Martians!!!!
• Martian meteorites
• Fossil evidence of life on Mars or
geochemical structure?
Early Evolution and Rise of O2
• First organisms had simple metabolism
• Atmosphere was O2 free, life must have been
anaerobic
• Probably chemoheterotrophs
– Obtained nutrients from organic material
– Obtained energy from inorganic material
 Modern archaea appear to be close to the root of the tree of
life
 Obtaining energy from chemical reactions involving
hydrogen, sulfur and iron compounds (all abundant on early
Earth)
Early Evolution
• Natural selection probably resulted in rapid
diversification
• Modern DNA has enzymes that reduce the rate
of mutations
• RNA is not so lucky, more likely to have copying
errors
• Higher mutation rate in early evolution than now