Transcript Common Characteristics of Life?

We are now mostly finished with planetary
environments, so..
Before proceeding to life…
A brief visit to a missed topic…
The Origin of the Earth’s Moon
The Earth-Moon
double planet does not fit
well into the nebular theory
planetesimal accretion
predicts both should have
the same chemical
composition.
They don’t - there are
subtle but significant
differences
Moon is composed of less dense material than Earth
The general view is that
the Earth’s moon was
created as a result of the
impact of a large object,
perhaps as large as
Mars, with the Earth very
early in its existence.
The moon was formed
from the debris of this
collision, which included
lower density “mantle”
material from the Earth.
“Life”
 How is life defined?
What is needed for
life?
How hard it is for life
to form?
What environments
are suitable for life?
How is “LIFE” defined?
This is extremely difficult. We can look at
commonalities of what we have defined as living…
 Order - life has structure
How is “LIFE” defined?
This is extremely difficult. We can look at
commonalities of what we have defined as living…
 Order - life has structure
 Reproduction
 Growth & development
 Energy utilization
 Senses & reacts to environment
 Evolutionary adaptation
All six properties of life are important,
but biologists consider evolutionary
adaptation to be the most important.
Evolution: “change with time”
Organisms need to be able to
encode their structural
information in order to
reproduce.
In Earth-based life, this
encoding is accomplished
through DNA.
DNA Replication
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Steps:
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Complete double helix
Strands separate into 2 helices
Free floating bases join open strands
Fill in deoxyribose and a phosphate group
Two identical copies of the DNA in the cell
Cell division: one copy to each daughter cell
Complexity:
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Enzymes: a dozen or more needed
Heredity: ensured by exact copying, but
Errors: occur occasionally -> evolution
Origin of Life: need simpler mechanism (RNA?)
Will Life Elsewhere Use DNA?
 Heredity and evolution are essential
 DNA does the job on Earth today
 RNA may have been the first mechanism
 Who’s to say that the same complex mechanism is
universal?
 Some type of molecule has to provide the
mechanism for heredity and evolution
ERRORS ARE IMPORTANT!
Changes (mutations) in this encoding
will lead to changes in the organism.
Mutations and Evolution
 Causes of mutations:
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Ultraviolet (UV) light
Chemical agents (carcinogens)
Nuclear radiation (mostly natural cosmic rays)
 Effect of mutations:
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Harmless
Fatal
Evolution
If the change produces an organism better suited
to its environment, it is more likely to be passed
on, i.e., the organism changes (evolves).
Natural selection
Artificial selection
What are the necessities of life?
Nutrient source
Energy (sunlight, chemical reactions, internal heat)
Liquid water (or possibly some other liquid)
Hardest to find on
other planets
Common Characteristics of Life?
 Carbon based
 Have a protective membrane
 Need water
 Use energy to maintain internal state
 Can get energy from environment
 Conduct metabolic processes (use stuff, make waste)
 Responds to stimuli
 Grow, reproduce (replicate)
 Evolve and adapt to the environment as a population
Obtaining Energy
Living organisms can obtain energy through
 “eating”, energy & nutrients from other
organisms
 extraction from chemical reactions in the
environment (black smokers - ocean vents)
 extraction from radiative energy (e.g.,
photosynthesis)
Metabolism
Metabolism: chemical reactions within living
organisms. It takes place within cells.
Why in cells? Chemical reactions much faster
than in the open
Collects the raw materials for the chemical
reactions
Provides the energy for the reactions
Provides enzymes to catalyze the reactions
Instructions for enzymes encoded in DNA
 Synthesis reactions = “building up” reactions
Example: Making starch from glucose - requires energy
 Decomposition reactions = “breaking down”
reactions
Example: Breaking glucose into CO2 & H2O - releases
chemical energy
Metabolism and Cells
 Metabolism:
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Four forms of metabolism defined by:
Sources of carbon (direct or indirect)
● Sources of energy (light or chemical)
The four forms of metabolism are quite general and
should apply to life anywhere
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 Cells:
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Needed environment for metabolism at acceptable
rate
 Origin of Life (on Earth and elsewhere):
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Look for cells as sites of metabolism
Why Carbon based?
 Can bond to as many as 4 atoms at a time.
 Can form skeleton of long chains of atoms
(polymers).
 The complexity of life requires complex
molecules.
Silicon can also form 4 bonds and is
relatively abundant, however…
 Bonds are weaker than those of carbon (fragile:
complex Si-based molecules don’t last long in
water)
 Does not normally form double-bonds like
Carbon; this limits the range of chemical
reactions and molecular structures.
 Carbon is more mobile in the environment - it
can travel in gaseous form, e.g., CO2
Environmental limits to life
(as we know it) ?
Is the planet of interesting missing any of the key
ingredients? (water, energy, nutrients)
Are temperatures below –15 or beyond +115 C?
Is it really cushy? – does it have an atmosphere
Importance of Water
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Importance:
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Contact: organic chemicals float in the cell and find
each other
Transportation: bring chemicals in and out of cells
Participant in reactions:
● ATP
● Photosynthesis
Necessity:
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Life on Earth: all use water
Dormant without water: for a limited time only
Elsewhere: need a liquid
What are the alternatives?
Water
Liquid water plays a fundamental role in life:
 Make chemicals available (dissolved)
 Transports chemicals
 Plays a role in many metabolic reactions
Cells
 All life on Earth is made of cells - microscopic
units in which living matter is separated from the
outside world by a membrane.
 All cells on Earth share common characteristics
(e.g., use of ATP, DNA, …), leading to
conclusion that they share a common ancestor
 All cellular life is carbon based (organic
molecules)
Components of Cells
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Carbohydrates: energy needs and structures
 Lipids: Source of energy & major component of
membranes. Lipids can spontaneously form membranes
in water.
 Proteins: participate in a vast array of functions;
structural, enzymes, catalysts. Built from long chains of
amino acids.
 Nucleic acids: instructions for reproduction
 70 amino acids known to
exist; only 22 are found in
life on Earth.
 Only left handed versions
are found in living
organisms
 Both of these traits
suggest a common
ancestor for life on Earth.
Based upon the cellular structure of an
organism, living cells come in two types:
Prokaryotes
Eukaryotes
The prokaryotes
 lack a cell nucleus
 Most are unicellular
 two domains: bacteria &
archaea
 asexual reproduction
The Eucaryotes
 cells are organized into complex
structures enclosed within membranes.
Have a nucleus.
 typically much larger than prokaryotes
 May be unicellular, as in amoebae, or
multi-cellular, as in plants and humans.
 both sexual and asexual reproduction
Tree of Life
Cells & Metabolism
Metabolism takes place in cells.
Four different approaches to metabolism
have been identified.
Carbon and Energy Sources
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Carbon:
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Heterotroph: eat other organisms
Autotroph: self-feeding by converting atmospheric CO2
Energy:
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Photoautotrophs (plants): photosynthesis: CO2 + H2O +
sunlight
sugar
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Photoheterotrophs (rare prokaryotes): carbon from food
but make ATP using sunlight
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Chemoheterotrophs (animals): energy from food
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Chemoautotrophs (extreme prokaryotes): energy from
chemicals and not sunlight
Extremophiles
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Life that exists under “extreme”
conditions, conditions that until recently
were thought to be inhospitable to life.
Extremophiles
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Volcanic vents:
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Water temperature
reaches 400°C
(750°F), possible
because of the large
pressure
Black smokers: mixed
with volcanic chemicals
Extremophiles
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Antarctic dry valleys:
 Microbes in small pockets of water in rocks
Extremophiles
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Lithophiles (rock lovers):
 Several kilometers below the surface
 Chemical energy from rocks
 Carbon from CO2 filtering down
Extremophiles
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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
 Oxygen for eukarya on Earth for only ~10% of its life
 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
Origins of Life
Certain chemical processes are energetically
favored given specific circumstances, i.e.,
presence of specific elements, energy, 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 (as will
be discussed)
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Searching for the Origin of Life
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When Did Life Begin?
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DNA Molecules as Living Fossils
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Where Did Life Begin?
How Did Life Begin?
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Early Organic Chemistry
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Chemistry to Biology
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Migration of Life to Earth?
Early Evolution and the Rise of Oxygen
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Early Microbial Evolution
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Photosynthesis and Oxygen
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Rise of Oxygen
When Did Life Begin?
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Stromatolites
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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
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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
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13C/12C
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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
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Last sterilization: 3.9-4.2 byr
ago
The evidence indicates life formed
quickly after the Earth formed.
Within a few
100 million
years,
Perhaps as
short as 100
million years
Where Did Life Begin?
Unlikely on land
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Solar UV radiation: protection today by ozone (O3)
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But no atmospheric oxygen in the early Earth
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In water: no problem, UV absorbed effectively
Shallow ponds
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First evidence from Miller-Urey experiment
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Recent evidence: incorrect atmospheric content
Thermophiles
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DNA evidence shows early thermophiles
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Have advantage of more chemical energy
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Deeper sea vents better protected against bombardment
Early Organic Chemistry
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No atmospheric oxygen
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Helps: Oxygen destroys many organic compounds
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Atmosphere is reducing, not oxidizing
Miller-Urey experiment
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Can form amino-acid soup from methane (CH4) and ammonia (NH3) with electric
energy (mimicking lightning)
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Current thinking: early atmosphere was dominated by CO2
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Low yield
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Shallow ponds close to surface -> UV sterilization
Alternative sources of amino acids
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Extraterrestrial: amino acids are abundant in meteorites
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Deep sea vents: abundant chemical energy & protected from UV
Chemistry to Biology
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Organic soup of amino acids
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Has to be the initial step, however the amino acids formed
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Perhaps in special locations, not ubiquitous initially
Short strands of RNA
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Required physical catalysts: clay or other minerals
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Some had to be self-replicating
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Initial simulations in the laboratory
Spontaneous membranes  “pre-cells”
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Protect chemicals and allow faster reactions
Slow initial natural selection
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Gradual increase of complexity
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Fast mutation but slow natural selection at first
Rapid natural selection
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Complexity  risk  faster natural selection
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Probably the stage when DNA formed and took over
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Against:
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No atmosphere or water
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Solar and stellar radiation
Migration of Life
to Earth?
For:
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Fact: amino acids found in meteorites
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Question is not “could” but “did”
Difficult formation of life
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Argument: too little time to form life on Earth
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Counter argument: other sites in the solar system no better
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Another stellar system?
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Travel time much longer and more dangerous
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All meteorites found in the solar system have the same age
Migration of Life to Earth?
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Easy formation of life
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Life could have formed on Earth, but also elsewhere
in the solar system
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Life was then transported to Earth, before life on
Earth had a chance to form
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Who came first?
Early Microbial
Evolution
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Fossils of Eukarya go back only 1-2 byr
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But hard to detect cell nuclei in fossils
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Tree of life suggests that prokaryotes came first, but how much earlier?
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Whoever came first, microbes still rule the Earth, even by mass
Early microbes
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Rudimentary metabolism: at least a few enzymes
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Anaerobic: there was no oxygen
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Chemoautotrophs: obtained energy from inorganic chemicals
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Thermophiles favored: Abundant energy & chemicals (H, S, Fe)
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Simple creatures -- faster mutation and evolution
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Complexity: hard to form but also more stable and survives
Photosynthesis and Oxygen
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Photosynthesis:
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Key step to harnessing sunlight
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May have begun as a pigments protecting against
UV radiation, then started using the energy
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Some bacteria, even today, use H2S instead of H2O
in photosynthesis
Cyanobacteria
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Exist today (blue-green algae)
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Earliest fossils resemble modern cyanobacteria
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Release oxygen in photosynthesis
Rise of Oxygen
 O is highly reactive: would disappear from the atmosphere in a
few million years
 To maintain oxygen, it therefore needs to be replenished
constantly
 Today: living creatures consume most of the O
 Early Earth: inorganic reactions, mainly rusting iron, suffice,
and for a long time prevented the rise of O
 Banded iron formations (2-3 byr old) suggest very low
atmospheric O (less than 1% of today)
 Rise of O began about 2 byr ago
 Evidence for O at today’s levels: only 0.2 byr ago
 Intermediate O level uncertain