Chapter 24
Lif in
Life
i the
th Universe
U i
ASTROBIOLOGY
A FIELD OF SERIOUS STUDY
• NASA ASTROBIOLGY
Life on Earth
Our goals for learning
• When did life arise on Earth?
• How did life arise on Earth?
• What are the necessities of life?
When did life arise on Earth?
Earliest Life Forms
• Life probably arose on Earth more than 33.85
85
billion years ago, shortly after the end of
heavy bombardment
• Evidence comes from fossils, carbon
isotopes.
Fossils in Sedimentary
y Rock
• relative ages: deeper layers formed earlier
• absolute ages: radiometric dating
Fossils in Sedimentary
y Rock
• Rock layers of Grand Canyon record 2
billion years of Earth’s history
Earliest Fossils
• Oldest fossils show
that bacteria-like
organisms were
present oover
er 33.5
5
billion years ago
• Carbon isotope
evidence pushes
origin of life to
more than 3.85
billion years ago
The Geological
g
Time Scale
How did life arise on Earth?
Origin of Life on Earth
• Life evolves through time.
• All life on Earth shares a common ancestry.
ancestry
• We may never know exactly how the first
organism arose,
arose but laboratory experiments
suggest plausible scenarios.
The Theory of Evolution
• The
h fossil
f il recordd shows
h
that
h
evolution has occurred through
time.
• Darwin’s theory tells us HOW
evolution occurs: through
natural
t l selection.
l ti
• Theory supported by discovery
of DNA: evolution proceeds
p
through mutations.
• WE WILL EXAMINE
EVOLUTION ON EARTH IN
MORE DETAIL LATTER!
Tree of Life
• M
Mapping
i genetic
ti
relationships has led
biologists to discover
this new “tree of life.”
• Plants and animals are
a small part of the tree.
• Suggests likely
characteristics of
common ancestor.
• These genetic studies suggest that the earliest life on
Earth may have resembled the bacteria today found
near deep ocean volcanic vents (black smokers) and
ggeothermal hot springs
p g .
Laboratory Experiments
•
Miller-Urey experiment (and more recent experiments) show that building
blocks of life form easily and spontaneously under conditions of early Earth.
Stanley
y Miller :
Synthesized
several amino
acids which are
Monomers
But molecules of
g
are
organisms
Polymers which
are linked
monomers
Microscopic, enclosed membranes or “pre-cells”
have been created in the lab.
Proteinoids-200
Proteinoids
200 linked amino acid polymer
shows organic monomers can link –
synthesized by Sidney Fox
Fox ‘ss experiments demonstrate that proteinoids will spontaneously
Aggregate into units called microspheres which grow &divide but
do not reproduce!
Another veiw of
MICROSPHERES WHICH
DEVELOP DOUBLE LAYER
BOUNDARIES SIMILAR TO
CELL WALLS. THEY ARE
AN INTERMEDIATE STAGE
BETWEEN COMPLEX
MOLECULES AND CELLS
WITH MOLECULES
CONTAING REPRODUCING
GENETIC MATERIAL
Chemicals to Life?
NO EVIDENCE OF EARLIER SIMPLER LIFE FORMS-THAT
ARE?
ANAEROBIC REQUIRE NO FREE OXYGEN
ANAEROBIC:
HETEROTROPHIC: EXTERNAL SOURCE OF NUTRIENTS
(ATP THEN FERMENTATION)
PROKARYOTIC: LACK CELL NUCLEUS AND OTHER STUFF
MODERN CELLS ARE CALLED EUKARYOTIC CELLS
STAMATOLITES
Layered mats of mineral structures built by
communities of Photosynthesizing bacteria or
Blue green algae =sophisticated life forms!
Blue-green
EARTH A BILLION YEARS LATER (3.5 billion years ago)
Brief History of Life
• 4.4 billion years - early oceans form
• 3.5 billion years - cyanobacteria start releasing
oxygen.
• 2.0 billion years - oxygen begins building up in
atmosphere
• 540-500 million years - Cambrian Explosion
• 225-65 million years - dinosaurs and small
mammals (dinosaurs ruled)
• Few million yyears - earliest hominids
Thought Question
You have a time machine with a dial that you can spin
to send you randomly to any time in Earth’s history. If
you spin
y
p the dial, travel through
g time, and walk out,
what is most likely to happen to you?
A. Y
A
You’ll
’ll be
b eaten
t by
b dinosaurs.
di
B. You’ll suffocate because you’ll be unable to
breathe the air.
air
C. You’ll be consumed by toxic bacteria.
D. Nothing:
g yyou’ll pprobablyy be just
j fine.
Thought Question
You have a time machine with a dial that you can spin
to send you randomly to any time in Earth’s history. If
you spin
y
p the dial, travel through
g time, and walk out,
what is most likely to happen to you?
A. Y
A
You’ll
’ll be
b eaten
t by
b dinosaurs.
di
B. You’ll suffocate because you’ll be unable to
breathe the air.
air
C. You’ll be consumed by toxic bacteria.
D. Nothing:
g yyou’ll pprobablyy be just
j fine.
Origin of Oxygen
• Cyanobacteria
paved the way for
more complicated
life forms by
releasing oxygen
into atmosphere via
photosynthesis
Necessities for Life
• Nutrient source
• Energy (sunlight,
(sunlight chemical reactions,
reactions
internal heat)
• Liquid water (or possibly some other liquid)
Hardest
H
d t tto find
fi d on
other planets
Information Storage and Duplication
All information guiding all
processes of life are
stored in long spiral
molecules of DNA
(Deoxyribonucleic Acid)
Basic building blocks are
four Amino acids: Adenine,
Cytosine Guanine
Cytosine,
Guanine, and
Thymine
Information is encoded in
the order in which those
amino acids are integrated
i th
in
the DNA molecule.
l
l
DNA
Deoxhribonucleic Acid
Sugars & Phosphates
Rungs are “bases”
Cytosene-Guanine
Thymine Adenine
Thymine-Adenine
Sequences are the
Genetic codes
One Human cell
Contains
1.5 meters of DNA
=4.5 billion pairs of
Bases
W consist
We
i t off
60 x 10 12 cells
Or we have
600 AU of DNA
HUMAN GENOME
PROJECT
In any event, the first living cells were of this type- Prokaryotes
In fossils from 3.5 to 2.1 billion years ago.
Note: no nucleus and other stuff!..simple
Processes of Life in the Cell
Information
stored in the
DNA in the
nucleus is
copied over to
RNA
(ribonucleic
acid) strands
strands,
which acts as a
messenger to
govern the
chemical
processes
p
ocesses in
the cell.
A Modern Cell
Code patterns are used to construct
PROTEINS
ENZYMES are special controlling proteins
e.g. growth
h
DNA stays in cell nucleus and builds
A messenger molecule that travels into
the cell to build proteins out of AMINO ACIDS
The messenger molecule is built from long
Chains of carbon and is called RNA(ribonucleic
(
Acid) or Messenger RNA
Duplication and Division
In the course of cell
division, the DNA
strands in the nucleus
(chromosomes) are
duplicated by splitting
the double-helix
strand up and
replacing the open
bonds with
ith the
corresponding amino
acids
Process must be
sufficiently accurate,
but also capable of
occasional minor
mistakes
(MUTATIONS!) to
allow for evolution.
The Origin of Life on Earth
• Life develops into more complex forms through
gradual evolution, spanning many thousands of
generations.
generations
• Life began
g in the
sea as single-celled
creatures.
• Those as well as
early multi-celled
creatures had no hard
parts to leave fossils.
Earliest, microscopic fossils
date back ~ 4 billion years.
Could life have migrated to Earth?
• Venus, Earth, Mars have exchanged tons
of rock ((blasted into orbit by
y impacts)
p
)
• Some microbes can survive years in
space...
space
Extraterrestrial Origin of Life on
E th
Earth
• Alternative theory: Most
primitive living entities
transported to Earth in
meteorites or comets.
• Some meteorites do
show traces of amino
acids.
• Theory of extraterrestrial
origin of life is currently
untestable.
untestable
24.2 Life in the Solar System
Our goals for learning
• Could there be life on Mars?
• Could there be life on Europa or other jovian
moons?
Could there be life on Mars?
Searches for Life on Mars
Insert image, filename:
MarsSurface.jpg
• Mars had liquid water in the distant past
• Still has
h subsurface
b f
ice;
i possibly
ibl subsurface
b f
water near sources of volcanic heat.
In 2004,, NASA Spirit
p and Opportunity
pp
y Rovers sent home new
mineral evidence of past liquid water on Mars.
The Martian Meteorite debate
composition indicates
origin on Mars.
• 1984: meteorite ALH84001 found in Antarctica
• 13,000
13 000 years ago: ffell
ll tto E
Earth
th in
i Antarctica
A t ti
• 16 million years ago: blasted from surface of Mars
• 4.5
4 5 billion years ago: rock formed on Mars
Microscopic carbonate
Globules resemble terra
Bacterial deposits
Resemble
Earthly
Bacteria
features
• Does the meteorite contain fossil evidence
of life on Mars?
… most scientists not yet convinced
Viking experiments looked for life on Mars by scooping up
Soil samples and experimenting to see if they contained
Life forms or there by-products.
Could there be life on Europa or
other jovian moons?
Possible abode of life is the warm ocean under the ice of
Jupiter’s moon Europa!
• Ganymede, Callisto also show some evidence for
subsurface oceans.
• Relatively little energy available for life, but
still…
• Intriguing prospect of THREE potential homes for
lif aroundd Jupiter
life
J i alone…
l
Ganymede
Callisto
Titan
• Surface too cold for liquid water (but deep underground?)
• Liquid ethane/methane on surface
24.3 Life Around Other Stars
Our goals for learning
• Are habitable planets likely?
• Are Earth-like planets rare or common?
Are habitable planets likely?
H bi bl Planets
Habitable
Pl
Definition:
A habitable world contains the basic
necessities for life as we know it,
it including
liquid water.
• It does
d
nott necessarily
il have
h
life.
lif
Constraints on star systems:
1) Old enough to allow time for evolution (rules
out high-mass
g
stars - 1%))
2) Need to have stable orbits (might rule out
binary/multiple
y
p star systems
y
- 50%))
3) Size of “habitable zone”: region in which a
planet of the right size could have liquid water
on its surface.
Even so… billions
E
billi
off stars
t in
i the
th Milky
Milk Way
W seem
at least to offer the possibility of habitable worlds.
The Life Zone
around other stars.
Based on life as we know it
depends on where we might
Find Liquid Water!
The more massive the star,
the larger the habitable
zone — higher probability
of a planet in this zone.
Finding them will be hard
Recall our scale model solar system:
y
• Looking for an Earthlike planet around a
nearby star is like standing on the East
Coast of the United States and looking for a
pinhead on the West Coast — with a VERY
bright grapefruit nearby.
nearby
• But new technologies should soon show the
way…
• Kepler
p ((2008 launch)) will
monitor 100,000 stars for
transit events for 4 years.
Later: SIM and TPF
interferometers to obtain
spectra and crude images
of Earth-size
Earth size planets.
Spectral Signatures of Life
Venus
Earth
Mars
oxygen/ozone
Are Earth-like planets rare or
common?
Elements and Habitability
• S
Some scientists
i ti t argue
that proportions of
heavy elements need
to be just right for
formation of habitable
planets
• Iff so, then
h Earth-like
h lik
planets are restricted
to a galactic habitable
zone
Impacts and Habitability
• S
Some scientists
i ti t argue
that Jupiter-like
planets are necessary
to reduce rate of
impacts
p
• If so, then Earth-like
planets
l
are restricted
i d
to star systems with
Jupiter like planets
Jupiter-like
Climate and Habitability
• S
Some scientists
i ti t argue
that plate tectonics
and/or a large Moon
are necessary to keep
the climate of an
Earth-like planet
stable enough for life
The Bottom Line
We don’t yet know how important or
negligible
li ibl these
th
concerns are.
Three Questions About the
E l ti off Life
Evolution
Lif
1) Could life originate on another world if
conditions were suitable?
Miller experiment etc
etc. indicate: probably yes
yes.
2) Will life always evolve toward intelligence?
If intelligence favors one species over another:
probably yes.
3) How common are suitable conditions
for the beginning of life?
 Investigate conditions on other planets and
statistics of stars in our Milky way
Some Requirements of Life
• Liquid water (for chemical reactions and
as transport
p medium).
)
• Atmosphere (to avoid rapid vaporization of
water; gasses needed for organic compounds)
• M
Moderate
d t ttemperatures
t
(k
(keep
water
t liquid;
li id
avoid disintegration of organic compounds;
activate complex chemical reactions)
• Time for life to evolve from simple organic
compounds
d iinto higher
hi h lif
life forms:
f
severall billion
billi
years.