Phys 214. Planets and Life
Dr. Cristina Buzea
Department of Physics
Room 259
E-mail: [email protected]
(Please use PHYS214 in e-mail subject)
Lecture 21. Origin and evolution of life. Part I
March 3rd, 2008
Contents
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Textbook pages 190-197, 203-206
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Conclusions on life at the extremes
Origin of life
Evidence towards the origin of life
(Stromatolites, Microfossils, Isotopic evidence)
When did early life begin and how it looked like
Panspermia - the possibility of life migration from nearby
planets
Conclusion - Life at the extremes
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The extreme conditions that limit the growth or prove lethal to most organisms are favorable
for others.
Extremes of temperature, high and low pH, high salt concentration, high levels of radiation kill
the majority of organisms on Earth.
However, there are organisms from all three domain of life that have adapted to these extreme
conditions.
Probably high temperature, low pH, high salinity persisted throughout Earth history (and icy
environments). These environments are not rare on Earth today as well.
There are very few natural environments on Earth where life is absent!
Life is a rule rather than an exception!
The limits of life are yet to be discovered.
Evidence towards the origin of life
Old rocks have been hidden or
destroyed by geological processes
like burial, erosion, subduction
back into the mantle.
The geological record becomes incomplete as we look back in time, and no known rocks
survive from Earth`s first half-billion years. Geological record cannot tell us how life
originated!
However, there are three lines of evidence to indicate that life arose quite early on Earth.
1. Stromatolites
2. Microfossils
3. Isotopic evidence
1. Stromatolites
Stromatolites are layers of sediment that once contained colonies of ancient microbes.
Stromatolites date back to approximately 3.5 billion years ago.
• They have a distinctive layered structure.
• Regarding size, shape, interior structure – ancient stromatolites look identical to mats
formed today. They also contain chemical and isotopic evidence of biological origin.
• Layers of sediments intermixed with microbes. Microbes at the top generate energy via
photosynthesis, the microbes beneath use the organic compounds (waste) of the
photosynthetic microbes.
• They grow over time as sediments are depositing over, forcing them to migrate upward
in order to perform photosynthesis.
1. Stromatolites
Pre-Cambrian stromatolites, Glacier National Park.
In 2002 W. Schopf published a paper in the scientific
journal Nature arguing that geological formations such as
this possess 3.5 billion year old fossilized algae microbes,
the earliest known life on earth.
1. Stromatolites
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If the microbes in the ancient stromatolites are like in the modern ones, this
means that at least some produced energy by photosynthesis
Photosynthesis is a sophisticated metabolic process – it took a long time for
this process to evolve
Therefore primitive life should have already existed some long before 3.5
billion years ago!
2. Microfossils
Second life of evidence for the early life on Earth – microfossils – highly
controversial.
Microfossil evidence suggests that life existed before about 3.0 billion years
ago and may well have existed before 3.5 billion years ago.
Difficulties:
- Finding old fossils is very difficult – the oldest rocks have been altered by
geological processes destroying microfossils. When searching for evidence of
past life, sedimentary environments are considered the most suitable because
they are often formed in association with water, a fundamental requirement for
life. There are only three known locations that host exposures of ancient
sediments: Greenland, which are 3.8 to 3.7 billion years old (Ga), in Australia
(3.5 to 3.3 Ga), and South Africa (3.5 to 3.3 Ga).
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Contamination of ancient rocks by recent endolithic microorganisms
further complicates the task of identifying the original signatures of life!
- The claim of the discovery of microfossils is controversial because it is not clear
whether they are of biological or mineral origin.
2. Microfossils
Shape and organic content suggests this is a microfossil of ancient living cell dated
back 3.5 billion years ago in Australia; chemical analysis shows the presence of
organic carbon. The rock presumed to be a sedimentary rock from a shallow sea
and the fossils of a photosynthetic microbe. However, the rocks analysis indicates
is from a deep-sea volcanic vent, similar to a black smoker; therefore the fossil
cannot be of a photosynthetic microbe.
Microbial biofilms and mats assumed to be
using a primitive form of photosynthesis
(anoxygenic) - filamentous, coccoid, or
rodshaped microbes—have been found in
sediments that formed in shallow-water basins
environments, 3.5 - 3.3 billion years old
(Walsh, Precambrian Res. 54, 271, 1992; Tice
and Lowe, Nature 431, 549, 2004).
2. Microfossils
2. Microfossils
2. Microfossils
2. Abiotic microfossils?
Phylogenetic tree - Time scale
Phylogenetic tree of life with dates indicating the
minimum age of selected branches based
on fossil evidence and chemical biomarkers.
The length of the branches has no temporal
scale - related only to evolutionary
distance, not geological time! We can
show some ages estimated from the
fossil record.
The earlies presence of eucaryotes indicated by
steroids (sterane precursors - rigidify molecules
within the lipid layer in the cell membrane - give
ability to engulf large particles, allows endosymbiosis
(living inside) of organelles.
3. Isotopic evidence
Studies of carbon isotopes can be used to detect the
presence of past biological activity in rocks.
The earliest evidence for life comes from chemical
remnants of biological processes in rocks.
Many fundamental biochemical processes involving
the extraction of carbon from an abiotic reservoir
preferentially take up 12C rather than 13C.
e.g an enzyme that captures carbon from carbon
dioxide, preferentially removes 12C from CO2
because of the weaker carbon-oxygen bond
strength in the lighter isotope.
Hence, remnants of organic carbon (processed
though cells) should be enriched in 12C/13C
relative to the contemporaneous atmospheric
CO2.
Carbon isotope evidence from rocks found in
Greenland, although controversial, suggests that
life may have existed 3.85 billion years ago.
Ancient rock formation of the coast of
Greenland.
Rocks of similar age (3.8 billion
years old) show similar carbon
isotopic ratios to Greenland rocks.
Life can also alter isotopic ratios
of other elements (Fe, N, S). These
isotopes are present in
characteristic ratios for life within
the ancient rocks, confirming the
existence of life as early as 3.85
billion years ago.
3. Isotopic evidence
Sedimentary carbon isotope values through time (middle dashed line is the mean and outer two
lines are one standard deviation)
Carbonate carbon Organic carbon
Carbon isotope ranges of major groups of
higher plants and micro-organisms compared
with the respective ranges of eh principal
inorganic carbon species in the environment.
(Schidlowski 1983)
When did early life begin
Life arose considerable earlier than 3.85 billion
years ago.
The young Earth probably experienced
numerous large impacts during the heavy
bombardment, some being large enough to
extinguish any life, or eat least life near the
surface.
Conclusion: Current geological evidence
suggests that life appeared very soon after the
heavy bombardment in a short time period of
perhaps only 200 million years.
We could expect life to arise as fast on any
other world with similar conditions (many
other worlds with similar conditions).
What did early life look like
The earliest living organisms presumably went extinct long ago.
The current organisms most closely related to the common ancestor of all life on Earth
would be located very closed to the root of the tree of life.
Organisms close to each other on the “Tree of Life” are genetically very similar, while
the ones far from each other are genetically very different.
As life on Earth evolved, its DNA became gradually more complex.
Present-day living organisms closest to the root of the Phylogenetic tree are
hyperthermophiles.
The Phylogenetic tree suggests that present day life on Earth evolved from a
hyperthermophile ancestor, even though earlier life might have had greater variety
(but became extinguish by a near-sterilizing impact.
Universal phylogenetic tree features
hyperthermophilic (grow >90o), and
cold adapted species – phychrophilic
(blue lines), or psychrotolerant (violet
lines) of Bacteria and Archaea
The melting temperature of a protein whose thermal stability is
correlated with the growth temperature of the host organism, versus
the geological time. Solid lines are temperature curves inferred from
maximum 18O.
Where did life begin
1. On land surface? NO Life probably did not originate on the land surface because there was no ozone
layer to shield out harmful UV rays. (only Deinococcus Radiodurans –like organisms could survive)
2. Shallow ponds? NO full of organic compounds; as the water evaporated they became more
concentrated making it easier to combine into more complex molecules.
However, shallow ponds would have lacked a source of chemical energy.
In addition, life in ponds would have suffered lethal effect of UV radiation and or high salt concentration.
3. Deep-sea vents? Probably. – DNA evidence suggest early life was hyperthermophile.
Offer plenty of chemical energy to fuel metabolism, protected from the Sun`s UV rays by the water
above, safe from impacts that partially vaporized the oceans during the heavy bombardment – the
safest environment!
Universal phylogenetic tree features
hyperthermophilic (grow >90o), and cold
adapted species – phychrophilic (blue lines), or
psychrotolerant (violet lines) of Bacteria and
Archaea
The origin of life
Abiogenesis - Scientific consensus is that
life on Earth might have emerged
from non-life between 4.4 billion
years ago, when water vapor first
liquefied, and 3.9 billion years ago,
when the ratio of stable isotopes of
carbon (12C and 13C ), iron and sulfur
points to a biogenic origin of
minerals and sediments.
Panspermia - Life could have an
extraterrestrial origins of life.
The most likely candidates for a source of
organisms in the early Solar System
are Mars and Venus.
Mars
Venus
Microstructures in martian meteorite ALH84001 claimed
to be of biogenic origin
1n 1996 NASA made the announcement that a martian
meteorite ALH84001 contains fossils of alien
micro-organisms.
The meteorite Alan Hills ALH84001 has a fascinating
history:
4.5 Ga crystallized from magma on Mars
4.0 Ga battered by not ejected by an asteroid impact
3.6-1.8 Ga altered by water to prodice carbonate
minerals
16 Ma blasted into space by an asteroid impact
1984 Discovered in Antarctica
The possibility of migration
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Among the 20,000 meteorites identified, chemical analysis revealed that about 30 came from
Mars. These meteorites were blasted by large impacts, then followed orbital trajectories that
caused them to land on Earth.
• During the last 4 billion years, the inner planets (Venus, Earth, Mars) have exchanged
many tons of rocks - giving the possibility of microscopic life hitchhiking on meteorites!
The microbes had survive
- The impact that blasted the surface on its home world
- Time spent in the outer space (cold, UV, radiation, cosmic rays)
- The impact in the new world
The interior of many meteorites shows minimal disruption, suggesting that a microbe inside the rocks
could survive the two impacts.
The chance of surviving the trip between planets would depend on how long the meteorite spends in
the outer space. Many meteorites will orbit for many millions of years before reaching Earth,
however 1 in 10,000 meteorites may travel from Mars to earth in a decade or less.
We have seen that microbes can be revived after thousands of years of being frozen, therefore
panspermia from a neighborong planet is possible!
Panspermia from other stars systems is unlikely, because of the large distances
between stars.
Reasons to consider migration
Venus
Earth
Mars
Why do we have any reason to suppose that life originated elsewhere rather than on Earth?
1) If life does not form easily, at least under the conditions on early Earth. Did some other world
in our Solar System has better conditions for life? We have no reason to believe this. Also,
they did not had more time for life to appear, because the planets formed at the same time.
2) If life forms very easily that we should expect life to originate on any planet under suitable
conditions. Life occurred on whichever planet got the conditions first. In this scenario life did
not originate on Earth because life from another planet got here first.
Venus had once oceans and a habitable climate, however now is so hot that any fossil record would
be destroyed by heat and geological activity.
Conclusion: life arose on Earth quite soon after the conditions allowed it.
Implications of migration to the search for life beyond Earth
Venus
Earth
Mars
It is almost certain that microbes from Earth traveled to the Moon, Venus, Mercury,
and Mars.
If life survived on any of these worlds, we should find it there. Probably it did not
survive on the Moon, Mercury, and probably Venus.
Mars might have habitats that could provide a temporary refuge to terrestrial microbes.
Therefore, if we find life on Mars, we will ask: is it native or it came from Earth? The
biochemistry will provide the answer.
Organic compounds in meteorites
Organic compounds in interstellar space
Some of the 120 compounds detected in the interstellar medium,
mostly by microwave spectroscopy (courtesy Lucy Ziurys).
Next lecture
Evolution of life. Part II.
Assignment: due on March 17th
Imagine you are attending a conference on
Astrobiology and you have to write an
abstract for the conference (300-400 words
the body of the text).
The subject: any subject of astrobiology.
Be creative! Think out of the box! Try to bring
your own ideas! Look for controversies!
Your text should have between 3 to 5 references
to scientific publications.
10 of the best abstracts will have oral
presentations during one of the class!
The rest will be posted on the Phys 214 web.