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The origin of life
Goals
The fossil record for very early life
• Chemical evolution—amino acids to DNA
• The advance of life
• Cambrian Explosion
•
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First inklings of life
We can never know the age of the oldest life on Earth, but we can determine
upper and lower limits for its age.
Upper limit: the age of the Earth.
–Lower limit: the age of the oldest fossils or signs of life we can find
The more skilled we become at detecting ancient life on Earth, the better we
will be at detecting it on other planets.
As in many things in science that are
ultimately unknowable, we can at least
make our best guess, and support this with
sound theory and observations.
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The earliest three lines of evidence of life
Stromatolites
Stromatolites are relatively simple bacterial communities that live in
high-saline environments.
In these communities, layers of blue-green bacteria (cyanobacteria)
are producers (photoautotrophs).
The cyanobacteria overlie other bacteria (chemoheterotrophs) which
feed upon them. Sediment embeds in the community, resulting in the
formation of large mats with a characteristic shape.
Fossils are frequently found that look just like stromatolites in
structure, chemical, and isotopic composition.
Fossil stromatolites date back to at least 3.5 b.y. old, i.e., the
Archean eon.
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The earliest three lines of evidence of life
2. Microfossils
What about actual fossils from ancient life? They will probably be…
– ancient;
– tiny;
– easily destroyed;
– hard to identify.
The oldest, from Australia, are 3.465 b.y. old, from the Apex chert
deposits in Warrawoona, Western Australia.
Originally thought to be remnants of photosynthetic bacteria, but are
now thought to be related to ancient undersea volcanic activity (i.e.,
black smokers). Perhaps they are nonbiological in origin?
Even so, they still look like fossil organisms.
Controversial fossils have also been found in Swaziland, South Africa.
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The earliest three lines of evidence of life
2. Microfossils (continued)
Fossils up to 3 b.y. old are not very controversial.
Fossils older than 3.5 b.y. are questionable and under scientific
debate. Perhaps they are pseudo-fossils that were created by some
peculiar events of water seepage through rocks, etc.
The fact that they occur in widely separated rock from the same era
suggests, though that they are indeed ancient fossils.
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The earliest three lines of evidence of life
3. Isotopic evidence
The atom carbon occurs in several isotopes.
12C
is the most common; it has 6 protons and 6 neutrons.
13C
is rarer by a factor of about 89; it has 6 protons and 7 neutrons.
These isotopes are stable—they do not decay from one form to
another.
In rock, 13C to 12C ratios are around the typical 89:1.
Since the complex biochemical reactions of photosynthesis
discriminate against 13C, the rarity of this isotope is heightened in
plant tissues.
(Note to botanists: it is rarer in C4 and CAM plants than C3 plants).
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The earliest three lines of evidence of life
3. Isotopic evidence (continued)
Akilia Greenland has rock outcrops estimated to be 3.85 b.y. old.
Apatite crystals in the rock contain graphite (carbon).
Isotopic ratios of 13C to 12C in the graphite suggest life was active
when the rocks were being created.
But are these rocks really sedimentary, and therefore fossiliferous?
Currently, the feel within the scientific community is that the Akilia
evidence is highly divided, but perhaps leaning towards believing it.
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First inklings of life
What can we infer from these lines of evidence?
Life is probably 3.5 b.y. old on Earth (stromatolites, microfossils);
Life may be 3.85 b.y. old on Earth (shakier isotopic evidence);
(These are both lower limits for the age of life on the planet; life
may be older still).
If the late heavy bombardment ended 3.9 b.y.a., it appears that life
evolved within only a few to several hundred million years after the
Earth’s surface stabilized.
This is pretty speedy. It suggests that on Earth, life appeared readily.
Can we extrapolate this to other planets? Does life appear rapidly, given a
chance?
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Steps towards life
Let us look at how life might have developed on Earth.
In increasing complexity, the key players are
Simple carbon-containing molecules
Complex organic molecules (amino acids)
RNA
DNA
Life
Simple carbon-containing molecules are common in the Universe. But
how did the more complex structures come to be?
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The Miller-Urey experiment
In 1953, researchers at the University of Chicago ran a set of experiments
designed to recreate the conditions of early Earth.
– They started with various primitive compounds in the mix;
– The system included a heat source, a cooling source, and an electrical
source;
– Cook the experiment for a week!
– They ran many different types of simulations.
What did they find?
– The simple molecules had turned into complex organic molecules.
– Miller and Urey detected the formation of five amino acids.
– 10-15% of the carbon was in organic compounds!
Modern perspectives on the experiment
– Better model atmospheric composition decreases the yield
significantly.
– Reexaminations have shown 22 amino acids in the mix!
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The Miller-Urey experiment
More to think about...
– There are many amino acids, but the ones that were created by the
Miller-Urey experiment are those found in life on Earth.
– Looking at the oldest parts of genes on life (the genes that all life
forms share), we find they are mostly made out of a subset of amino
acids—the ones preferentially synthesized by Miller-Urey
experiment.
– The Miller-Urey experiment did not make life, but his experiment
was tiny and lasted only for a week.
– Creationists seize upon the lack of life’s creation in the Miller-Urey
experiment as evidence that the natural creation of life is impossible.
I disagree! The experiment did not set out to create life; furthermore,
it showed that the creation of amino acids is very easy!
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Sources of organic compounds…
Before life developed, the Earth must have had a source of complex
organic molecules.
What was this source?
1. Atmospheric processes as suggested by Miller?
2. Undersea volcanic vents, ultra-hot, and rich in simple molecules,
similar to the ingredients in the Miller-Urey experiment?
(Note: one of the Miller-Urey experiments that produced the
richest blend of organic molecules involved a stage that squirted
steam at the electrical spark—now we see this is like a
simulation of hydrothermal vents.)
3. Meteors and cometary material often includes organic
compounds such as amino acids. The Murchison meteorite
contains 90 amino acids, including 19 of the 20 used by life on
Earth.
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So you have amino acids, what next?
We have determined that simple molecules can easily be assembled
into complex organic molecules, like amino acids.
What is the next stage of complexity?
Certain types of fine-grained silicate materials called “clays” have a
natural repeating surface grain.
It has been shown that amino acids can adhere to these clays, and then
become ordered into short lengths of RNA.
Strands of RNA as large as 100 bases have formed in small-scale
laboratory experiments!
Presumably, on a planet-wide scale, and with millions of years to play,
you could make larger nucleic acids, such as moderately complex RNA
molecules!
Then what?
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Chemical evolution
Cool RNA facts
Nucleic acids such as RNA and DNA need special molecules called
enzymes to replicate themselves.
It has been observed that RNA itself can act as a kind of enzyme (called a
ribozyme).
It has also been observed that RNA itself can, at least partially, catalyze (i.e.,
cause to happen) its own replication.
Suppose…
Suppose, long ago, moderately complex RNA molecules developed on
clays.
If some of the RNA molecules had the ability to replicate themselves, they
would start increasing in numbers.
RNA molecules that replicated themselves more rapidly and reliably would
increase in numbers, and would thus dominate the chemical composition.
This is called “chemical evolution.”
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RNA to DNA
In a world of chemical evolution, random variations in molecules, and errors
introduced in RNA replication, would be the basic fuel for changes in the RNA
chemistry.
One set of changes would change RNA to DNA (by doubling the strands).
→ Now you have all the pieces of life: RNA, DNA, and enzymes.
In modern cells, these three elements are all present and work together
– DNA and RNA need enzymes to replicate;
– Enzymes are created by RNA, following instructions from DNA.
– (To make enzymes you need DNA/RNA; to make DNA/RNA you need
enzymes!)
Critics (who have not followed the research regarding RNA formation in clay)
cite modern cells as a system that cannot have formed naturally—how do you
break into the DNA-RNA-enzyme cycle?
Now we know at least one way—RNA might have developed on clay matrices!
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The RNA world
Here is a possible chain of events
1. Simple molecules assembled into organic molecules such as
amino acids;
2. Organic molecules accumulate on lattice-like silicate clay
deposits;
3. Clay-facilitated reactions can cause the creation of RNA;
4. Replicating RNA entered a stage of chemical evolution;
5. Lipid cells group RNA, improving the effectiveness of the
chemistry;
6. Lipid cells evolve membranes;
7. Membranous cells evolve into biological cells.
Somewhere, the chemistry becomes biology.
At some point above, RNA evolved into DNA. If life evolved before
DNA evolved, there might have been a stage during which life was
being run by RNA—an “RNA world,” instead of today’s DNA
world.
RNA is less stable than DNA, with more transcription errors.
Evolution in an RNA world would be more rapid.
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Implications for extraterrestrial life
Notice that at no point in these theories have we assumed anything that is too
particular about the Earth.
Examples
– No assumption of a single large moon;
– No assumption of only 1 a.u. from the Sun.
This chain of logic for the formation of life is plausible for any other world
with the basic compounds and conditions amenable for life.
And remember, it seems that when given the chance, life evolved fast!
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Panspermia and interplanetary sneezes
Astronomers have detected organic compounds and amino acids in
meteorites and comets in space.
Nucleic acids have not been found in space. But even so, could life
itself be in space already?
Could meteorites be carrying life? Are we descended from life alien
to Earth?
Arrival of alien life via asteroids is conceivable. Yet, life had to
develop someplace, at some point.
Since planets communicate with each other via meteoric ejecta,
planetary ecologies are coupled, or at least interact at some level!
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Life becomes complicated
Once prokaryotes evolved, what next?
– Prokaryotes developed a tolerance for the waste oxygen that began to
accumulate (Proterozoic Eon);
– Eukaryotic life forms developed (Proterozoic Eon);
– Multicellular life developed (Phanerozoic Eon).
Let us look at these three stages…
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The tale of toxic waste oxygen: Part I
The initial archean atmosphere did not have oxygen.
1. The first life forms (chemoautotrophs) were anaerobic, such as iron
or sulfur-based. Evidence for the anaerobic conditions are from
banded iron formations, which cannot form with O2;
2. The sulfur-based chemoautotrophs evolved photosynthetic
metabolisms based in sulfur, becoming photoautotrophs;
3. Photosynthesis switched from using H2S to H2O; perhaps 2.7 b.y.a?
4. The waste oxygen was locked into surface soils and rocks. After 350
MY, the soils and rocks became saturated, and began to accumulate
in the atmosphere.
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The tale of toxic waste oxygen: Part II
5.
About 2.35 b.y.a., atmospheric oxygen levels rose above
0.002%. This is called the great oxidation event;
6.
Aerobic life developed in response to the oxygen;
7.
Oxygen levels reached perhaps 10% of current levels 540 m.y.a.
(“Cambrian explosion”);
8.
Oxygen levels peaked in the carboniferous period 360-300
m.y.a. (charcoal occurs in the fossil record). At this time, O2 was
as high as 35% (currently it is 21%), which allowed for giant
invertebrates and amphibians. (Meganeura dragonfly’s
wingspan ~50-75cm!)
We have evolved in response to a polluted atmosphere.
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Prokaryote to Eukaryote
Bacteria, Archaea, Eukarya are all about the same age;
Eukarya absorbed organelles around 2.1 b.y.a.
– Nuclei store genetic material for the entire cell.
– Mitochondria are powerhouse factories that create ATP.
– Chloroplasts, found only in phototrophs, photosynthesize.
– Multicellular life makes its appearance.
Some eukarya organelles (mitochondria and chloroplasts) have
their own DNA, which is bacterial.
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Advent of multi-celled life
545 MYA—Phanerozoic eon begins;
– Macroscopic life appeared on Earth;
– Many phyla (basic body plans) of life appeared, more than
we have today;
– This Cambrian Explosion of life took only about 40
million years.
475 MYA—Alga evolved into land plants (e.g., Cooksonia);
400 MYA—Animals were fully established on land.
Why the Cambrian Explosion?
–
–
–
–
Cellular/genetic complexity had reach some critical level;
Oxygen levels had begun to rise;
Climate change, such as an ending snowball Earth phase;
Minimal predation.
We don’t see many of these innovations today because the
many sophisticated competitors and predators would interfere
with new forms of early life.
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What about the supernatural?
Where is God in all this?
The influence of a supernatural force does not seem to be required in the
development of life…
…or at least, there appears to be a plausible way for life to have developed,
without needing to invoke supernatural forces.
Even so, supernatural influences are NOT prohibited by these scenarios.
The most parsimonious scientist would say that, following Occam’s Razor,
unnecessary elements should be trimmed from a theory. Hence, supernatural
forces would not be a necessary feature of the model.
…Yet, it cannot be ruled out. Religion, by its nature, cannot be falsified.
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