The origin of life

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The origin of life

• Goals
• The fossil record for very early life
• Chemical evolutionamino 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

• 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

• 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

• 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 stablethey do not decay from
  one form to another.
• In rock, 13C to 12C ratios are around the typical
  891.
• 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

• 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 Earths
  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
  acidsthe 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 lifes
  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?
• Atmospheric processes as suggested by Miller?
• 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 sparknow we see
  this is like a simulation of hydrothermal vents.)
• 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
  naturallyhow do you break into the
  DNA-RNA-enzyme cycle?
• Now we know at least one wayRNA might have
  developed on clay matrices!

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The RNA world

• Here is a possible chain of events
• Simple molecules assembled into organic molecules
  such as amino acids
• Organic molecules accumulate on lattice-like
  silicate clay deposits
• Clay-facilitated reactions can cause the creation
  of RNA
• Replicating RNA entered a stage of chemical
  evolution
• Lipid cells group RNA, improving the
  effectiveness of the chemistry
• Lipid cells evolve membranes
• 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
  RNAan RNA world, instead of todays 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.
• 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
• The sulfur-based chemoautotrophs evolved
  photosynthetic metabolisms based in sulfur,
  becoming photoautotrophs
• Photosynthesis switched from using H2S to H2O
  perhaps 2.7 b.y.a?
• 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

• About 2.35 b.y.a., atmospheric oxygen levels rose
  above 0.002. This is called the great oxidation
  event
• Aerobic life developed in response to the oxygen
• Oxygen levels reached perhaps 10 of current
  levels 540 m.y.a. (Cambrian explosion)
• 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
  dragonflys 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 MYAPhanerozoic 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 MYAAlga evolved into land plants (e.g.,
  Cooksonia)
• 400 MYAAnimals 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 dont 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 Occams 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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