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THE ORIGIN
OF LIFE
By Abby Wilson TSW 4
In the beginning, there was methane, and there was ammonia, and there
was no free oxygen …
WHAT DO WE KNOW?
Living organisms are incredibly diverse
* ~1.5 million species identified so far
* Many more remain unidentified
1.Primordial soup
• Life on Earth began more than 3 billion years
ago, evolving from the most basic of microbes
into a dazzling array of complexity over time.
But how did the first organisms on the only
known home to life in the universe develop from
the primordial soup?
2. Electric Spark
• Electric sparks can generate amino acids and sugars
from an atmosphere loaded with water, methane,
ammonia and hydrogen, as was shown in the famous
Miller-Urey experiment reported in 1953, suggesting
that lightning might have helped create the key building
blocks of life on Earth in its early days. Over millions
of years, larger and more complex molecules could
form. Although research since then has revealed the
early atmosphere of Earth was actually hydrogen-poor,
scientists have suggested thatvolcanic clouds in the
early atmosphere might have held methane, ammonia
and hydrogen and been filled with lightning as well.
3. Community Clay
• The first molecules of life might have met on
clay, according to an idea elaborated by organic
chemist Alexander Graham Cairns-Smith at the
University of Glasgow in Scotland. These
surfaces might not only have concentrated these
organic compounds together, but also helped
organize them into patterns much like our genes
do now.
WHAT DO WE KNOW?
• All living organisms share common ancestry
– Populations of
organisms change
through time
– Change (evolution)
may be slow or
relatively rapid
– Given life,
evolution is
inevitable
4. Deep-Sea Vents
• The deep-sea vent theory suggests that life may
have begun at submarine hydrothermal vents,
spewing key hydrogen-rich molecules. Their
rocky nooks could then have concentrated these
molecules together and provided mineral
catalysts for critical reactions. Even now, these
vents, rich in chemical and thermal energy,
sustain vibrant ecosystems.
5. Chilly Start
• Ice might have covered the oceans 3 billion
years ago, as the sun was about a third less
luminous than it is now. This layer of ice,
possibly hundreds of feet thick, might have
protected fragile organic compounds in the water
below from ultraviolet light and destruction from
cosmic impacts. The cold might have also
helped these molecules to survive longer,
allowing key reactions to happen.
6. RNA World
• Nowadays DNA needs proteins in order to form, and
proteins require DNA to form, so how could these have
formed without each other? The answer may be RNA,
which can store information like DNA, serve as an
enzyme like proteins, and help create both DNA and
proteins. Later DNA and proteins succeeded this "RNA
world," because they are more efficient. RNA still
exists and performs several functions in organisms,
including acting as an on-off switch for some genes.
The question still remains how RNA got here in the
first place. And while some scientists think the
molecule could have spontaneously arisen on Earth,
others say that was very unlikely to have happened.
7. Simple Beginnings
• Instead of developing from complex molecules
such as RNA, life might have begun with
smaller molecules interacting with each other in
cycles of reactions. These might have been
contained in simple capsules akin to cell
membranes, and over time more complex
molecules that performed these reactions better
than the smaller ones could have evolved,
scenarios dubbed "metabolism-first" models, as
opposed to the "gene-first" model of the "RNA
world" hypothesis.
HISTORY OF THE UNIVERSE
• 12 – 15 billion years ago
– “Time zero”
– Everything compressed into volume of sun
• Incredibly dense, incredibly hot
– Big bang
• Matter and energy very rapidly distributed throughout
•
•
•
universe
Temperatures dropped
Fusion reactions created light elements
Resulting background radiation is still detectable
HISTORY OF THE UNIVERSE
• First billion post-big bang years
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Gaseous particles collide, condense under force of gravity
First stars are formed
As stars grew, nuclear reactions ignited
Heat and light liberated
Heavier elements formed
Explosive deaths of stars released these heavy elements
Released elements incorporated into newly forming stars and
orbiting planets
– Still heavier elements formed
• New star formation currently visible in dust clouds of Orion, etc.
ORIGIN OF THE EARTH
• Contracting cloud formed our solar system
– H2, H2O, Fe, Silicates, HCN, NH3, CH4, H2CO, and
other small inorganic and organic molecules present
• Planets formed ~ 4.6 – 4.5 billion years ago
• Earth was hot
– Asteroid impacts, internal compression, radioactive
decay of minerals
– Much of rocky interior melted
– Many heavier elements moved toward interior
– Lighter elements floated toward surface
EARTH
• Crust
– Surface zone
– Basalt, granite, and other low-density rocks
• Mantle
– Interior to crust
– Intermediate-density
rocks
• Core
– High-density, partially molten nickel and iron
EARTH
• Earth 4 billion years ago
– Thin-crusted inferno
• Earth ~3.8 billion years ago
– Life arose, but how did this happen?
EARLY ATMOSPHERE
• 1950s: Stanley Miller & Harold Urey recreated the
assumed early atmosphere
– Contained H2O, H2, CH4, NH3
– Lacked free O2
– Energy input in forms of heat
and electrical sparks
• Mimic geothermal heat and
lightning
ORIGIN OF LIFE
• Can we identify the physical and chemical
conditions that prevailed on the Earth when life
originated?
• Do the known principles of physics, chemistry,
and evolution support or disprove the hypothesis
that organic molecules formed spontaneously
and evolved into molecular systems with the
fundamental properties of life?
• Can we design experiments to test the hypothesis
that living systems emerged through chemical
evolution?
ORIGIN OF LIFE
• Unfortunately, our understanding of the origin of
life is incomplete
• Many laboratory experiments lend support to an
abiotic origin of life through chemical evolution
EARLIEST LIFE
• Life arose ~3.8 billion years ago
• The earliest cells were prokaryotic
– Lack a membrane-bound nucleus
• Early in the history of life, populations diverged
into two major lineages
–  bacteria
–  archaea & eukaryotes
EARLIEST LIFE
• How do we know that domain Eukarya is more
closely related to domain Archaea than to
domain Bacteria?
– Analysis of rRNAs
and other highly
conserved genes and
proteins provide the
strongest evidence
TIMELINE