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Beginning Of Life Essay

2150 words - 9 pages

AST 309
part 2:
Extraterrestrial Life

The Origin and Evolution of Life on Earth

• The formation of Earth
• Pre-biotic chemistry (Miller-Urey exp.)
• First evidence for early life
• The evolution of life
• Extreme life on Earth: lessons for astrobiology

A timeline for the very early history of the Earth

The formation of Earth:

The Earth formed over ~50 Myr via planetesimal accretion

Earth differentiation:
The iron "drops" follow gravity and
accumulate towards the core. Lighter
materials, such as silicate minerals,
migrate upwards in exchange. These
silicate-rich materials may well have
risen to the surface in molten form,
giving rise to an initial magma ocean

Early Earth ...view middle of the document...

A timeline for the very early history of the Earth

In order to be able to find life outside our Earth, we have to
understand life in our own planet. The chemistry of life and the
different processes during the formation and evolution of the
Earth have played a crucial role.

Is life on Earth a very special thing ?
Can life spawn spontaneously elsewhere ?

Tiny zircons (zirconium silicate crystals) found in ancient stream deposits indicate that
Earth developed continents and water -- perhaps even oceans and environments in
which microbial life could emerge -- 4.3 billion to 4.4 billion years ago, remarkably soon
after our planet formed. The presence of water on the young Earth was confirmed when
the zircons were analyzed for oxygen isotopes and the telltale signature of rocks that
have been touched by water was found: an elevated ratio of oxygen-18 to oxygen-16.

A timeline for the very early history of the Earth

How to understand an astrobiologist (or any
Monomer: usually a small molecule that can bind chemically to form a
polymer (amino acids are monomers)
Polymer: a macro-molecule composed of repeating structural units. Proteins
and nucleoacids (RNA & DNA) are polymers
Protein: a biochemical compound that facilitates a biological function
Encyme: are proteins that catalyze (e.g. increase rates) of chemical reactions
RNA: Ribonucleic acid, a macromolecule made of long chains of nucleotides (1
base, 1 sugar and a phosphate group). Single strand, can carry
genetic info.
DNA: Deoxyribonucleic acid, double-helix shaped macromolecule made of
nucleotides. Carries genetic information.

The Miller-Urey experiment
In the 1930s, Oparin and Haldane independently suggested
that ultraviolet radiation from the sun or lightning discharges
caused the molecules of the primordial atmosphere to react to
form simple organic (carbon-containing) compounds. This
process was replicated in 1953 by Stanley Miller and Harold
Urey, who subjected a mixture of H2O, CH4, NH3, and H2 to an
electric discharge for about a week. The resulting solution
contained water-soluble organic compounds, including several
amino acids (which are components of proteins) and other
biochemically significant compounds.

Problem: The assumed atmospheric composition

The experiments only give large yields of interesting
organics (amino acids, nucleic acids, sugars) if the gas is
H-rich (highly reducing). If the early atmosphere was CO2
+ N2 (mildly reducing), as many suspect, the yields are

Atmosphere from
volcanic outgassing?

This would give atmosphere
rich in CO2, N2, and H2O.
Not the composition that
favors Miller-Urey synthesis.

Could the original atmosphere have been delivered to the Earth from
comets, asteroids, …? Perhaps then the composition would be H-rich.
What was the source of the early Earth’s atmosphere? Not necessarily “endogenous” (there from the
start). Outgassing from the crust due to volcanoes (top two), or planetesimal impact (lower left),...

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