The Origins of Life
• The Big Bang occurred about 18 billion
years ago, creating the universe.
• The Earth formed about 4.6 billion years
• Life on Earth probably originated between
3.5 and 4 billion years ago.
Evidence of First life
• Stromatolites are
mounds of banded
sediment formed by
• They are found in the
warm waters of
Australia and off the
coast of Baja, Mexico.
Evidence of First Life
• Stromatolites have
been found that date
back to 3.5 billion
• They are strikingly
similar to those
produced by modern
Evidence of First Life
• Fossilized filamentous
bacteria has also been
found that dates back
to 3.4 billion years.
• This fossilized
prokaryote was found
in a 3.4 billion year
old stromatolite in
• When the Earth first formed, it was very
different from its current state.
• It was filled with reducing gases, like H2,
NH3, CH4, CO, and CO2.
• Most notably, the atmosphere of early Earth
was completely anaerobic.
• It lacked a protective ozone layer.
• In the 1920’s, Oparin
and Haldane predicted
that the conditions of
early Earth favored the
of organic compounds.
• In 1953, Miller and
Urey tested this
From Molecules to Cells
1. Small organic monomers formed in
2. Monomers joined to form polymers –
binding sites in clay helped concentrate
the monomers, while metal ions acted as
3. Polymers aggregated to form protobionts.
4. A molecule of heredity originated.
• Protobionts are
aggregates of molecules
that have some cell-like
qualities such as:
– Response to osmotic
– Ability to store a
– Engulfing and splitting
• RNA as genetic material
• Heterotrophic – absorbed organic molecules from
• Anaerobic – oxygen was not present in
atmosphere until it was put there by
photosynthetic bacteria as a waste product (began
accumulating about 2 billion years ago
Origin of photosynthesis
• Some time after the first living things appeared,
cells developed that could perform photosynthesis.
• These organisms put oxygen into the air as a waste
• About 1.5 billion years ago, oxygen still only
made up about 1% of the atmosphere
• Accumulation of oxygen in the atmosphere is
thought to have ended chemical evolution
Open for debate
• Some scientists believe organic molecules came to
Earth from outer space. Meteors have been found
containing amino acids and nucleic acids
(including several amino acids nonexistent on
• There may have been a simpler hereditary
molecule that preceded RNA, or maybe RNA was
never genetic material.
• Maybe life began near hydrothermal vents (and
still forms there?)
Taxonomy – the classification of
• From that first cell,
life has diversified
into millions of
• These forms are
divided into five major
Protista, and Monera.