Transcript Where Did Life Come From?

Where Did Life Come From?
Early thoughts about where life came
from:
• For a long time, it seemed as if life just
appeared.
• As far back as Aristotle (4 B.C.) people
believed in spontaneous generation, the
idea that non-living objects could give life
to living organisms.
Spontaneous Generation
Recipe for Bees
(Roman poet, around 20 A.D.)
1. Kill a bull during the first
thaw of winter.
2. Build a shed.
Beetles in cow dung
Maggots found on meat
3. Place the dead bull on
branches and herbs inside
the shed.
4. Wait for summer. The
decaying body of the bull
will produce bees.
Mice in grain
Francesco Redi, 1668
• Maggots often seen on rotting meat
• Logical conclusion at the time: maggots must come
from meat
• Redi set up an experiment:
• Conclusion: Maggots came from flies, not meat.
Spontaneous generation did not occur.
Invention of Microscope
• Anton van
Leeuwenhoek, 1674
• Allowed scientists to
see living things that
they couldn’t see before
Lazzaro Spallanzani, 1765
• Boiled 2 flasks of broth
– One open to the air:
microorganisms grew
– One sealed: no
microorganisms grew
Conclusion: living things came from the air. Supported Redi’s hypothesis, however,
many thought there was a “life force” in the air.
Louis Pasteur, 1864
• Designed a flask with a
curved neck
– Allows air in
– Does not allow
microorganisms in
• Boiled broth
• Stayed clear!
Conclusion: Living things only come from other
living things. Theory of Biogenesis.
How did the first life form arise?
We don’t know, but we know the conditions of the early earth
Formation of Earth
• Swirling gas and dust
• Some dust collapsed and formed
the sun and planets
• Meteorites hit earth and created
liquid lava
• As liquid settled, heavy elements
sunk toward core, lighter ones
floated on top. Light gases
formed the atmosphere.
• As the earth cooled, water
condensed to form ocean 3.8
billion years ago.
• Earliest rocks with fossil formed
3.5 billion years ago.
ircamera.as.arizona.edu
Conditions of the early Earth
• Anaerobic
• Contained ammonia,
methane, hydrogen gas, and
water
• High temperatures, above
100 degrees C
• Amino acids formed and
collected in the water as the
Earth cooled
mwsu-bio101.ning.com
Miller and Urey’s Experiment
We can make predictions about how the first organic molecules
arose.
• Stanley Miller and Harold Urey conducted experiments in
the 1950s that suggest how mixtures of the organic
compounds necessary for life could have arisen from
simpler compounds present on the early earth.
• Carl Sagan describes Miller and Urey’s experiment
http://www.hulu.com/watch/63327/cosmos-one-voicein-the-cosmic-fugue
(47:30-51:35)
Proteinoid Microspheres
• Large organic molecules can
sometimes form these tiny
bubbles
• Not cells, but have some
characteristics of cells, like
membranes, and means of
storing energy.
• Some hypothesis suggest
that structures like these
may have been precursors
to living cells
(image courtesy of Cornell University)
Early life forms
• Fossils show that singlecelled prokaryotes lived 3.5
billion years ago, in the
absence of oxygen
• Photosynthetic bacteria
lived 2.2 billion years ago
http://parts.mit.edu/igem07/index.php/Image:Organisms.jpg
http://www.sciencewithmrmilstid.com/wp-content/uploads/prokaryote.png
What does
photosynthesis
produce?
Oxygen!
How did the living organisms respond to the rise of
oxygen in the atmosphere?
• Some went
extinct
•Some found new
places to live
http://whyfiles.org/coolimages/images/csi/nur04506.jpg
•Some developed
ways to use
oxygen and
protect
themselves from
oxygen’s reactive
nature…
Life as we know it: Eukaryotes
Cells with a nucleus
and other organelles
which enable them to
perform more
complex functions.
How did they get
here?
The Endosymbiotic Theory
Eukaryotic cells arose from
associations formed between
prokaryotic organisms.
http://www.biology.iupui.edu/biocourses/n100/2k2endosymb.html
•Certain prokaryotic cells became ingested
by another cell and formed a symbiotic
relationship which is not harmful to either
organism.
•The interior prokaryotes were
mitochondria and chloroplasts.
This discovery was made about 100
years ago when scientists observed
that the membranes of these
organelles looked like bacterial
membranes.
http://www.cartage.org.lb/en/themes/Sciences/Zoology/AboutZoology/SymbioticTheory/EndosymbioticTheory/symb.jpg