Download Brief Summary of Life on Earth

Brief Summary of Life on Earth
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Earth is about 4.6 billion years old.
o Earth at the start was hot, regularly rocked by asteroids and by lightning storms, covered in
liquid rock, under immense radiation, and surrounded by an atmosphere of hot gases
o Earth cooled, and water vapour condensed, causing massive rainstorms for 100s of years,
which formed the oceans.
Life appears to have originated on Earth about 3.5 billion years ago.
o Urey and Miller created an apparatus to represent early earth, and found that large carbonbased organic molecules formed (nucleotides, amino acids, glucose, etc)
o Others have shown that simple proteins can form spontaneously, and that certain RNA can
replicate themselves (not perfectly, but making mistakes (mutations) is important for life!)
o Lipid membranes can form spontaneously (drop of oil on water)
o Possibly, this is how life began.
Oldest known fossils of cells on Earth are 3.465 billion years old! They are very similar to anaerobic
photosynthetic bacteria that live today. But life must have started before that.
Probably, the first organisms were anaerobic, prokaryotic, autotrophs that depended on
chemosynthesis of molecules in their environment (like some archaebacteria still do today) and
could withstand high temperatures. As they reproduced, the molecules they used in
chemosynthesis would decrease, so any cell that was able to use simple inorganic molecules and an
alternative energy source (i.e. Photosynthesis) would have had an advantage.
Photosynthetic bacteria would release oxygen into the atmosphere, which would have killed off
many of the anaerobic organisms. Some of this oxygen would reach the upper atmosphere and
form an ozone layer, which would limit the amount of radiation reaching Earth’s surface.
Any organisms that were able to consume other organisms (heterotrophs) would be successful.
These earliest cells probably gave rise to the eubacteria and archaebacteria.
Prokaryotes differ from eukaryotes in that prokaryotes have no membrane-bound organelle
(nucleus, mitochondrion, etc). The first membrane bound organelle probably arose as the cell
membrane folded in, which would increase surface area. As this folding increased, a nucleus,
endoplasmic reticulum, vacuoles and golgi bodies formed.
Evidence suggests that eukaryotes arose from a line of archaebacteria that ingested but did not
destroy eubacteria that were very good at cell respiration (breaking down sugar into usable
energy). An endosymbiotic relationship was formed, where the eubacteria provided energy to the
archaebacteria, and the archaebacteria provided the eubacteria a place to live. Eventually, neither
cell could live without the other, and the eukaryote was born.
o Evidence that supports the endosymbiotic theory:
 Double membranes on mitochondria and chloroplasts – as though they were ingested
through phagocytosis
 They have their own DNA, distinct from that of the cell.
 They replicate and divide on their own, independently of the cell.
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Some of their proteins are more similar to bacteria than to the proteins produced by the
cell they’re in.
o Other organisms alive today perform endosymbiosis, including humans. We ingest bacteria (in
yogurt), let it live in our gut and help us digest our food.
Multicellular organisms evolved about 1.5 billion years ago. This represents a big step in evolution
because it means that cells need to communicate with each other so they can work together.
Plants, fungi and animals evolved less than 750 million years ago, in water. The move to land took
another 300 million years!
Analysis:
On a One Month Calendar, the major events would be…
Week
M
T
W
Th
1 (4.5 –
3.5 bya)
2 (3.5 –
2.5 bya)
3 (2.5 –
1.5 bya)
4 (1.5 –
0.5 bya)
5 (0.5 bya
– present)
F
S
Su
Evolution depends on time, mutations, luck and selective advantages. For each major event listed above
(there are 6), determine what selective advantage the “new” organism would have had. Four of the
major events tell you the advantage, but the last 2 you need to figure out.