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Review of Lecture 8: In the beginning: The origins of life


1. Conditions on earth after it was formed about 4.5 billion years ago were very different from what we have today: No O2 and N2 gas. Early atmosphere: H2, He, CH4, NH3, H2O.

2. UV radiation was high (no ozone layer); energy from lightning and internal heat sources; lots of chemical building blocks in the environment.

3. Organic compounds formed naturally through random process in environment. Lipids, amino acids and proteins, nucleotides and nucleic acids all formed from environmental interaction.

4. Self-assembly of lipids into liposomes was the beginning of cell evolution. Nucleic acids are self-replicating, amino acids can spontaneously aggregate into polypeptides. Lipids can engulf other molecules.

5. Early cells were the results of millions of years of natural selection for self-replication. Fossil record reveals the earliest cells formed at least 3.5 billion years ago. Early life on earth was prokaryotic (and that is still the dominant life form today).

6. Early life used the energy available in the environment -- ingestion of organic molecules and chemosynthesis -- similar to photosynthesis but electron donor is sulfur and not oxygen. Early life was anaerobic -- oxygen was poisonous.

7. Oxygen began to accumulate in the atmosphere after prokaryotic cells evolved to use sunlight as an energy source. There was a major change in the atmosphere -- going from a "reducing" atmosphere to an "oxidizing" atmosphere. The iron ions present in the oceans reacted with oxygen to form ferric oxide (rust) and over several million years of time the iron all precipitated out of the oceans to form deep sediment layers -- that's how we know about when it happened on earth.

8. Autotrophism began with photosynthesis (cyanobacteria). The next major innovation in life was the development of the eukaryotic cell.

9. ENDOSYMBIOTIC ORIGIN OF EUKARYOTIC CELLS: One cell engulfed another cell and stabilized the engulfed cell. Each cell contributed to the organism. Examples: 1. photosynthetic cell engulfed by heterotrophic cell -- the photosynthetic cell functioned the same way that chloroplasts do today. 2. Aerobic prokaryotic cell engulfed by another -- function similar to mitochondrion.

10. Evidence for endosymbiotic origin: 1) chloroplasts and mitochondria are anatomically similar to prokaryotic cells, 2) have similar types of ribosomes as prokaryotes, 3) react to similar antibiotics as prokaryotes, 4) have similar biochemical pathways, 5) have separate genomes from the nucleus, 6) nucleic acid sequences are most similar to prokaryotes, and 7) we can observe endosymbiosism today -- e.g., the sea slug stabilizing chloroplasts from algae.


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Last updated April 18, 1997.