|Lecture Notes: 5 September||
The oldest fossil evidence for life on Earth dates to about 3.7 by (billion years ago). The Earth itself formed about 4.5 by with the formation of our solar system. It is thought that the Earth was too hot and chaotic to support life until perhaps 3.8 by (intense bombardment of the earth did not end until 3.9 by, thus life arose quite quickly, essentially as soon as possible!
How did this occur? Obviously guess work - no one was there, and there is no record in the rocks that we could even be certain of. However, we have good guesses as to Earth's early environment (atmosphere of H2O, NH3, CO2 and smaller amounts of CH4, NH3, SO2, and H2. If you treat such an atmosphere with any high energy source in the laboratory (as was first done by Miller in 1953, text Figure 1-33) you will get a mixture of organic molecules including many important to organisms today. Interestingly, we also find small precursor molecules all over the Universe - in ancient rocks, meteors, comets etc. Evidence of small precursor molecules (amino acids, nitrogenous bases etc.) also occurs in interstellar space, the atmospheres of carbon stars, gas giant planets etc.
The formation of polymers is more problematic. A major difficulty is that biopolymers are all thermodynamically unstable relative to their hydrolysis products. Some theories, but no certainty as to how polymers may have formed, though polymers have been synthesized under conditions which may have occurred on the early Earth.
The biggest problem for the origin of life is the issue of how we go from polymers to living "systems."
Pre-Cambrian Life:1 (text Figure 1-35)
DNA vs. Fossils
1A slightly enhanced treatment, with photos of specimens at our natural history museum is available by clicking on the link.
Last modified 5 September 2008