Humboldt State University ® Department of Chemistry

Richard A. Paselk

Chem 431	Biochemistry	Fall 2007
Lecture Notes: 5 September	© R. Paselk 2007
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Origin of Life

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."

• Consider the problem of protein biosynthesis:
  • Going from DNA (information archive) to RNA (usable information) requires a rather complex system to assure accurate transcription today. However, we could assume a much simpler system in the early, low-competition, Earth.
  • Going from RNA to protein incredibly complex:
    • Need adaptor molecules (t-RNA) because there isn't any natural relationship between RNA codes (sequences) and particular amino acids.
    • Need enzymes to match specific amino acids to specific tRNA's because even the tRNA's are not specific to amino acids in term of recognizing them.
      • as a result, need 20 tRNAs + 20 proteins!
    • Need a complex molecular machine, the Ribosome, to read off the mRNA message using the tRNAs and make proteins:
    • Ribosome consists of
      • 1-2 small RNAs + two large, complexly folded, RNAs,
    • 50-100 accesory proteins.
  • The chance of such a system arising spontaneously is truly infinitesimal.
• "RNA World" has been potulated to solve these dificulties. (text Figure 1-34)
  • In this scenario RNA-based life preceeds "modern" life.
  • First "life" combined information and catalytic properties in single RNA-protein molecules.
  • Later there was a transition where proteins took over much of machinery over time.

Pre-Cambrian Life:¹ (text Figure 1-35)

• 3.8 - microfossils?
• 3.46 - stomatolites (Australia) [3.5-3.3 microfossils of cyanobacteria?]
• 2.8 kerogens with low ¹³C/¹²C indicative of O₂ use by methanogens
• 2.7-2.5 2-methyl-hopane "fossils" (cyanobacterial markers); steranes (eukaryotic molecular marker, oxygen needed for synthesis)
  • Origin of eukaryotes (text Figure 1-36)
• 2.5 -> stromatolite reefs rivaling modern reefs in size and architecture
• 0.6 Ediacaran fauna
• 0.54-0.53 "Cambrian explosion"

DNA vs. Fossils

¹A slightly enhanced treatment, with photos of specimens at our natural history museum is available by clicking on the link.

Pathway Diagrams

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Last modified 5 September 2008