Genesis: The Scientific Quest For Life's Origin, Robert M. Hazen, c. 2005

576.8 HAZ

pp. 63+

Cells: employ a remarkably quirky synthesis pathway
Isoprene: linear, C-H molecule; 5-carbon
Squalene: six isoprene molecules joined together, folded up -- to form a 4-ring sterol backbone, precursor to cholesterol

take-home lesson:
life is exceedingly choosy about its chemistry
of the millions of known organic molecules with up to a dozen carbon atoms, cells typically employ just a few hundred
this selectivity is perhaps the single most diagnostic characteristic of living versus nonliving systems
if ancient rock contains a diverse and nondescript suite of organic molecules, we can't make much of it
but, if the ancient rock is found to hold a highly selective suite of carbon-based molecules -- predominantly even-numbered hydrocarbon chains or left-handed amino acids, for example -- then that's strong evidence that life was involved
biomolecules must be stable over long periods of geologic time
large protein molecules won't last long; and, neither will the 20 amino acids that comprise the building blocks of proteins
over time, water attacks the bonds of these biomolecules, breaking them into smaller units of no diagnostic use
but polycyclic compounds, like sterols, degrade more slowly and might degrade over geologic spans of time
therein lies a possible top-down path to the discovery of life that is distant in space or time

Hopanes
distinctive sterol-derived polycyclic hydrocarbon molecules
5-ring molecules
2.7 billion year old shale -- shattered time line by about 1 billion years, back in 1999
2-methylhopanoid: a hopane variant known to exist in cyanobacteria
cyanobacteria: ancient; oxygen-generating

Biosignatures
distinctive molecules essential to cellular processes
stability: able to survive billions of years
must be common and relatively abundant

Polycyclic aromatic hydrocarbons
phenanthrene and anthracene ratio
life: 10:1
unambiguously biological specimens never seem to be less than 2:1 ratio

Mars, like Earth formed 4.5 billion years ago

Part II: Stanley Miller and Harold Urey

Miller-Urey lightning in a bottle, and voila! Life.

Part III: the great gap -- from molecules to cells

• ribose: a 5-carbon sugar
• Carl Woese
• prior to Woese, five kingdoms
• animal
• plant
• fungi
• proctoctists (complex single-celled organisms with a membrane-bound nucleus
• Woese
  the Eukarya: animals, plants, fungi, eukaryotes: similar biochemical processes
• prokaryotes: Bacteria and Archae
• so, three groups: Eukarya, Bacteria (well recognized because of association with disease); Archae (relatively unstudied)
• extremophiles

Water / self-organization
amphiphiles: elongated molecules with both water-attractant and water-resistant ends
pyruvate
minerals and rocks
Cairns-Smith
crystals: left vs right