Biology 20C - Fall 1998

Biology 20C - Fall 1998

ECOLOGY AND EVOLUTIONARY BIOLOGY

Essentials Precursors for Origin of Life:

1.	Temperature	< 450^oC (most organic molecules stable); > -70^oC (CO₂ and NH₃ are gases)
2.	Liquid water	Restricts temperature to 0^oC - 100^oC; unless at high pressure or in concentrated solutions
4.	Energy	Solar or chemical
3.	Carbon	In simple organic molecules
5.	Nutrients (N,P..	In simple soluble inorganic molecules or ions

The order of events and processes are now widely accepted, but the timing, details and relative importances of particular steps remain under intense discussion, and continue to be revised. Life probably evolved almost immediately after appropriate conditions were established (4.2 Gya or even earlier) on an Earth that:

was warmer than today's tropics (due to greenhouse effects of high CO₂);
was extremely active volcanically;
had a strongly reducing, oxygen-free atmosphere;
was exposed to intense solar radiation (including UV);
was still experiencing declining bombardments by rocky asteroids and icy comets.

Evaporative concentration of molecules in shallow pools probably completed creation of conditions most likely to favor most of the stages necessary for living cells to evolve from simple inorganic molecules:

Molecular and Structural Stages
	Self-assembling monomeric molecules (amino acids, sugars, lipids, purines, pyrimidinesÖ)
	Polymers and colloids (polypeptides, proteinoids, nucleotides, RNAÖ)
	Self-assembling protobionts:
		- microspheres (protein membranesÖ)
		- liposomes (lipid bilayers)
		- coacervates (proteins, lipids, polysaccharides, nucleic acids, enzymes (absorb substrates, release products, grow, divide …)
	Self-replicating molecule (RNA?) catalyst (but unstable)
	Shift to DNA stable information transfer ( = inheritance)

Accumulation of these pre-living molecules and structures probably exhausted available precursor inorganic molecules and chemical energy (free ATP), leading to intense competition and natural selection favoring those protobionts best able to sequester organic molecules at low concentrations, make energy storage molecules, and retain them (within membranes).

The subsequent histories of the earth and of life have been closely integrated, linked by:

physical/chemical environmental changes either leading to and/or being caused by major macro-evolutionary steps;
increasing modification and buffering of physical and chemical environments as a result of biological activity;
increasing competition among organisms for resources, nutrients and energy.

The cumulative effects of the environmental Earliest Metazoan fossils known (Ediacaran fauna)modifications produced by living organisms ensure not only that today's Earth is very different from the one on which life arose, but also that it is now one on which it would probably be impossible for life to arise.

Timeline of Earth and Life History
(ages in Ga = billion years ago)

>14 Ga	"Big Bang" origin of the universe
13	Origin of galaxy ("Milky Way")

4.6	Supernovae create inter-stellar cloud of cold gas and dust (= nebula)
4.55	Supernova initiates gravitational collapse of nebula into a spinning cloud
		center (He-Xe, H, C, N, O) increases in mass and temperature until many elements and minerals vaporized
		rest of nebula collapses into a spinning disc in equatorial plane
		gravity pulls heavier elements (metals) and highly refractory minerals (silicates) inwards until balanced by their increasing angular momentum
		center reaches critical mass and temperature and ignites sun
		solar wind pushes lighter molecules outwards
		low velocity collisions and accretion planetesimals create (<1 km) small asteroids 10-100km), protoplanets (<500km),and planets (>1000km)

4.5	Solar System established
		Sun: mainly gases (He-Xe, H, C, N, O); 99.7% of system mass
		Inner Planets (M, V, E, M, asteroids): high density; hot; mainly metals and refractory minerals (Ca, Al, Fe, Mg, Si)
		Inner Gas Giants (J, S): low density: cool; similar to sun (He-Xe, H, C, N, O)
		Outer Gas Planets (U, N): very cold; mainly gas(H,C,N,O) and ice; very cold
		Pluto: high density; very cold; an asteroid with an unusual orbit?
		Comets: outer solar system; organic/inorganic ice (CO_2,H₂O, NH₃, CH_4,HCN...
4.5		Asteroid bombardment - high velocity collisions heat, melting, volcanics,
		degassing 1^o atmosphere (CO_2,CO, H₂, N₂) reducing environment
4.3		Cometary bombardment of ices 2^o atmosphere (CH₄, H₂, CO, CO_2,H₂O, NH_3,HCN, CH₃CN, N₂...) oceans organic molecules

4.2	ORIGIN of LIFE? - (reducing atmosphere; seismically active; high UV; high CO₂)
4.1	Oldest chemical "fossil" traces of life
3.85	Oldest cellular fossils known (Prokaryota)

3.55	Oldest biological "reefs" known (Stromatolites)
	Photosynthesis has evolved, probably in organisms similar to Cyanobacteria
		oxygenation of atmosphere begins
		gradual shift from reducing to oxidizing atmosphere
		gradual reduction of CO₂ and gradual lowering of global temperatures
		precipitation, accumulation of mineral oxides (especially Fe, Al, Mg, Ca) and carbonates leading to permanent changes in ocean chemical composition
		production and accumulation of atmospheric O₃ and increasing UV shielding
		3^o atmosphere (N₂ , O₂ ) [Greatest pollution event in history?]
		elimination of most (all?) other lineages (can't tolerate oxidizing environment) [Greatest mass extinction in history?]

2.7	Earliest glaciation?
2.5	Oxygenation accelerates; aerobic organisms become the norm

2.1	Oldest fossil Eukaryota known (unicellular) - [follows glaciation?]
1.0	Multicellular animals (Metazoa) and plants (Algae) - [follows glaciation?]
0.59	Earliest Metazoan fossils known (Ediacaran fauna) - [follows glaciation?]
0.55	Evolution of Skeletons (start of Cambrian) - [follows glaciation?]
0.47	Plants invade land (Devonian); animals follow

"Systéme Internationale" or S.I. Units

Units andAbbreviations:

	Distance:	1 meter	m
	Mass:	1 kilogram	kg
	Time:	1 second	s
	Temperature:	1 degree Kelvin	o K

Common S.I. prefixes and symbols:

G = Giga = 10⁹

M = Mega = 10⁶

k = kilo = 10³

c = centi = 10²

Logarithmic vs. Arithmetic time scales: