Chapter 14: History of Life (on Earth) Modern Biology © 2009. Holt, Rinehart, & Winston, pp 278-290.

Biogenesis vs. Spontaneous Generation Current view All life comes from preexisting life Precept of the Cell Theory Spontaneous Generation Popular before 1600s Life could arise from nonliving things Maggots from rotting meat Fish from dried ponds

Francesco Redi (1626-1697) Redi noted that maggots arose where adult flies had previously landed Maggots turned into sturdy oval cases Flies emerged from those oval cases Did 1st experiment in 1668 showing that maggots only arose from uncovered meat. Concluded flies gave rise to maggots, not the meat

Lazzaro Spallanzani (1729-1799) Tested if microorganisms arose spontaneously Had 2 separate groups of broth-filled flasks Boiled broth in all flasks Left 1 group open to the air but sealed others Only broth in the open flasks became cloudy Opposition claimed air’s “vital force” was absent from closed container & this prevented microorganisms’ growth

Louis Pasteur (1822-1895) Addressed Spallanzani’s experimental design Air’s “vital force” must have access to broth Used broth in curved-necked flasks Curve allowed air exchange with the room Curve prevented solid particles’ entry to flask Boiled broth to kill microorganisms in flasks Left flasks undisturbed for a year  clear Broke off curved neck allowing access to air After 1 day  broth was cloudy

Life in a New Solar System Our Solar System’s History Initially part of swirling mass of stellar dust & gas Gravity pulled most together  our sun Occurred around 5 x 109 years ago Our Planet’s History Dust & gas continued revolving around sun’s mass Debris continually collided with itself  planets Earth formed over ~ 4 x 108 yr span of collisions

Estimating Age Geological history = stratification of surface Radiometric dating Different isotopes have different half-lives Radioactive isotopes have constant half-lives C-14 = 5.7 X 103 yrs U-238 = 4.5 x 109 yr U-235 = 7 x 108 yrs Th-232 = 1.4 x 1010 yrs Calculate age of sample by comparing quantities of stable element with its isotope

Earth’s History Oldest rocks & crystals = ~ 4 x 109 yrs Collisions with space debris ceased Proposed early atmosphere NH3 or N2, H2, H2O, & CH4 or CO2 Synthesis of Organic Compounds Miller-Urey Apparatus tested Oparin’s hypothesis Treated compounds with heat & electricity Produced amino acids; later ATP & nucleotides too

Molecules to 1st Cell-like Structures Fox, et al, mixed simple organic compounds Spontaneous formation of cell-like structures Microspheres Protein molecules Membrane organization Coacervates Collections of droplets Composed of lipids, amino acids, & sugars No response to natural selection No hereditary material

Cyanobacteria structure

First Life Forms RNA may have been critical macromolecule Ribozyme; self-replication; basic biochemistry 1st cells? anaerobic, heterotrophic prokaryotes Ate up spontaneously formed organic molecules 1st autotrophs likely did chemosynthesis CO2 = C source; energy from oxidizing inorganics Archaea/ archaebacteria: hostile environments Photosynthetic prokaryotes @ 3.5 x 109 yrs Fossils look like our current cyanobacteria Produced O2  109 yrs  our atmosphere today

Earliest Evidence for life on Earth

Cyanobacteria

First Eukaryotes Differences Theory of Endosymbiosis Evidence Larger cells; DNA in chromosomes inside nucleus Membrane-bound organelles Theory of Endosymbiosis Aerobic prokaryotes engulfed by eukaryote Modern Chloroplasts & Mitochondria Evidence Independent replication Contain some unique DNA which is circular