How? How do you know? NaturalDon’t KnowSupernatural Autonomous Don’t KnowExternal NaturalAutonomous SupernaturalExternal

The Origin of Life on Earth: Story Elements The Hadean Eon witnessed the accumulation of water and chemicals from space (CH 2) Two hypothetical origins –Space Earth was seeded with cells from elsewhere (CH 3) –Chemical evolution Life arose from non-life by the combination of chemicals on the surface of early Earth

The Origin of Life on Earth: Story Elements Chemical evolution –Random inorganic interactions produced molecules able to form similar molecules –Information in simple molecules enabled the synthesis of larger, stable, complex shapes –Larger, stable, complex molecules could participate in new kinds of chemical reactions (CH 1)

The Origin of Life on Earth: Story Elements Chemical evolution –Interacting systems of molecules became enclosed in compartments –Control was exerted over the entrance, retention and exit of molecules and the reactions that were taking place –This doesn’t happen any more (CH 1)

The Origin of Life on Earth: Story Elements Chemical evolution –Experimental support? Stanley Miller –Strongly reducing atmosphere »CH 4, NH 3, H 2, H 2 O –Electrical discharge –Building blocks of life: amino acids (CH 3)

The Origin of Life on Earth: Story Elements Chemical evolution –The atmosphere was not strongly reducing CO 2, H 2 S, SO 2, N 2 –Optically active isomers were selected from racemic mixtures –Building blocks had to be polymerized Clay minerals Deep ocean vents (CH 3)

The Origin of Life on Earth: Story Elements Chemical evolution –Which came first - DNA or protein? –Or RNA? RNA enzymes (ribozymes) can perform some catalytic functions Peptide bonds are formed by the large ribosomal subunit DNA is made from RNA by reverse transcriptase (CH 3)

The Origin of Life on Earth: Story Elements Chemical evolution –Doesn’t happen today - as far as we know The strongly oxidizing atmosphere favors the reverse process Hungry heterotrophs would consume any reduced compounds Comparable energy sources are not present (CH 3)

The Origin of Life on Earth: Story Elements Life from chemicals –Life requires compartmentation Lipids form bubbles in water –Lipids + prebiotic chemistry + RNA world = the bubble theory of life’s origin (coacervates) (CH 4)

Video Introduction Remember your biochemistry - –life is based on reduced carbon compounds oxidized carbon (CO 2 ) is reduced by NADPH in the Calvin Cycle today, the atmosphere is strongly oxidizing (C as CO 2 ).

Video Introduction “Life” implies reproduction of “like” offspring - a hereditary mechanism –amino acids and nucleotides can assemble in any order with equal likelihood a 100 a.a. polypeptide can be assembled in ways (1.27 x ) a 300 a.a. polypeptide can be assembled in ways (2.04 x ) –that’s raw probabilities

a (specific) hereditary mechanism “Life” implies reproduction of “like” offspring - a hereditary mechanism –amino acids can assemble in several ways other than amino-carboxyl chemical synthesis of a.a. produces both D and L stereoisomers; proteins have only L non-protein amino acids occur in nature under ideal conditions, odds are small that’s chemistry

a (specific) hereditary mechanism solution? –“an infinite number of monkeys typing on an infinite number of keyboards will eventually produce all of the great works of literature” –with infinite time, random combinations of building blocks will assemble functional proteins …but

a (specific) hereditary mechanism earth has been habitable for ~ billion years (plenty of time for Darwinian evolution) life has been present for ~ 3.85 billion years (apparently) –life appeared in ~ million years that’s really fast…

Video Introduction The Origin of Life: Researching the Possibilities –Contains excerpts from “The Great Computer” a parable from The Hitchhiker’s Guide to the Galaxy. –Originally broadcast by the BBC under the title, Life is Impossible