Part II of course: Origin of LifeThe basic idea we will be discussing assumes that life arose as asequence of increasingly complex chemical reactions, driven by the varietyof energy sources present in the early Earth environment. In this model thebasic atoms used by life here on Earth, provided by nuclear reactions instars, became more and more complex molecules (“monomers”--likesugars, amino acids, nucleic acids), which finally made the (difficult)transition to functioning biological molecules (“biopolymers” like lipids,proteins, RNA, DNA or their equivalents). This is sometimes called the“chemical evolution theory,” but it is more like a working hypothesis, andthere are several different theories, related to a lot of empirical andlaboratory evidence, for the details of how this came about. atoms planetary temperatures molecules energy [C, N, O [methane, ammonia (??); from stars; water vapor, carbon dioxide, H from big bang] hydrogen cyanide HCN, …] ?? more complicated proteins coupled molecules (e.g. amino membranes DNA,RNA, acids, lipids, nucleic a self-replicating proteinsacids) molecule (RNA?) (prokaryotic) evolutionary process complex (eukaryotic)cells multicellular life (~0.5 billion yr) (~3 billion yr)Here are two visual representations of the general process we have inmind, although details of the order of events (e.g. cells, or metabolism,before genomes? a pre-RNA-self-replicator?Here is a more detailed (and colorful!) picture of the kind of developmentsthat we must be able to account for, from monomers (bottom) to complexcells (top):What is “life”? (Not a trivial question!)Historically and cross-culturally, several ways of viewing this: reductionism, emergentism, vitalism, animism, mechanism holism panpsychism, pantheismNearly all the ideas you will read about are to the far-left in this sequence, since that isthe realm of science (although there is a little leaning toward the center).Some suggested attributes of life (or a living process): (no special order;compare with list suggested in textbook) First, notice that life is a pattern in space and time, not a specific materialobject, e.g. most of our cells are replaced many times. So maybe more like aprocess or a function or an interaction. metabolism (“eating”) increase in order--life locally halts or reverses entropy increase reproduction morphogenesis-growth, differentiation(w/mutation?) and evolution of form sensitivity to possession of a “genetic” programexternal stimuli death maintenance of a structural boundary excretion storage of energy for later use processing of matter utilization of external energy and information and materialsNotice that no single attribute defines life, and every attribute has acounterexample. Reproduction with genetic-like evolution are todayusually considered the primary attributes—as in your text.Other important considerations: 1. Information storage of a self-representation, e.g. earth organisms storea description of themselves in DNA molecules, interpreted byprotein/RNA “machinery.” [This is mostly for repair and reproduction. . .But we also have other representations---our sense of “identity,” “self-consciousness.”] 2. Functional interactions with the environment---responds to oranticipates changes in environment.Problematical example: "Artificial life" cellular automata (discussed later)The Chemical Basis of LifeCompare abundances of elements in life, Earth, and rest of Universe.• Life---4 types of atoms account for >97% of the atoms in living things: hydrogen (H), oxygen (O), carbon (C), nitrogen (N) ~60% ~25% ~10% ~2%+phosphorus, sulfur, and calcium (about 0.1% each)+trace elements (e.g. iron, zinc, magnesium, manganese, . . .) <0.01%• Earth---mostly oxygen (47%) + silicon (28%) + Fe, Mg, Al, S, Ni, Na, Ca, P,... C,... NBut this includes the land masses; if we only include oceans andatmosphere, H, O, C, N may have been most abundant.Abundances of trace elements in bacteria, plants, land animals, may correlatewith abundances in sea water. [Not so easy to demonstrate quantitatively!] Τhis suggests that life began in oceans (not so certain) and that life arose onEarth, not elsewhere (“panspermia”). Or that water and life were delivered together!(Comets, asteroids, …) We’ll return to evidence related to this later.• Sun + Stars---remarkable uniformity [abundances derived from analyses of spectrallines]. [These are abundances by number, not mass.]H 93% [*He, Ne are “noble gases,” unreactive]4He 7% everything O 0.06% *Ne 0.02% else ~0.1% N 0.01% C 0.03 ........ . So H, C, N, O most abundant in stars and universe as well as in life.→ Suggests that our type of life, HCNO (+important trace elements), is the most likelytype in the universe, based on availability.A pie chart showing the composition of the human body is shown on the next page.More support for this conclusion:• discovery of amino acids and other biologically-related molecules inmeteorites (350, compared to 46 on earth)*; sugar discovered in 2001. See Table below.You will soon become familiar with the types of molecules listed!• discovery of complex prebiological molecules in interstellar space (discussedin Part I of the course).*1994:Biologist Carlisle claims at least 2 of them are dandruffMolecules of LifeA. Basic chemistry nucleus (pos. charge) protons (+) and neutrons (no charge)atoms electrons (negative charge)Atomic number = number of protons (e.g. 6 for carbon C): unique characteristic of eachelementAtomic mass (or weight) = number of protons plus neutrons (e.g. 12 for 12C).1H4He12C14N16O# protons: 1 2 6 7 8If atom is in its (normal) neutral state, then the #electrons = #protons. If the #electrons isgreater or less than #protons, then we refer to it as a (negative or positive) ion.Isotopes: same element but different number of neutronse.g. 2H (deuterium), 3He, 13C (~1% of C), 14C (about 10-12 of C in earth’s atmosphere).Too many neutrons: isotope is
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