HISTORY OF LIFE WRITTEN IN THE ROCKS (geological record): notice how at first no life, very simple if for billions of years, complex life only recently 600 mya In these chapters, two primary themes: History of Life How Life first began on the Earth First, an supershort History of the Earth by Eon 1
HADEAN EON: (Hades = Hell ): 4.5 BYA to 3.8 BYA (Billions of Years Ago) Formation of earth by accretion of cosmic debris earth hot, molten, no geological (rock) record dates for the eons come from dating of meteorites, moon rocks (assumed to be similar age) ARCHEAN: ( Ancient ): 3.8 to 2.6 BYA formation of major earth features (continents, ocean basins (oldest dated rocks) evidence of first (microbial) life at 3.5 MYA good bacterial fossils at 3.0 BYA (abundant bacterial life) PROTEROZOIC ( FIRST LIFE ): 2.6-0.7 BYA abundant bacterial life (cyanobacteria and the stromatolites) photosynthesis produces free oxygen in oceans, atmosphere for first time first eukaryote fossils (ancestors of multicellular life) PHANEROZOIC ( visible life ): 0.7 BYA (700 MYA) fossils of macroscopic life (visible to naked eye, e.g., shells, bones, plant impressions etc.) gradual increase with time in complexity of life, first fossils of invertebrates, fishes, then terrestrial vertebrates and plants 2
Memorize Table 20.1, pp. 380-381: Earth s Geological History know names, sequence, dates of eons, eras, periods know major physical changes, major biological events associated with each time segment GEOLOGICAL TIME PERIODS: first distinguished on basis of characteristic assemblages of fossils in rock layers ( strata ) formed from sedimentation and volcanic activity 3
Relative Dating of Rock Strata (and their fossils): younger strata lie above older strata (deposited from bottom to top) Each stratum shows different periods of sedimentation in different environments (marine, lake, glacial) etc. with organisms (fossils) living and dying at that time We know that younger rock strata lie on top of older strata: But what are the actual ages of these strata (and the fossils in them)??? ABSOLUTE DATING: actual ages determined by RADIOMETRIC (RADIOISOTOPE) DATING 4
A radioactive isotope of an element decays into a stable isotope of another element EXAMPLE: 92 U238 (uranium 238) DECAYS to 82 Pb 206 (lead 206) Parent (radioactive) isotope DECAYS to the Daughter (stable) isotope HALF-LIFE: the time it takes for 1/2 of the parent isotope to the daughter isotope For 92 U 238 to 82 Pb 206, 4.5 billion years but can be millions of years, thousands of years, a few seconds, depending on parent-daughter isotopes A Rock Layer with a mineral whose chemical composition contains the parent isotope can be used for dating The only source of the daughter isotope in the mineral is the decay of the parent isotope So, if we know the proportion of daughter isotope to parent isotope and the half-life of this parent daughter pair, can calculate the true or actual age of the sample 5
Now, back to the beginning of the earth ATMOSPHERE OF THE EARLY EARTH: N 2, NH 3, CO 2 (much more than now), CO, H 2 0, CH 4, H 2 gases now emitted from volcanoes, like in the early molten earth BUT NO FREE OXYGEN (0 2 )!!!!!! Chemically, a REDUCING not an OXIDIZING atmosphere (like today) Lack of free oxygen in the first 2 billion years of the earth seen in deposition of minerals that can only form in ANAEROBIC (oxygen-free) environment SEQUENCE OF EVENTS LEADING TO MACROSCOPIC LIFE IN THE PHANEROZOIC (1) Abiotic formation of organic molecules (2) Aggregation of organic molecules into metabolizing, self-replicating systems (first living organisms) first life forms were prokaryotic (bacteria) with heterotrophic nutrition; at first, the abiotic formation of organic molecules formed to base of the food chain So, at first, food chains and biomass (amount) of life were constrained by limited primary production 6
(3) Evolution of photosynthesis (autotrophs): tremendous increase in primary production, allowed increase in amount of life that planet could support Oxygen emitted as by-product of photosynthesis poisonous to most living beings (bacteria) but those which evolved an ability to use the oxygen to oxidize foodstuffs had tremendous advance in metabolism Now, vast populations of photosynthetic bacteria and stromatolite reefs in shallow seas and other aquatic environments; extensive primary production supports more varied, complex bacterial food chains Oxygen builds up in oceans, then atmosphere to present levels (2 BYA to 0.5 BYA) Oxygen in atmosphere allows formation of ozone shield, reduces intensity of dangerous UV light, protects life in productive shallow seas and on land Aerobic metabolism allows evolution of eukaryotic one-celled organisms (protists), then algae, primitive invertebrates, then advanced invertebrates, fishes, terrestrial plants and animals 7
HOW DID FIRST LIFE ON ARISE ON THIS PLANET: CHEMICAL EVOLUTION Formation of organic compounds by inorganic process on early earth Stanley Miller and others experiment with simulated primitive atmospheres: get amino acids (building blocks of proteins); nucleotides (building blocks of DNA, RNA Basics of Life (protein assembly and genetic materials) created in test tube; shows how first life began Miller s Experiment: simulates processes in the reducing (oxygen-free) atmosphere of early earth ENERGY SOURCE (electric spark, simulating lightning) passed through N 2, NH 3, CO 2, H 2 0, CH 4, H 2 = ORGANIC MOLECULES 8
ENERGY SOURCES RESPONSIBLE FOR ABIOTIC FORMATON OF ORGANIC MOLECULES ON EARLY EARTH: LIGHTNING (ELECTRIC); UV LIGHT IN SOLAR RADIATON; HEAT IN UNDERSEA VOLCANOS (HYRDOTHERMAL VENTS) What are most important organic molecules on the road to first life on early earth? PROTEINS: from amino acids; serve as structural molecules & enzymes (catalysts for biochemical reactions necessary for life) NUCLEIC ACIDS: DNA, RNA: contain information to construct proteins and to transmit this information via replication (reproduction) 9
FIRST LIVING BEINGS WERE PROKARYOTIC SINGLE-CELLED ORGANISMS ALL CELLS HAVE : A SURROUNDING CELL MEMBRANE (isolates cell contents from outside environment; controls movement of materials in and out of cell) CYTOPLASM (contents of the cell, where metabolism occurs; genetic material controls reproduction of the cell from one generation to the next) HOW DID THE COMPLEX CELL SYSTEMS FORM FROM A DILUTE SOLUTION OF ORGANIC MOLECULES IN EARLY SEAS????? Oparin (Russian, 1920 s Origin of Life : CELL MEMBRANE and cell from droplets (coacervates) in colloidal solutions COACERVATE droplets can concentrate materials from outside solution; as in a cell, materials can enter, react, and exit cells (see next slide) BUT COACERVATES ARE NOT ABLE TO SELF- REPLICATE (REPRODUCE); have cell form but are not yet living systems 10
Coacervate Droplets Formed In Laboratory From Starch And Gelatin (protein) Colloidal Solutions: CAN HAVE A PRIMITIVE METABOLISM: RNA first genetic material: assembled from nucleotides formed by abiotic processes RNA not only can replicate itself BUT ALSO can form proteins (e.g., enzymes) from amino acids in surrounding solution In modern organisms, more stable DNA is genetic material, RNA relegated to vital role of production of proteins 11
SYNTHESIS AND SUMMARY OF FIRST LIFE: RNA s in primitive soup of organic molecules in early seas trapped in coacervate droplets RNA produces enzymes from amino acids, metabolism begins, matter and energy can be accumulated, growth, self-replication = LIFE (single-celled prokaryotes = bacteria are first life) PARADOX: today, all life comes from reproduction of pre-existing life But first life must have evolved from non-living matter (chemical evolution) Why isn t such spontaneous generation occurring today??? 1. Chemical formation of life possible in reducing, but not oxidizing atmosphere and hydrosphere (today s world) 2. Today s life forms would quickly utilize any protolife as a food source before level of living organism attained 12