1.Life on earth probably didn't begin until 3.8 - 3.5 billion years ago when the planet had cooled enough such that water was able to condense from the primitive atmosphere.

2. The oldest known life forms were cyanobacteria (blue-green algae) whose fossils are preserved as tiny filaments in rocks about 3.5 b.y. old. Thus, the earth was already 1 billion years old before the first signs of life appeared.

3. Once established, the early life forms continued as simple, unicellular bacteria and cyanobacteria over the next 1.5 b.y.

4. The first multicellular animals did not appear in the fossil record until 600-700 m.y. ago, almost 3 b.y. after the first evidence of life.

1. It has been realized for some time that the earth's present atmosphere is too oxidizing and corrosive to allow simple organic compounds to combine into more complex molecules. A reducing, oxygen poor environment is required.

2. This criteria was met 3.5 b.y. ago when the earth's atmosphere was rich in CO₂, methane, ammonia and N₂, but not free O₂. There also was no ozone layer back then, so the earth was constantly bombarded by ultraviolet radiation that helped catalize the breaking of bonds in simple organic compounds to form more complex molecules.

3. Charles Darwin (mid-1800's) suggested that organic compounds that were the precursors of life must have formed in a warm pond containing ammonia and phosphoric salts that were exposed to light, heat and electricity.

4. In the 1920s, Russian biochemist (A.I. Oparin) and British geneticist J.B.S. Haldane proposed that an earth with a reducing atmosphere and abundant methane would have been an ideal "primordial soup" for the origin of life.

5. A reducing (anarobic) environment is required to form C-H compounds. Any free O₂ would prevent this by immediately oxidizing carbon to CO₂ or CO₃²⁺

Figure 9.2: The first breakthrough in reconstructing the origin of life came in 1953 when a simple experiment by Stanley Miller and Harold Urey produced organic compounds in a flask of ammonia and methane subjected to sparks from electrodes. The organic compounds produced included cyanide (HCN), formaldehyde (H₂CO) and small quantities of four amino acids. Later experiments produced the 12 most common amino acids of the 20 known to occur in life.

1. A variety of experiments have been conducted which link simple organic molecules into more complex molecules that are essential for the existence of life.

2. Amino acids are the building blocks of proteins. Experiments demonstrate that amino acids are not difficult to produce provided there is a reducing environment and a source of simple chemicals.

3. The next important step in creating life is to link simple organic molecules into complex chains called polymers. An important question is how to string together amino acids to form proteins?

4. Sidney Fox (1950s) showed that splashing amino acids under hot, dry conditions caused them to instantly polymerize into proteins. Other experiments utilizing cyanide, clays and heat were successful at triggering polymerization of amino acids into proteins.

5. Figure 3.11: Nucleotide bases (adenine, guanine, etc), the essential components of the nucleic acids RNA and DNA, can be synthesized in the laboratory using aqueous solutions of ammonium cyanide or hydrogen cyanide subjected to heating or bombardment with ultraviolet radiation. Scientists, however, have not yet been able to create entire RNA or DNA molecules in the laboratory.

6. Fatty acids, that form the outer membrane of simple cells, are easily synthesized in the laboratory and have even been found in meteorites.

7. Figure 9.3: Combining fatty acids and alcohol forms lipids, the chemical building block of most fats and oils. Lipids are polar molecules with a head made of glycerol and two tails made of fatty acid chains. The glycerol head is attracted to water whereas the fatty acid tails are repelled by water. Therefore when lipids are surrounded by water, they tend to line up with their heads facing the water and tails pointing away. As a result, lipids bead up when surrounded by water. Organic molecules in the immediate vicinity could be entrained by the beading process and trapped within a lipid membrane.

1. It's one thing to be able to create amino acids, proteins and other polymers in the laboratory and encase them in a lipid membrane, but quit another to create an actual living cell. Living cells have the capability of encoding genetic information and reproducing by means of RNA and DNA.

2. Several experiments have provided possible insights into how early cells may have emerged in the primitive earth.

3. We found that when fatty acids are dried and concentrated, and then wetted again, they spontaneously condense into spherical balls that can trap any DNA present. Therefore, it is easy to surround DNA with a lipid membrane.

4. Figure 9.4: Sidney Fox and A.I. Oparin were able to experimentally produce small droplets or spheres of protein called colloidal particles or proteinoids. When colloidal particles are surrounded by water, they form coacervate drops that can selectively absorb and release certain compounds in a process similar to bacterial feeding and excretion.

5. These colloidal particles (protocells?) discovered by Fox and Oparin, however, were incapable of genetic coding and replication and therefore could not be regarded as actual living organisms.

6. Genetic information in modern cells is carried by the nucleotides RNA and DNA. Neither have been synthesized in the laboratory, so much speculation occurs as to how they first came into existence. (a) Some scientists conjecture that proteins formed first and that the resulting proteinoids collected nucleic acids that could then polymerize into RNA and DNA. (b) Other scientists argue that nucleic acids formed first, possibly as single-stranded RNA, and afterwards assembled the protein polymers.

1. Several theories addressing the polymerization of organic molecules to form protocells utilize the idea of templates. Templates are surfaces that serve to line up organic molecules in close order, thus allowing them to bond into more complex molecules.

2. Clays have a structure that can absorb organic molecules and catalyze their breakdown and synthesis into other products. If clays and associated organic molecules could somehow become trapped in an organic membrane, then the resulting protocell could have the makings for early life. Later, newly formed nucleotides could then take over the early functions of the clay.

3. Zeolites, complex silicate minerals formed as an alteration product of volcanic glass, also have the complex, repeating structures like clays and can catalyze organic reactions.

1. Box 9.1: Perhaps the most fascinating hypothesis proposes that life originated near hot volcanic vents along deep oceanic spreading centers. Hydrothermal vents possess the elements and energy source necessary for synthesis of organic molecules. In fact, amino acids have been detected in hydrothermal vent solutions.

2. The mineral Pyrite, (Fe-Sulfide) which occurs in great abundance around deep-sea hydrothermal vents, have crystal surfaces that could attract phosphate complexes contained in many organic molecules, particularly nucleic acids. These organo-phosphate compounds could line up in close order on the pyrite surface. The crowding together of these molecules may eventually cause polymerization via organic bonds. Once polymerized, these new organic complexes could detach from their pyrite template and become free organic molecules. In this way, nucleic acids and even cell membranes may have evolved.

3. Deep-sea hydrothermal vents are known to be the home of primitive Archaebacteria which live on H₂S, on CH₄ or in hot salty springs. The primitive Archaebacteria may represent the earliest life forms on earth.

1. At least 74 amino acids have now been found in chondritic meteorites. In addition, fatty acids have also been discovered in meteorites leading some scientists to suggest that life on earth was seeded by organic molecules supplied by bombarding meteorites early in the history of the earth.

1. Figure 9.6: The oldest fossils show that life had already split into two groups by 3.5 b.y. ago. One group, the Eubacteria, include true bacteria plus cyanobacteria. The other group comprised the Archaebacteria which can live in extremely hot, anoxic water and include microbes that feed off sulfur compounds or methane.

2. Figure 9.8: The oldest known fossils are found in 3.4 -3.5 b.y. old rocks from northwestern Australia and South Africa and consist of spherical microfossils arranged in strings and resembling cyanobacterial filaments. These fossils are thought to represent primitive cyanobacteria (blue-green algae).

3. Figure 9.7: Cyanobacteria, a member of the kingdom Eubacteria, are prokaryotes (single celled) whose genetic material is not organized into a descrete nucleus. Cyanobacteria undergo photosynthesis, a process that converts light, water and CO2 into complex organic substances. Free oxygen (O₂) is a byproduct.

CO₂ + H₂O + light = (CH₂O) + O₂

4. Figure 9.7: Eukaryotes (real algae), which emerged much later at around 1.8 b.y. ago, are also single celled but have a discrete nucleus.

5. Cyanobacteria form layered mats called stromatolites, which are the only megascopic fossils in rocks from 3.5 billion to 700 million years in age. Stromatolites can form cabbage-like domes and a readily recognized in the fossil record.

6. It is envisioned that by 3 billion years ago, the shallow waters of the earth's surface were filled with stromatolitic, cyanobacterial mats producing abundant free O₂.

1. By 1.8 b.y. ago, atmospheric oxygen released from photosynthesis of cyanobacteria reached 1% of the present level. Aeraobic bacteria developed while anaerobic bacteria sought niches within reduced environments.

2. Figure 9.9: The eukaryotes appeared around 1.8 billion years ago as evidenced by the appearance of acritarchs (organic walled) which may represent the cyst stage of early eukaryotic algae. The eukaryotes presently include all single-celled organisms with nuclei as well as all plants, animals and fungi.

3. Figure 9.10: Eukaryotes may have evolved from earlier colonies of prokaryotes formed through symbiosis of both archae- and eubacteria beginning around 1.75 billion years ago.

1. Figure 9.13: Life remained fairly simple until about 600-700 m.y. ago when there was a dramatic change in life forms with the appearance of metazoans.

2. Figure 9.11: Impressions of soft-bodied, 600 m.y. old metazoans (multi-cellular) are found in Ediacara Hills of Southern Australia. These metazoans included jellyfish, arthropods (animals with joint legs and segmented bodies) and worm-like creatures up to 1 meter long. Similar fossils are also found in other parts of the world.

3. Between 600 to 550 million years ago, the world was dominated by the Ediacaran,soft-bodied, forms. These Vendian animals disappeared in the Early Cambrian and were replaced by small, shelly creatures that thrived in the warm, shallow waters of continental shelves.

1. Figure 9.13: Trace fossils of burrowing organisms appear in the Early Cambrian.

2. Figure 9.16: The Earliest Cambrian marked the appearence of archaeocyathids (Lower to Upper Cambrian). Archaeocyathids were colonial organisms resembling modern sponges in that they were filter feeders and were encased in cone-like structure perforated with pores. Cambrian reefs were composed of archaeocyathids in addition to stromatolites formed by cyanobacteria.

3. Figure 9.17: Brachiopods (Cambrian to Recent) are bivalve filter feeders that attach to the seafloor with a long, fleshy stalk called a pedicle.

4. Trilobites (early arthropods) dominated the Cambrian and Ordovician periods but were extinct by the end of the Permian. Shells were composed of calcite and organic chitin. Segments include head (cephalon), body (thorax) and tail (pygidium). Most grazed and burrowed the seafloor of shelf regions.

5. Molluscs (Cambrian to Recent) were the ancestors of modern clams and snails.

6. Figure 9.18: Echinoderms (Cambrian to Recent) are represented today by starfish, sea urchins, sand dollars, crinoids and sea cucumbers.

7. Figures 9.19 & 9.20: The Cambrian not only included hard-shelled animals, but also soft-bodied animals that could only be preserved under exceptional circumstances. The middle Cambrian Burgess Shale in the Rocky Mountains, British Columbia, preserves an abundance of unusual soft-bodied animals including arthropods, worm-like animals and others unlike anything living today.

There are several hypotheses proposed to explain the Cambrian Explosion:

(a) Retreat of Varangian glaciers during a major global warming period.

(b) Atmospheric oxygen reached 6%-10% of present levels which allowed production of CO₃²⁺. The abundance of carbonate allowed organisms to secrete calcitic hard shells.

(c) Rifting of supercontinent created large areas of shallow marine continental shelf.

(d) Rifting and volcanic activity released large amounts of nutrients such as calcium and phosphate originally trapped in the deep ocean.

(e) Abundant stromatolite mats in the Early Cambrian provided food for primitive molluscs, leading to almost complete disappearance of these mats during the Cambrian.

(f) Natural selection kicked in, causing rapid diversification of animals and the development of a food chain.