I. Abiotic Assembly of Organic Macromolecules ● All living things are dependent on the following biological macromolecules to maintain the structure & metabolism of their cells: 1) 2)

3)

4) Figure 1: Biological Macromolecules & Monomers

● As indicated in figure 1, biological macromolecules are assembled from preexisting subunits (i.e. monomers). ● It is currently understood that only living things can synthesize biological macromolecules. ● When considering the early history of life on Earth, this relationship becomes seemingly paradoxical since:

➢ Biological macromolecules could not come into existence BEFORE the appearance of the first cells. ➢ The first cells could only have arisen AFTER the formation of these vital macromolecules.

● To resolve this paradox, it must be assumed that biological macromolecules must have initially come into existence Abiotically, or outside of a living system (i.e. cells).

II. Oparin-Haldane Hypothesis ● The Oparin-Haldane Model attempts to explain how the organic molecules necessary for life could have formed abiotically from inorganic raw materials. Figure 2: Hypothesized Chemistry of Early Earth Atmosphere

● The atmosphere at this time can be characterized as Reducing, for there was no free O2 to potentially “attack” chemical bonds & extract electrons. Some of the reduced atmospheric gasses may have, in turn, reduced others to abiotically form the first organic molecules. The activation energy (Ea) for such reactions may have been provided by UV rays & lightening. These molecules subsequently began to collect in the primordial oceans via precipitation. Figure 2.1: Testing the Oparin-Haldane Hypothesis: Miller-Urey Experiment (1950’s)

● In the 1950’s, Stanley Miller & Harold Urey tested the validity of the Oparin-Haldane model. The components of their experimental setup was as follows: a) The reducing “atmosphere” in their experimental setup consisted of H2O, H2, CH4, & NH3. b) A warmed flask of water simulated the primordial sea. c) Sparks were discharged in the “atmosphere” to mimic lightening & provide activation energy for gasses to react. d) A condenser cooled the atmosphere, “raining” water & any potential dissolved compounds back into the “sea”.

● After a week, Miller & Urey analyzed the contents of the collection flask & found many of the molecules of life: amino acids, nucleic acids, sugars, lipids, & hydrocarbons. The results of this experiment demonstrated it was possible that organic compounds could arise from inorganic raw materials in a reducing atmosphere.

III. Origins of Life ● Protobionts are random collections of macromolecules enclosed within a lipid bilayer. These assemblages are thought to be the forerunners of true living cells & exhibit the following characteristics: a) Because they are surrounded by a membrane, protobionts can establish & maintain an internal environment that is

chemically distinct from that of their surroundings. b) May exhibit a rudimentary (simple) metabolism. c) Can reproduce, but lack a genetic material. Thus “offspring” are not faithful copies of the original.

Figure 4: Protobionts (Pre-Cells)

Protobionts to Cells: Step 1 ●The first step in the formation of true, living cells from protobionts would be the appearance of self-replicating molecules. The following molecules serve as the most likely candidates for the 1st self-replicators …

Protobionts to Cells: Step 2 ● As RNA molecules made copies, they invariably made errors leading to a population of slightly different molecules all competing for the limited nucleotide resources in the surroundings. In this environment, only some variants would successfully replicate. The existence of the competing RNA variants initiated the mechanism of Natural Selection as early as 4 billion years ago!

● Some RNAs may have been absorbed by protobionts that protected them from degradation by other molecules in their environment. Consequently, these RNA variants survived to out-reproduce all others ...

Protobionts to Cells: Step 3 Figure 4.3: Evolution of a Hereditary Mechanism

●Some RNA’s coded for proteins & enzymes that allowed some protobionts to carry out a more efficient metabolism than others. Copies of this RNA could be passed onto “offspring” when it divided, giving rise to another protobiont with the ability to produce the SAME proteins & carry out the SAME metabolism. ●It can be argued that the point at which a hereditary mechanism was established was, in fact, the point at which protobionts became the first living cells.

Protobionts to Cells: Step 4 Figure 4.4: RNA to DNA

RNA Survival “Machines” DNA Survival “Machines” Modern Cell

●Over time, RNA may have mutated to form double-stranded copies (DNA) that proved to be more stable & thus more appropriate for storing coded information.

IV. Evidence of the First Cell Figure 5: Stromatolite Formations

● Stromatolites are rock-like formations containing the oldest known microfossils (3.5 billion years old!) Are formed by the activities of Cyanobacteria colonies that become lithified (turned to rock) as they absorb minerals from seawater. Over time, new colonies become established over previous lithified ones that eventually add more layers to the stromatolite formation. ● The microfossils found in stromatolite formations resemble certain cyanobacteria that still exist today. For bacteria so complex to evolve by 3.5 billion years ago, life must have originated much earlier, perhaps as early as 3.9 BYA. These 1st organisms are thought to have exhibited the following characteristics: a)

b) c) Figure 6: Divergence from LUCA to Major Branches of Life

● The first split from LUCA (last universal common ancestor) is thought to have occurred around 3.8-4 BYA & gave rise to the Bacteria & Archaebacteria. The archaebacteria, in turn, diverged to give rise to the Eukarya 2.5-3 BYA, modern members of which include protists, fungi, plants, & animals.

● The evidence in support of LUCA & the relatedness of all life includes the following: 1) 2) 3) 4)

V. Rise of Eukaryotes Figure 7: Eukaryotic Origins

● The first eukaryotes descended from certain archaebacteria that acquired internal membranes from infoldings of the plasma membrane that would later form the endomembrane system. They also established endosymbiotic relationships with certain prokaryotes. Both of these events allowed for a more complex & efficient metabolism. Cambrian Explosion ● An “explosion” of eukaryote diversity occurred during the Cambrian Period (543-488 MYA). During this time, animal diversify & complexity significantly increased. Events that likely initiated this “explosion” of animal diversity include: a) A global freeze that occurred between 700 & 600 million years ago. During this Snowball Earth period, ice sheets

would have eroded the underlying bedrock to release a variety of minerals into the seas as the glaciers melted. b) Phosphates washing into the seas would have resulted in large algal blooms leading to an increase in O2 levels to

modern-day concentrations (21%)…

● The increase in atmospheric O2 during the Cambrian would have been both problematic & beneficial to life at the time (see below)… Problems w/Rising Free Atmospheric O2: Reactive O2 Species Figure 8: Reactive Oxygen Species (ROS)

● O2-derived free radicals, especially the hydroxyl radical, can react with vital biological molecules (DNA) to form other free radicals. The ensuing chain reaction (see figure 8.1) can damage a host of vital molecules within the cell to disrupt its metabolism (often to the point of cell death). Figure 8.1: ROS Chain Reactions

● As seen in figure 8.1, the hydroxyl radical reacts with an unsaturated lipid (common within the plasma membrane). As a result, a lipid radical is formed which can react with other molecules to form additional damaging free radicals. This free radical chain reaction usually ceases when an unreactive species is eventually formed.

*As will be discussed in coming chapters, cells have evolved various methods to minimize free radical damage.

Benefits of Rising O2: Metabolic Efficiency & Multicellularity ● In the presence of O2, cells can produce significantly more ATP to power cellular reactions. This excess energy could be invested to promote increased metabolic diversity.

● Increased O2 levels may have selected for the first multicellular organisms, for their reduced surface area would have meant less exposure for O2 uptake & the potential for the production of free radicals …

Unicellular: More Free Radical Uptake Multicellular: Less Free Radical Uptake

VI. Animal Ancestry ●No one knows what the oldest animal ancestor was, but we do have evidence as to what our closest living relatives may be. Choanoflagellates, are a groups of single-celled & colonial aquatic filter-feeders that are found all over the world in both marine & fresh water.

●Choanoflagellates exhibit the simplest of all body plans (cells arranged radially about a central stalk) & share many characteristics with animals. Thus they are considered to be the closest relatives to modern animals (sponges especially) whose common ancestor existed approximately 700 million years ago… Figure 9: Animal Phylogenic (“Family”) Tree

Figure 10: Evidence of Animal-Choanoflagellate Common Ancestry

●Evidence suggesting a close common ancestry between choanoflagellates & modern animal groups includes … 1)

2)