The Origin and Evolution of Life on Earth


Why do we start the discussion of biology with a discussion of geology? Here are some reasons why one would do this:


The Big Bang theory of the formation of the universe

All material in the universe was created in a huge "explosion," creating and defining matter and space. The sudden cooling of the superheated ejecta facilitated the combination of atomic components into atoms and molecules. These clouds of gasses eventually cooled and formed the principle components of galaxies - including stars and planets.

Other theories have been postulated (i.e. the oscillating universe theory which states that the universe expands and contracts in a cycle every 100 billion years) and these are hotly debated


Location of the Solar System

Our solar system is located on a spiral arm of the Milky Way galaxy. We are approximately 2/3 of the distance from the core to the outer rim of the galaxy. This distance places us in a lower-density region of the galaxy - there are some stars near to ours, but this region is nowhere as congested as the galactic core.


Formation of the solar system

A. The earth formed approximately 4.6 BYA (billion years ago.) Initially, there was a cloud of gasses and dust particles, possibly originating from the ejected particles of a nearby supernova.

B. The cloud gradually contracted and flattened, concentrating about 99% of its mass in the center with the rest rotating counterclockwise in a flattened disk.

C. As the disk rotated, turbulence was created, causing condensation of the disk into small, turbular eddies. These gradually accreted together to form protoplanets.

D. These protoplanets further accreted, creating the mature planets of the solar system.

The sun also accreted, pulling in most of the mass. As these accumulated, the pressure and temperature caused the initiation of thermonuclear fusion. This thermonuclear fusion is what provides the ultimate source of energy for all life on earth.

In the hot accretions of planetesimals, iron-rich elements condensed first, creating the cores (the inner and outer cores). Next, the lower-density silicates began to condense and aggregate, forming the mantle and the crust. Further differentiation of the crust was fueled by the energy output from radioactive decay deep within the earth


Structure of the earth

There are three clearly defined regions of the earth:

  • Cores (inner (1250 km) and outer (2100 km)) - composed primarily of iron (85%) and nickel. Other heavy elements (such as radioactive elements) are also found here
  • Mantle (2900 km) - a "fluid" region primarily composed of oxygen and silica - derived minerals
  • Crust (5-70 km) - thin film of "crud" which has floated to the surface. We are simply passengers, living on this geologic flotsam.


Early Earth Conditions

Theory of Early Earth Conditions-Hot and violent


What is life?

What is life? Life is everywhere, but it resists a simple one-sentence definition. Some of the properties of life are given below:


Chemical Evolution Occurred Early in Earth's History

Radiometric dating technique determines age of rocks. Estimations of the Time when Chemical Evolution Occurred
Building Blocks of Chemical Evolution
Chemical Modeling of Chemical Evolution Reactions
Reduction of carbon was a key step in chemical evolution.
For carbon to be reduced, early atmosphere must have contained CH4, H2, and NH3 (molecules that can give up electrons).
Once organic molecules formed, only heat was needed to drive formation of more complex organic molecules to complete chemical evolution.
Conditions in the Oceans at the Time of Chemical Evolution


Chemical Evolution-Theory and Hypothesis Testing

Oparin and Haldane Testing the Predictions of the Chemical Evolution Theory Experimental evidence indicates step 1 of the Oparin-Haldane chemical evolution hypothesis is plausible-Small organic molecules could have formed in the atmosphere and hydrothermal vents of early Earth.


The Origin of Small Organic Molecules in the Prebiotic Soup

The Second Step in the Oparin and Haldane Theory

Evidence/observations that support the occurrence of amino acids in prebiotic soup:


Sugars and the prebiotic soup


Nucleotides and the prebiotic soup


The Origin of Macromolecules in the Prebiotic Soup

The Third Step in the Oparin and Haldane Theory

Macromolecules Polymerize from Monomers

Challenges of Polymerization Modeling Polymerization Reactions on Ancient Earth


Proteins-Highly Variable Macromolecules, Some of Which Act as Catalysts

Catalyst

Proteins and the Prebiotic Soup


DNA: A Stable, Information-Containing Molecule That Can Act as a Template

DNA and the Prebiotic Soup


RNA - An Information-Containing Molecule That Can Act as a Template or a Catalyst

Differences between the Structures of RNA and DNA

RNA can function as a template

RNA can function as a catalyst.

RNA and the Prebiotic Soup

Experiment 1 (Bartel and Szostak)-Can a ribozyme capable of catalyzing phosphodiester bond formation be created? Experiment 2 - In-vitro selection of ribozymes: Can ribozymes with improved activity be created by inexact copying and selection in vitro?


Natural Selection of the First Living Entity in the Prebiotic Soup-A Model for the Transition from Chemical Evolution to Biological Evolution

A self-replicating RNA molecule appears in the prebiotic soup as the result of chemical evolution.

The self-replicating RNA makes copies of itself using free ribonucleotides in the prebiotic soup.

Natural selection occurs. Chemical evolution gives way to biological evolution over time.