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Biology 1010

Fall 2009


ORIGIN OF LIFE

Life - the state characterized by the capacity for metabolism, growth, reaction to stimuli, and reproduction.

I. Formation of solar system

A. Big Bang Theory - all matter was originally concentrated in one mass that blew apart about 12 - 15 billion years ago 

B. Smaller atoms, hydrogen and helium, which make up about 99% of all matter, fused to produce the heavier elements found in our solar system

C. Stars were borne out of huge masses of interstellar gases several light years across

Click here for an explanation of the formation of stars from interstellar gases

D. A more recent image, here is an explanation

E. Our sun was formed about 5 billion years ago

F. About 4.6 billion years ago the planets of our solar system formed by the condensing of peripheral gases and matter around our sun

II. Formation of the earth

A. Earth formed about 4.6 billion years ago

B. Heat from gravitation and radioactivity formed the earth in four layers:

  1. an inner, solid core of mostly iron and nickel.
  2. an outer molten core of iron and sulfur
  3. a "plastic" mantle composed of iron (Fe), magnesium (Mg), aluminum (Al), silicon (Si), and oxygen (O) silicate compounds.
  4. a thin crust which solidified about 4.1 billion years ago

C. Atmosphere forms

1. The earth's size provides a gravitational field strong enough to hold an atmosphere

2. Primitive atmosphere was formed by volcanic outgassing

3. Primitive atmosphere consisted of water vapor, nitrogen, carbon dioxide, hydrogen and carbon monoxide

4. Primitive atmosphere contained little or no free oxygen (O2) and was a reducing atmosphere as opposed to the oxidizing atmosphere of today. Recall atmosphere currently contains about 21% oxygen

a. Reducing atmosphere lacks free O2 and allows formation of complex organic molecules

b. Oxidizing atmosphere contains free O2 and inhibits formation of complex organic molecules

5. The earth was so hot that H2O only existed as a vapor in dense, thick clouds

6. As the earth cooled, H2O vapor condensed to form liquid H2O, and rain collected in oceans, etc.

7. The earth's distance from the sun allows H2O to exist in all phases: solid, liquid, and gas

III. Origin of life 

A. Chemical evolution - increase in complexity of chemicals led to first cells 

B. Small organic molecules evolve

1. Oparin (1938)  postulated that reducing atmosphere, coupled with vast amounts of free energy from volcanic activity, lightning, radioactive minerals and the sun (there was no ozone layer back then) could facilitate the formation of organic molecules from the primitive atmospheric gases

2. Miller (1953) duplicated these early earth conditions in the lab:

a. Constructed an artificial system which contained an "atmosphere" and "ocean" 

b. Introduced hydrogen, methane, ammonia and water into the system, turned on an electric spark as energy supply

c. In one week amino acids and other small organic molecules formed

3. Other scientists repeated their work, eventually producing other amino acids, ATP, glucose and other sugars, lipids and the bases which form RNA and DNA, and adenine the key component of ATP and NAD

4. Over a long period of time the lack of oxidation and decay allowed organic molecules to form a thick, warm organic "primordial soup"

C. Macromolecules via polymerization. Three hypotheses:

1.  RNA-first hypothesis:

a. Only the macromolecule RNA was needed at the beginning to form the first cell

b. RNA can be both a substrate and an enzyme (ribozyme)

c. Also, some viruses have RNA genes

d. RNA would carry out processes of life associated with DNA (in genes) and protein enzymes

2. Hydrothermal vents:

a. superheated mineral rich waters in deep ocean vents promoted polymer formation

3. Clay hypothesis

a. Clay attracts small organic molecules and contains iron and zinc atoms serving as inorganic catalysts for polypeptide formation

b. Clay collects energy from radioactive decay and discharges it if temperature or humidity changes; could have been source of energy for polymerization to take place

c. RNA nucleotides and amino acids became associated so polypeptides were ordered by and helped synthesize RNA

D. Protocells evolve

1. Protocell - cell-like structure with a lipid-protein membrane

2. Recall how phospholipids spontaneously form membranes in water

3. Oparin demonstrated a protocell could have developed from coacervate droplets:

a. Coacervate droplets - complex spherical units that spontaneously form when concentrated mixtures of macromolecules are held in the right temperature, ionic composition, and pH

b. Coacervate droplets absorb and incorporate various substances from the surrounding solution

c. In a liquid environment, phospholipid molecules spontaneously form liposomes, spheres surrounded by a layer of phospholipids; this supports a semipermeable-type membrane.

d. A protocell could have contained only RNA to function as both genetic material and enzymes

E. First protocells were heterotrophs

1. First protocells probably were heterotrophic fermenters living on the organic molecules in the primordial soup that was its environment; this suggests heterotrophs preceded autotrophs:

a. Heterotroph - an organism that cannot synthesize organic compounds from inorganic substances and therefore must take in preformed organic compounds, e.g. animals

b. Autotroph - an organism that makes organic molecules from inorganic nutrients, e.g. plants

2. First protocells may have used preformed ATP, but as supplies dwindled, natural selection favored cells that could extract energy from carbohydrates to transform ADP to ATP

3. As there was no free O2, it is assumed that protocells carried on a form of fermentation

4. First protocells had limited ability to break down organic molecules; it took millions of years for glycolysis to evolve

5. Microspheres have some catalytic ability and coacervates incorporate enzymes if they are available in the medium

F. Self-replication (reproduction) evolves

1. In living systems, information flows from DNA --> RNA --> protein; this sequence developed in stages

2. RNA-first hypothesis suggests that the first genes and enzymes were RNA molecules:

a. These genes would have directed and carried out protein synthesis

b. Ribozymes are RNA that acts as enzymes

c. Some viruses contain RNA genes with a protein enzyme called reverse transcriptase that uses RNA as a template to form DNA; this could have given rise to the first DNA

3. DNA is more stable than RNA and eventually took over carrying the genes

4. Once the protocell was capable of reproduction, it became a true cell and biological evolution began

IV. History of life 

A. History of earth can be displayed as a 24-hour span starting at midnight

1. Prokaryotes do not appear until 5 A.M.; eukaryotes are present at 4 P.M.; multicellular forms start at 8 P.M.; land is invaded at 10 P.M.; and humans appear 30 seconds before the end of the day.

2. This relative timetable corresponds to the fossil record spread over 4.5 billion years.

B. Fossils and dating were previously discussed

C. Life began in the Precambrian from 570 million years ago to 4.6 billion years ago

1. Precambrian encompasses 87% of the geologic time scale

2. Early bacteria probably resembled archaea that live in hot springs today

3. 3.5 billion years ago the first prokaryotic cells appear in stromatolites

4. By 2 billion years ago, the first oxygen-releasing cells appeared

5. Oxygen-releasing photosynthesis caused the atmosphere to become oxidizing rather than reducing

6. Accumulation of O2 caused extinction of anaerobic organisms and rise of aerobic organisms, recall relative efficiencies of both types of respiration

7. O2 also contributed the ozone shield, blocking ultraviolet radiation from reaching the earth's surface to allow organisms to live on land.

D. Eukaryotic cells arise

1. The eukaryotic cell of 2.1 billion years ago is aerobic and contains a nucleus and organelles

2. Arose via endosymbiosis: cells engulfed prokaryotes that became various organelles  (recall the structure and DNA of mitochondria and chloroplasts)

3. Nucleus formed by infolding of the plasma membrane to protect genetic material

4. By 600 million years ago, multicellular organisms appeared

E. Complexity increases in Paleozoic

1. Invasion of land by plants took place about 440 million years ago, probably in intertidal zones or mud flats 

2. Once land plants evolved they provided the food necessary for higher animals to evolve on land

F. Dinosaurs ruled the Mesozoic

1. Late in the Mesozoic flowering plants spread and increased in dominance; coniferous trees declined; placental animals appeared; modern insect groups appeared

2. A mass extinction occurred at the end of the Mesozoic eliminating dinosaurs

G. Mammals take over in the Cenozoic

1. 24-66 million years ago mammals diversified tremendously, beginning at the size of a mouse; flowering plants formed vast tropical forests

2. Beginning about 24 million years ago primates evolved into monkeys, apes, and then humans; major climatic shifts occurred; grasslands replaced forests, which put pressure on primates who were adapted to living in trees, causing some primates to evolve to a non arboreal existence

3. 6-24 million years ago, grasslands spread as forests contracted; apelike mammals and grazing mammals flourished

4. 2-6 million years ago, herbaceous flowering plants flourished; first hominids appeared

5. Pleistocene epoch (0.01-2 million years ago) was beginning Ice Age and contributed to significant mammalian extinction; herbaceous plants spread; modern humans arise and may have contributed to extinction

6. During last ice age sea levels were lower, allowing humans to migrate from Asia to North America via Bering Land Bridge

7. Events which led to evolution of humans

V. Plate tectonics

A. Plate tectonics (continental drift) has had a profound effect on the evolution of life

B. Wegener (1920) presents data from across disciplines supporting continental drift

C. Continental drift - movement of continents with respect to one another over the earth's surfaces

D. The plates of the earth’s thin crust are constantly moving apart at ridges and colliding at trenches, the land masses on earth have been drifting around almost since they were first formed 4 billion years ago

E. At one time all the continents formed one large land mass called Pangea

F. At another time there were two major land masses:

  1. Gondwana - the current southern hemisphere continents plus India (which was part of eastern Africa).
  2. Laurasia - the current northern hemisphere continents.

G. Biological Evidence:

1. Fossil Glossopteris (seed ferns) date back about 250 million years ago. Their distribution was: South  America, Antarctica, South Africa, Australia and India, classic Gondwana distribution

2. Three extant genera of lungfishes, one each in Australia, Africa and South America, classic Gondwana distribution

3. Lack of native placental mammals in Australia

a. Australia has no native placental mammals

b. They are replaced by marsupials - opossums, kangaroos, koalas and other mammals that complete their development in a marsupium (pouch)

c. Marsupials arose several million years before placental mammals

d. Later on placental mammals arose in Laurasia and spread southward into Gondwana. However, Australia had already drifted away from Gondwana before placental mammals got there