Sul Ross State University Sul Ross State University

ORIGIN OF LIFE ON EARTH

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Important Comparative Dates:
4.5 billion years (b.y.): age of the earth
3.87 b.y. (Science, 1996): carbon particles embedded in oldest rocks on earth
3.5 b.y.: earliest actual fossils, in rocks of western Australia
3.4 b.y.: evidence of photosynthesis in organisms (fossils in rocks)
2.0 b.y.: approximate date of divergence of prokaryotes and eukaryotes (Science, 1996)
2.0 b.y.: abundant free oxygen (O2) in atmosphere
1.8 b.y.: earliest evidence of eukaryotes (Science, 1995); 1.5 b.y. in Raven et al.
1.0 b.y.: eukaryotes well established; multicellular algae (Science, 1995)
650 million years (m.y.): multicellular organisms well established
450 m.y.: development of ozone (O3) layer, screening UV radiation
450 m.y.: multicellular terrestrial organisms
2 m.y.: humans
10, 000 years: agriculture

Early Earth Atmosphere, Before Ozone Layer:
Carbon dioxide (CO2), nitrogen (N2), emitted from volcanoes, and Water vapor (H2O)
These three molecules contain the chemical elements that make up 98 % of the material found in living organisms today (C, H,O, N).

Early Earth Atmosphere, After Ozone Layer:
Probably some combination of: Methane (CH4), ammonia (NH3), carbon dioxide (CO2), hydrogen sulfide (H2S), nitrogen gas (NH2), water vapor (H2O), hydrogen gas (H2), or carbon monoxide (CO).
No extensive molecular oxygen (O2) until about 2 b.y.

Present Day Earth Atmosphere:
Molecular oxygen (O2) = 21+% (declining)
Nitrogen (N2) = 78+%
Carbon dioxide (CO2) = .036% (increasing)

The Hypothesis of Chemical Evolution of Life on Earth:

The Major Players:

Oparin, A. I. (1924 to 1930's)--Russian who hypothesized about the composition of the ancient earth atmosphere and how life could have evolved. He postulated that if methane, ammonia, water, and hydrogen were subjected to high energy sources they could form the kinds of organic compounds upon which life is based. The early energy sources on earth were thought to be ultra violet light, lightening, volcanic heat, and/or ionizing radiation. Oparin's famous book, "Genesis and Evolutionary Development of Life" contains a detailed explanation of the hypothesis of chemical evolution.


Haldane, J. B. S. (1929)--Englishman who translated and elaborated on Oparin's original works in Russian. He postulated that if and when primeval organic molecules were formed, they would accumulate in seas, ponds, lakes, and that a dilute soup of these molecules ("primeval soup")would develop. The chance for chemical interactions in such a "soup" would be increased. Ultimately small structures that could mimic life, or could be precursors of life, would form.


Miller, S. L. (1953)--A graduate student of the famous Harold Urey at the University of Chicago, took the gases (hydrogen, water, methane, ammonia) postulated by Oparin to be present in the early earth atmosphere, enclosed them in an airtight glass container, passed electric discharges through the chamber, and carried out the experiment for 24 hours. When the container was examined, Miller found amino acids and other organic molecules. Although we know now that the major atmospheric gases on early earth at the time of organic evolution probably were carbon dioxide, nitrogen, and water vapor, the experiments by Miller were the first test of Oparin's hypothesis.


Fox, S. W. (1964)--A scientist at the University of Miami, took 18 amino acids, heated them to 170 degrees Celsius, and found that they formed chains of amino acids he called proteinoid polymers or thermal proteinoids. When these were placed in water they spontaneously formed small organized spheres that he called microspheres. The microspheres were separated from the surrounding solution by a two-layered membrane. The microspheres, later known as coacervates, were important because they resembled simple cells and exhibited some of the properties of life (they could grow, reproduce by budding, they were organized, and they carried out a simple metabolism in speeding up chemical reactions).



Meteors and Comets, Another Theory for the Origin of Chemical Precursors Essential to Life on Earth:
Early earth apparently was bombarded by meteors that might have carried organic molecules to earth from outer space. Study of the recent comet Hale-Bopp (July, 1995) supports this theory. Hale-Bopp spewed forth water and numerous kinds of organic molecules. This suggests that comet and asteroid dust has been filtering into the earth's atmosphere for billions of years.

Characteristics of Life:
1. Reproduction–the production of genetically similar offspring.
2. Growth–increase in cell size, cell volume, and in number of cells.
3. Metabolism–sum total of the many, complex chemical reactions that occur inside cells.
4. Response to Stimuli–organisms react to their environment.
5. Adaptation to the Environment–short-term response to environment or long-term change
     (a population at this point of evolution).
6. Movement–plants move, slowly.
7. Complexity of Organization–particular organisms are composed of subunits called cells.

The Scientific Method:
This is the manner in which ideas are tested by scientists.
The most simplified outline of the scientific method involves observing, hypothesizing, and testing.
A slightly more complete outline might involve the following: The scientist, gets an idea, makes pertinent observations, studies pertinent literature, develops an hypothesis or hypotheses, designs experiments to test the hypotheses, conducts experiments, keeps careful records, and evaluates results of the experiments. Experiments must be repeatable (testable). Usually there is more than one way to test a single hypothesis. Over time, well-tested hypotheses supporting a general truth may give rise to a theory.



PLANTS IN PERSPECTIVE


Taxonomic Organization of Life on Earth:
The most modern classification system involves three domains:

The Three Domains:
      Archaea
            Archaebacteria
      Bacteria
            Bacteria
      Eukarya
            Fungi
            Protista
            Plantae
            Animalia

The three domains include Prokaryotic and Eukaryotic organisms:
      Prokaryota (Prokaryotic Kingdoms)
            Archaebacteria (methanogens, extreme halophiles and thermophiles)
            Eubacteria (Bacteria and Cyanobacteria)
      Eukaryota (Eukaryotic Kingdoms)
            Mycota (Fungi)
            Protista (Slime Molds, Algae, Protozoans)
            Plantae (True Plants)
            Animalia (Animals)

Whittaker (1969) proposed the following five kingdom system: the five kingdom was followed by biologists until it was replaced by the six kingdom system classified in the three domains listed above.
            Monera (Prokaryotes, Bacteria and Cyanobacteria)
            Mycota (Eukaryotes, Fungi)
            Protista (Eukaryotes, Protozoans)
            Plantae (Eukaryotes, Plants)
            Animalia (Eukaryotes, Animals)

Prokaryotic Organisms:                                    Eukaryotic Organisms:
No nucleus or cell organelles                          Nucleus and organelles present
Asexual reproduction                                       Sexual or asexual reproduction
DNA in naked circular strand                          DNA with protein in chromosomes
Have 70s ribosomes                                          Have 80s ribosomes
Ancient, simple organisms,                              More highly evolved and advanced
usually unicellular or colonial                          organisms

Nutritional Organization of Life on Earth:
     Autotrophic (synthesize their own "food" or nutrition; self-nourishing)
          Photoautotrophic (green plants, undergoing photosynthesis)
          Chemoautotrophic (chemosynthetic bacteria)
     Heterotrophic (derive "food" or nutrition from dead or decaying organisms)
          Parasites (derive nutrition from other living organisms)
          Saprophytes (derive nutrition from dead organisms)
          Phagotrophic (derive nutrition through endocytosis of organic matter)
          Etc.
     Symbiosis (two or more organisms "living together", perhaps sharing nutrition)
          Parasites
          Mutualistic organisms such as lichens
          Etc.

trophic = pertaining to nutrition



SOME FIELDS OF BOTANICAL STUDY, THE SCOPE OF BOTANY

Ecology
Evolution
Taxonomy (Systematics)
Molecular Biology
Morphology
Phytochemistry
Anatomy
Economic Botany
Physiology
Forestry
Cytology (Cell Biology)
Biochemistry
Genetics
Phycology
Mycology
Bryology
Bacteriology
Microbiology
Agrostology
Palynology
Pharmacognosy