Unit Objectives: History of Life on Earth
There are several hypotheses about the natural origin of life on Earth, each with supporting scientific
evidence. At the conclusion of this unit, you should be able to:
1.
2.
3.
4.
5.
Describe Oparin’s hypothesis and the Miller-Urey experiments.
Describe the proposed sequence of development of life on earth.
Describe the reasons for revisions of scientific hypotheses of the origin of life on Earth.
Define/describe geological time scale, Pangaea, Lucy, punctuated and gradual equilibrium.
Describe the use of the following in the study of macroevolution; embryology, fossils, homology,
vestigial organs and biochemistry.
6. Distinguish between anagenesis and cladogenesis.
Vocabulary from previous chapters that will be useful to know:
Abiotic – non living
Biotic - living
Monomers – the subunit that serves as the building block of a polymer
Polymers – a long molecule consisting of many similar or identical monomers linked together. (DNA, proteins)
Amino acids – monomers of polypeptides (proteins)
Proteins – functional biological molecules that are made up of one or several polypatides. (Proteins are made
up of repeating units of amino acids. Amino acids are the monomers to this polymer).
Nucleic acids – a polymer consisting of many nucleotides. (ex. DNA and RNA)
Organic compounds – compounds that contain carbon
Macromolecules – a giant molecule formed by the joining of smaller molecules. (Ex. Polysaccharides,
proteins, and nucleic acids).
Chapter Notes:
Fossils discovered in other parts of the world tell us that past organisms were very different from those now
alive. The sweeping changes in life on Earth revealed by fossils illustrate macroevolution, the pattern of
evolution over large time scales. Specific examples of macroevolutionary change include the origin of key
biochemical processes such as photosynthesis, the emergence of the first terrestrial vertebrates, and the longterm impact of mass extinction on the diversity of life.
Goal of this chapter is to explain:
- Hypothesis regarding the origin of life
- Seminal episode – no fossils exists
- Look at fossil record to tell us about major events in the history of life, particularly to how those factors
shaped the rise and fall of different groups of organisms over time.
25.1 – Conditions on early Earth made the origin of life possible
A. Earliest fossils of microorganisms date back to 3.5 billion years ago. But how did they get here?
B. Chemical and physical processes, aided by Natural Selection, could be the answer. (4 step sequence)
1. The abiotic synthesis of small organic molecules, such as amino acids and nucleotides.
2. The joining of these small molecules into macromolecules, including proteins and
nucleic acids.
3. The packaging of these molecules into “protobionts,”droplets with membranes that
maintained an internal chemistry different from that of their surroundings.
4. The origin of self-replicating molecules that eventually made inheritance possible.
C. Synthesis of Organic Compounds:
a. Earth and the other plant of the solar system formed about 4.6 billion years ago, condensing
from a vast cloud of dust and rocks that surrounded the young sun. Life could not exist for the
first million years because Earth was being bombarded by huge rocks and ice. The collisions
generated energy heat to vaporize the available water and prevented seas from forming. It is
believed that this phase ended 3.9 billion years ago.
b. Atmosphere was probably made up thick water vapor, along with various compounds released
by volcanic eruptions, including nitrogen and its oxides, carbon dioxide, methane, ammonia,
hydrogen, and hydrogen sulfide. As Earth cooled, the water vapor condensed into oceans and
much of the hydrogen quickly escaped into space.
c. 1920s - Oparin and Haldane independently hypothesized that Earth’s atmosphere was a
reducing (electron-adding) environment, in which organic compounds could have formed from
simple molecules. Haldane suggested that the early oceans were a solution of organic
molecules, a “primitive soup” from which life arose.
d. 1953 – Miller and Urey tested Oparin-Haldane hypothesis by creating laboratory conditions
comparable to those that scientist at the time thought existed on early Earth. Their experiment
yielded a variety of amino acids found in organism today, along with other organic compounds.
Many experiments since then have produced similar results, even with different recipes.
e. Recent scientists now believe that the early Earth atmosphere lacked enough methane and
ammonia for it to be reducing. Evidence now suggests that the atmosphere was made up
primarily of nitrogen and carbon dioxide, which would be neither reducing or oxidizing
(electron removing). In any case, it is likely that small “pockets” were reducing, perhaps near
volcanic openings (so maybe not in the atmosphere, but near submerged volcanoes and deepsea vents, where hot water and minerals gush into the ocean from Earth’s interior.
f. Overall, Miller-Urey-type experiments demonstrate that abiotic synthesis of organic molecules
is possible.
D. Abiotic Synthesis of Macromolecules
a. The presence of amino acids isn’t sufficient for the emergence of life.
b. Cells are made up of numerous macromolecules, including nucleic acids, enzymes, proteins,
etc…
c. Scientists have dripped solutions of amino acids onto hot sand, clay, or rock, and have been
able to produce amino acid polymers. The polymers formed spontaneously, without the help of
enzymes or ribosomes. Maybe these polymers acted as weak catalysts for a variety of reactions
on Earth.
E. Protobionts – collections of abiotically produced molecules surrounded by a membrane-like structure.
a. Exhibit some properties of life; simple reproduction and metabolism.
b. Lab experiments have demonstrated that protobionts could have formed spontaneously from
abiotically produced organic compounds.
F. Self-Replicating RNA and the Dawn of Natural Selection
a. First genetic material was probably RNA
b.
25.2 The Fossil Record Documents the History of Life
A. The Fossil Record
B. How Rocks and Fossils are Dated
C. The Origin of New Groups of Organisms
25.3 Key Events in Life’s History Include the Origins of Single-celled and Mulit-celled Organisms and the
Colonization of Land
- Geological Record – three eons
1. Archaean
Together lasted about 4 billion years.
2. Proterozoic
3. Phanerozoic – the last half billion years, and encompasses most of the time that animals have
existed. It is divided into three eras and the boundaries between the eras correspond to major
extinction events seen in the fossil record.
a. Paleozoic
b. Mesozoic – the age of reptiles (dinosaurs)
c. Cenozoic
A. The First Single-celled Organisms
B. The Origin of Multi-cellularity
C. The Colonization of Land
25.4 The Rise and Fall of Dominant Groups Reflect Continetal Drift, Mass Extinctions, and Adaptive Radiations
25.5 Major Changes in Body Form can Result in the Sequences and Regulation of Developmental Genes
25.6 Evolution is not Goal Oriented