Name __________________________ 542- ____ Due Date: ______________
Review Book Topic D: Evolution
D.1 Origin of Life on Earth
Key facts
1. In order for life to be possible, four key problems had to be overcome on the early Earth.
a. Life as we know it is based on organic molecules such as amino acids, yet early Earth had only inorganic matter.
b. In order to build more complex molecules like proteins, monomers (single molecules like amino acids, simple sugars, and nucleotides) need to be connected together into polymers (long chains such as polypeptides, such as polysaccharides, proteins, or RNA).
c. In order to be considered alive, things must be able to reproduce. DNA, the molecule most used for the replication of living organisms, is a complex molecule that requires enzymes in its formation. Also, to get polypeptides to form from DNA, RNA is needed.
d. Water tends to depolymerize molecules. Water dissolves many organic compounds, making it difficult for molecules to become organized into polymers.
2. In 1953, Miller and Urey experimentally reproduced organic molecules from the atmospheric components thought to have existed on pre-biotic Earth.
3. Miller and Urey’s formed organic molecules from the “primordial soup”, but they did not create life. Amino acids cannot be described as alive.
4. The idea that life on Earth began in space is known as panspermia.
5. Comets (chunks of space ice) and meteorites can carry organic compounds. NASA studies reveal that organic molecules can survive impact and the impact could help to polymerize some polypeptides into amino acids. Earth underwent a major bombardment by comets about 4 billion years ago.
6. Hypotheses for the location and origin of organic molecules include formation in space and delivery to Earth by comets, and formation in intertidal zones, deep-sea hydrothermal vents, or volcanoes on Earth.
7. Evidence for the origin of organic molecules in space comes from studies of the spectral lines of distant clouds of cosmic dust, which have been found to contain glycine, the simplest amino acid.
8. Alternating wet/dry conditions on flood plains or in intertidal zones might have provided an environment for producing organic molecules. The drying of clay particles could have created catalyzing reactions and formed early organic molecules.
9. Stromatolites, one of the oldest forms of life on Earth, live today in intertidal zones.
10. The eruption of water vapor, gases, and various minerals in volcanoes and the warmth of eruptions could have provided conditions favorable to the formation of amino acids and sugars.
11. Deep sea hot water geysers called hydrothermal vents are rich in dissolved minerals are used by symbiotic bacteria to make food for tube worms.
12. The study of the origin of life on Earth is a very young science and much work still needs to be done.
13. The prebiotic world did not contain enzymes which are needed for DNA replication; therefore, it is unlikely that double-stranded DNA was the means of inheritance.
14. Single-stranded RNA can replicate itself without enzymes under certain conditions because it can also act as a catalyst, helping certain chemical reactions the way enzymes do. Therefore, single-stranded RNA may have been the first molecule capable of passing along hereditary information.
15. Protobonts are structures that may have preceded the first living cells, providing an protected internal environment for polynucleotides such as RNA.
16. Origins for protobonts include proteinoid microspheres (bubble-like structures formed when amino acids spontaneously join together as polypeptide chains in drying clay) and coacervates (microscopic spheres formed from lipids in water).
17. There was no oxygen on Earth 4 billion years ago; therefore, early forms of life on Earth were anaerobic cells like bacteria.
18. About 3.5 billion years ago, certain bacteria developed the capacity to photosynthesize (make their own food). Evidence for this is seen in rock formations rich in iron oxide. The bacteria converted iron in ocean water to iron oxide.
19. Photosynthesis altered the composition of the atmosphere as oxygen was released as a waste product. The non-photosynthetic bacteria that survived this “pollution” were those living in mud or other places protected from the oxygen rich atmosphere.
20. Bacteria were the only organism on Earth from about 3.8 billion to 2 billion years ago, when the first fossil cells of eukaryotes formed.
21. The endosymbiotic theory states that the first eukaryotic cells were formed when larger prokaryotic cells ingested, but did not digest, smaller prokaryotic cells, in exchange for their services.
22. The endosymbiotic theory can explain how membrane-bound organelles such as mitochondria or chloroplasts (which also) became part of another cell.
23. Mitochondria and chloroplasts have characteristics that make them more like independent prokaryotes: They:
a. have a double membrane;
b. have their own naked circular DNA, just like prokaryotes
c. can perform protein synthesis using small ribosomes;
d. can make copies of themselves when more are needed.
24. Two problems with the endosymbiotic theory is the question of how the host cell passed on the genetic information to offspring to synthesize the newly acquired organelle, and chloroplasts and mitochondria cannot survive on their own outside the cell.
Further notes
Complete the following.
1. What four processes are needed for the spontaneous origin of life on Earth?
You need a source of organic molecules, you need to form polymers (polymerization) such as polypeptides to build more complex molecules like proteins, you need to be able to reproduce, and you need to overcome water’s tendency to dissolve or depolymerize molecules.
2. What is polymerization?
Polymerization is the process of building polymers (chains of monomers).
3. Outline Miller and Urey’s experiment into the formation of organic molecules.
Using a glass sphere and tubing in the lab, they introduced methane, hydrogen, and ammonia (gases thought to be present in the Earth’s atmosphere at the time), introduced liquid water, heated it, evaporated it, and cooled to condense to simulate the water cycle; kept everything at a warm temperature. Exposed the apparatus to UV radiation (Earth had no protective ozone layer early in its history), and generated electric sparks to simulate electricity. Within a week, amino acids had formed. These were organic molecules but could not reproduce (were therefore not alive).
4. Why are scientists studying comets in the effort to understand the origin of life on Earth?
Scientists are studying comets as a potential source for initial organic molecules needed for life on Earth. They have found such evidence and have also conducted experiments showing that a comet impact could help to polymerize certain amino acids into polypeptides.
5. List four environments where the synthesis of organic compounds could have occurred.
a) in space. Glycine (simple amino acid) seen in spectral lines in distant clouds of cosmic dust.
b) alternating wet/dry conditions along flood plains or intertidal zones (drying of clay may have catalyzed reactions forming organic molecules)
c) near volcanoes (rich sources of raw materials plus heat)
d) deep ocean hydrothermal vents – symbiotic relationships between bacteria and tube worms show that life can exist in dark deep ocean
6. How might prebiotic polymers have solved the problem of depolymerization?
Prebiotic polymers (proteinoid microspheres or coacervates) protect polymers from the external destructive environment – shielding from uv radiation
7. Outline the theory of endosymbiosis.
Endosymbiosis is the theory that organelles inside cells today were once independent prokaryotes and were long ago engulfed by bigger cells which, rather than digest them, kept them alive in exchange for their services. Host cells lacking the enzymes to digest them kept them alive inside the cytoplasm and obtained an abundant source of energy.
8. List four characteristics of mitochondria and chloroplasts that make them seem more like independent prokaryotes.
a) they have a double membrane.
b) they have their own naked DNA, which is circular
c) can perform protein synthesis using small ribosomes
d) can make copies of themselves when more chloroplasts or mitochondria are needed