2. 12.3 Origin of Life KEY CONCEPT The origin of life on Earth remains a puzzle.

3. 12.3 Origin of Life Earth was very different billions of years ago. There have been many hypotheses of Earth’s origins. The most widely accepted hypothesis of Earth’s origins is the nebula hypothesis.  the solar system formed by a condensing nebula (a cloud of gas and dust in space)

4. 12.3 Origin of Life Ancient Earth Atmosphere contents: –Ammonia, H 2 O vapor, Methane, H 2, CO 2 Climate: –Extreme heat due to meteor impacts & volcanic activity Eventually Cooling: Water vapor condensed (oceans formed) Click on picture to play video clip

5. 12.3 Origin of Life Several sets of hypotheses propose how life began on Earth. There are two organic molecule hypotheses. –Miller-Urey experiment –meteorite hypothesis electrodes heat source amino acids water “atmosphere” “ocean”  organic compounds could be made by passing an electrical current (to simulate lightning), through a closed system that held a mixture of gases (to simulate the atmosphere)  amino acids may have arrived on Earth through meteorite or asteroid impacts.

6. 12.3 Origin of Life There are different hypotheses of early cell structure. –iron-sulfide bubbles hypothesis  biological molecules combined in compartments of chimney like structures on the ocean floor.  The compartments acted as the first cell membranes. –lipid membrane hypothesis  Lipid spheres, or liposomes, could form around a variety of organic molecules, acting as early cell membranes.

7. 12.3 Origin of Life RNA as Early Genetic Material A hypothesis proposes that RNA was the first genetic material. –Ribozymes are RNA molecules that catalyze their own replication. –DNA needs enzymes to replicate itself.

8. 12.3 Origin of Life Eukaryotic cells may have evolved through endosymbiosis. Endosymbiosis is a relationship in which one organism lives within the body of another. Mitochondria and chloroplasts may have developed through endosymbiosis.

9. 12.3 Origin of Life The evolution of sexual reproduction led to increased diversity. Genetic variation is an advantage of sexual reproduction. Sexual reproduction may have led to the evolution of multicellular life.

10. 12.3 Origin of Life How do we know how old some of these fossils are? Dating fossils: 1. Relative Dating: –compares placement of fossils in layers of rock –Estimates the time period that the organism lived

11. 12.3 Origin of Life neutrons protrons 2.Radiometric dating: –uses decay of unstable isotopes. –Isotopes are atoms of an element that differ in their number of neutrons. –Isotopes are unstable in their nuclei, therefore they decay –The rate of decay of any isotope is known as a half life (number of years it takes for half of the isotopes to decay). – 14 C has a half life of approximately 5700 years, and 14 N is a decay product. –Scientists look at the ratio of 14 C and 14 N to tell us how old something is.

12. 12.3 Origin of Life