KEY CONCEPT The origin of life on Earth remains a puzzle.

1. KEY CONCEPT The origin of life on Earth remains a puzzle.

2. 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)

3. Ancient Earth • Atmosphere contents: • Ammonia, H2O vapor, Methane, H2, CO2 • Climate: • Extreme heat due to meteor impacts & volcanic activity • Eventually Cooling: Water vapor condensed (oceans formed) Click on picture to play video clip

4. electrodes “atmosphere” water “ocean” heat source amino acids Several sets of hypotheses propose how life began on Earth. There are two organic molecule hypotheses. • Miller-Urey experiment • 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) • meteorite hypothesis • amino acids may have arrived on Earth through meteorite or asteroid impacts.

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

6. 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.

7. 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.

8. 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.

9. 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

10. protrons neutrons 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). • 14C has a half life of approximately 5700 years, and 14N is a decay product. • Scientists look at the ratio of 14C and 14N to tell us how old something is.