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

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

Eventually Cooling: Water vapor condensed (oceans formed) 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

Several sets of hypotheses propose how life began on Earth. There are two organic molecule hypotheses. Miller-Urey experiment electrodes heat source amino acids water “atmosphere” “ocean” Energy from lightning created organic materials from inorganic ingredients Result: amino acids and later nucleotides meteorite hypothesis amino acids may have arrived on Earth through meteorite or asteroid impacts (outer space).

There are different hypotheses of early cell structure. iron-sulfide bubbles hypothesis lipid membrane hypothesis 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.

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.