1. 12.3 Origin of Life KEY CONCEPT The origin of life on Earth remains a puzzle.

2. 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 (cloud of gas and dust) hypothesis.

3. 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”

4. 12.3 Origin of Life There are different hypotheses of early cell structure. –iron-sulfide bubbles hypothesis: hot iron-sulfide rising from below the ocean floor combines with the cooler ocean water to form chimneylike structures made up of many compartments. These were the first cell membrane that fostered life

5. 12.3 Origin of Life There are different hypotheses of early cell structure. –lipid membrane hypothesis: Liposomes (seen below) formed the first cell membranes.

6. 12.3 Origin of Life 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.