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 of Earth’s origins is the nebula hypothesis.

Several sets of hypotheses propose how life began on Earth. There are two organic molecule hypotheses. Miller-Urey experiment: Demonstrated that organic compounds could be made by passing an electrical current, to simulate lightning, through a closed system that held a mixture of gases. electrodes heat source amino acids water “atmosphere” “ocean”

There are two organic molecule hypotheses. Meteorite hypothesis: Amino acids may have arrived on Earth through meteorite or asteroid impacts. electrodes heat source amino acids water “atmosphere” “ocean”

There are different hypotheses of early cell structure. iron-sulfide bubbles hypothesis: Biological molecules combined in compartments of chimneylike structures on the ocean floor. The compartments acted as the first cell membranes.

There are different hypotheses of early cell structure. lipid membrane hypothesis: Lipid spheres, or liposomes, could form around a variety of organic molecules, acting as early cell membranes.

A hypothesis proposes that RNA was the first genetic material. Ribozymes are RNA molecules that catalyze their own replication. May have been the first genetic material of cells.