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 electrodes heat source amino acids water “atmosphere” “ocean” meteorite hypothesis

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

There are different hypotheses of early cell structure. lipid membrane hypothesis

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.