Origins of Life on Earth (College Board AP® Biology): Study Guide

Evidence for the origins of life on Earth

• Scientific research provides evidence that explains how life could have arisen on Earth over billions of years

• One source of evidence is geological research, including the study of:

  • fossilised organisms preserved in rocks that date back millions of years

  • rock composition that indicates early environmental conditions

• Geological evidence suggests that life on Earth originated between 3.9 and 3.5 billion years ago (bya); this can be determined from the following:

  • Earth formed about 4.6 bya

  • conditions remained too hostile for life until about 3.9 bya

  • the earliest fossil evidence for life dates to 3.5 bya

• There are several scientific hypotheses for the origin of life, one of which is known as the RNA world hypothesis

RNA world hypothesis

• The RNA world hypothesis proposes proposes that early life on Earth was based primarily on RNA molecules, before the evolution of DNA and proteins

  • It proposes that RNA could have been the earliest genetic material, as well as functioning as a catalyst for early chemical reactions

• Key assumptions of the RNA world hypothesis are that:

  • RNA functioned as hereditary material

    • Before DNA and proteins, RNA molecules would have needed to be able to replicate themselves, transferring genetic information from one generation to the next

  • base-pairing allows for accurate RNA replication

    • Just like DNA, RNA strands can build complementary copies using base-pairing

    • Without reliable base-pairing, accurate copying wouldn’t be possible, so this is essential for any system based on RNA heredity

  • enzymes were not needed in early chemical reactions

    • The hypothesis proposes that RNA was capable of both genetic and catalytic roles without help from proteins

    • This assumption is supported by the discovery of ribozymes; RNA molecules that catalyze reactions