Origins of life | Biology | Khan Academy

Introduction to the Origins of Life

• Evolution and natural selection are well-understood.

• The fundamental question remains: the origin of life on Earth.

• This leads to considerations of possible life beyond our planet.

Timeline of Earth's History

• Formation of Earth:

  • Earth formed about 4.6 billion years ago (BYA).

  • Hostile environment due to ongoing collisions and instability in the solar system.

• Early Conditions:

  • 4.5 BYA: Earth is unsuitable for even simple life.

  • Formation of the Moon resulted from a collision between proto-Earth and another planet-sized object.

  • Heavy bombardment continued until approximately 3.9 BYA, destabilizing conditions.

Evidence of Early Life

• Earliest Life Evidence:

  • Fossils of microorganisms, specifically stromatolites, found dating back 3.5 BYA.

  • These structures persist today and demonstrate the complexity of early life.

Key Questions about Life's Origins

• The When vs. The How:

  • Establishing a timeline (e.g., 3.7 billion years ago) raises the more critical question of how life originated.

Building Blocks of Life

• Simple Molecules Present on Early Earth:

  • Water (H2O), Carbon Dioxide (CO2), Molecular Nitrogen, Ammonia, Phosphate, and others.

• Formation of Organic Molecules:

  • Organic molecules (e.g., amino acids, nucleotides) are essential for life.

  • Evidence shows that these molecules can be formed abiotically (without life) from simpler ones.

  • Miller-Urey Experiment (1950s):

    • Demonstrated that organic molecules could form under conditions simulating early Earth's atmosphere combined with energy sources like lightning.

Advancement to Complex Life

• From Organic Molecules to Life:

  • Amino acids lead to proteins, and nucleotides form RNA and DNA, which are crucial for living organisms.

  • These processes can occur spontaneously with appropriate conditions and energy.

Hypotheses on Proto-life

• RNA World Hypothesis:

  • First proto-life consisted of self-replicating RNA molecules.

  • RNA serves dual functions: information storage and catalytic activity.

  • Possibility of organization into membrane-bound structures for separation from environment.

• Metabolism First Hypothesis:

  • Basic biochemical pathways existed before cellular life, in environments rich in organic molecules.

  • Self-organizing structures may have emerged as complexity grew.

• Combination of Hypotheses:

  • Future discoveries might reveal connections between these ideas.

Conclusion

• Current biology offers hints about the characteristics of the earliest life forms.

• Ongoing research helps provide insight into the nature and origin of life's complexity.