Overview
This lecture covers the origin of cells, early Earth conditions, hypotheses for the origin of life, the importance of membranes and genetic material, the concept of LUCA (last universal common ancestor), and evidence for the timeline and ancestry of life.
Early Earth Conditions
- Early Earth had a hot, high-radiation environment with little free oxygen or ozone.
- Greenhouse gases like carbon dioxide and methane were more abundant, making the planet harsh for life.
- Chemical reactions happened faster due to heat, as there were no enzymes to catalyze reactions.
Origin of Life Hypotheses
- Life is thought to have originated from non-living matter through a series of intermediate stages.
- Carbon compounds, especially those forming four bonds, are essential for the basis of life.
- Spontaneous generation is the theory that first cells arose from non-living substances, but full process is unproven.
Timeline and Evolution of Life
- The first life forms appeared about 3.5-4 billion years ago, with bacteria as earliest species.
- Complex life and humans appeared much later; humans are recent in Earth’s history.
Functions and Definition of Life
- Living things perform seven functions of life, including metabolism, reproduction, and response to stimuli.
- Viruses are not considered alive as they cannot perform all functions and lack cellular structure.
- The cell is the smallest unit of self-sustaining life.
Cell Theory and Spontaneous Generation
- Cell theory states all cells come from pre-existing cells; the first cell is an exception (spontaneous generation).
- Spontaneous generation remains untestable due to lack of direct evidence and inability to recreate early Earth.
Miller-Urey Experiment
- The Miller-Urey experiment simulated early Earth to produce organic molecules (amino acids) from inorganic gases.
- Strengths: closed, sterile system; simulated atmosphere and ocean.
- Limitations: overly simplistic, lacked land/varied environments, uncertain conditions.
Importance of Membranes
- Cells require membranes for compartmentalization and maintaining different internal chemistries.
- Phospholipids naturally form vesicles in water, forming primitive membranes needed for cell formation.
Genetic Material: RNA and DNA
- First cells needed genetic material to store and replicate information.
- Early genetic material was likely RNA because it can store information and act as a catalyst (ribozymes).
- All life today uses DNA, suggesting a common origin.
LUCA and Common Ancestry
- LUCA (last universal common ancestor) is the hypothesized single ancestor of all current life forms.
- Universal use of DNA in all living things supports LUCA theory.
- Not the only early organism, but it outcompeted others.
Dating Early Life and Evidence
- Fossils and radiometric dating (carbon, potassium-argon) help estimate ages but are limited for earliest life.
- The molecular clock estimates divergence times based on mutation rates in genetic material.
- Early LUCA likely lived in hydrothermal vents, was anaerobic and autotrophic, using chemosynthesis.
Key Terms & Definitions
- Free oxygen — oxygen not bound to compounds, available in the atmosphere.
- Spontaneous generation — theory that life arises from non-living matter.
- Vesicle — a membrane-bound sphere formed by phospholipids in water.
- LUCA — last universal common ancestor of all currently existing life.
- Molecular clock — method using mutation rates to estimate evolutionary divergence times.
- Ribozyme — RNA molecule with catalytic (enzyme-like) function.
Action Items / Next Steps
- Review the seven functions of life.
- Read provided key ideas and make note of questions.
- Prepare for next class by reviewing cell theory and origin of cells concepts.