Abiogenesis vs. Biogenesis: Origin of Life Theories
Spontaneous Generation Theory
Around 200,000 years ago, the Greek philosopher Aristotle proposed that life could arise spontaneously from non-living matter. He believed that certain materials contained an “active ingredient” capable of generating life.
Aristotle’s hypothesis suggested that a passive principle (dead matter) required an active principle to initiate life. This concept resembles the modern idea of energy.
In the 17th century, Belgian physician Van Helmont even provided a recipe for producing mice in 21 days, involving a sweaty shirt and wheat grains.
Biogenesis Theory
Biogenesis posits that living beings can only originate from other living beings, directly opposing spontaneous generation (abiogenesis).
Francesco Redi’s Experiments
Francesco Redi argued that life cannot arise from non-life. He demonstrated that maggots in decaying meat came from fly eggs, not spontaneous generation. Redi’s controlled experiments involved varying only one factor at a time.
Practice 1: Characteristics of Living Beings
Four characteristics identify a living being:
- Organized functional structure
- Distinct chemical composition
- Ability to incorporate energy
- Self-reproduction
Spontaneous Generation or Biogenesis?
For centuries, it was believed that non-living materials could directly transform into living materials.
Scientists Supporting Spontaneous Generation:
- William Harvey
- René Descartes
- Isaac Newton
- Van Helmont
Recipe for Mice:
Place wheat grains and a sweaty shirt in a box. The “active ingredient” in human sweat on the shirt, combined with the wheat (food for mice), supposedly generates mice.
Practice 2: Redi’s Experiment
Redi used two jars, one open and one sealed, to prevent contact with the outside environment.
Results:
The uncovered jar contained maggots on the decaying meat, while the sealed jar did not.
Objections to Redi’s Results:
Critics argued that sealed jars lacked the “active ingredient” (air) necessary for spontaneous generation.
Redi’s Modification:
Redi changed the jar coverings to mesh, allowing air in while keeping flies out.
New Experiment Result:
No maggots appeared on the meat, as flies were excluded.
Conclusion:
Living beings come from other living beings.
Position on Spontaneous Generation:
The theory is rejected in favor of biogenesis.
The Invisible World and Further Debate
After Van Helmont and Redi, the invention of the microscope revealed a previously unknown world of microorganisms. This discovery provided new arguments for both theories.
Oblot’s experiment showed that boiled hay infusion, when exposed to air, became populated with microorganisms, while a covered infusion remained sterile.
Needham vs. Spallanzani
Needham: Heated and sealed culture media still produced microorganisms, supporting spontaneous generation.
Spallanzani: Boiled sealed media for longer periods and observed no microbial growth, supporting biogenesis.
Controversy:
Needham claimed Spallanzani’s prolonged boiling destroyed the “active principle” in the air. Spallanzani argued Needham’s brief heating didn’t kill all microorganisms.
Pasteur’s Contribution
In 1862, Louis Pasteur definitively disproved spontaneous generation through rigorous experiments. He demonstrated that microorganisms are present in the air and contaminate sterile media, leading to microbial growth.
Pasteur used swan-neck flasks, allowing air in but trapping airborne particles, to refute Needham’s claim that heating destroyed a vital principle.
Needham’s Experiment
Briefly heated sealed media, observed microbial growth, and supported spontaneous generation.
Results:
Microorganisms appeared in the media after a few days.
Objections:
Spallanzani argued that brief heating didn’t eliminate all airborne microorganisms.
Theory Supported:
Spontaneous generation
Pasteur’s Swan-Neck Flask Experiment
Results:
Microorganisms only appeared when the swan neck was broken, allowing airborne particles to enter.
Significance of Swan Neck:
The swan neck trapped airborne microorganisms, preventing contamination of the broth.
Theory Supported and Rejected:
Rejected: Needham’s spontaneous generation
Supported: Spallanzani’s biogenesis
Spallanzani’s Experiment
Prolonged heating of sealed media, no microbial growth, supported biogenesis.
Origin of Earth and Early Conditions
Our solar system likely formed from a proto-star and a cloud of gas and cosmic dust. The sun and planets emerged from this material through gravitational contraction and accretion.
The early Earth consisted of a core (NIFE), a mantle (SIMA), and a crust (STAL). As the Earth cooled, water vapor condensed, forming oceans.
Volcanic activity and tectonic processes shaped the Earth’s surface. Early seas filled depressions, and mountains formed through erosion and tectonic uplift.
Forms of Energy and Synthesis of Organic Compounds
The sun’s atmosphere contains hydrogen, oxygen, nitrogen, and carbon, capable of forming simple molecules.
These molecules were driven away from the sun by thermal agitation and radiation pressure.
First Synthesis of Organic Compounds
The first organic compounds were similar to those required by simple cells today: sugars, fats, proteins, and nucleic acids.
Laboratory Synthesis:
1. Stanley Miller: Simulated early Earth conditions by subjecting a mixture of gases (primitive atmosphere) to electric sparks. After a week, he found amino acids and other organic compounds had formed.
2. Melvin Calvin: Used a cyclotron to provide energy to a mixture of primitive gases, synthesizing sugars, urea, amino acids, and fatty acids.
3. Joan Oro: Mixed hydrofluoric acid and ammonia in water, heated the solution, and obtained adenine, an essential component of nucleic acids and ATP.