Origin of Life on Earth

Abiogenesis is the concept that living organisms can arise naturally from non-living substances.

Autogenesis is another term for spontaneous generation, meaning self-origin of life from non-living matter.

Biogenesis is the idea that life arises only from pre-existing life.

• The universe is about 13.8 billion years old, and the origin of life is considered a unique event in the history of the universe.
• The universe consists of huge clusters of galaxies containing stars, gas, and dust.
• The Big Bang theory explains the origin of the universe as a singular huge explosion, after which the universe expanded and its temperature decreased.
• Hydrogen and helium formed later, and these gases condensed under gravity to form galaxies.
• Earth was formed about 4.5 billion years ago in the solar system of the Milky Way galaxy.
• Early Earth had no atmosphere.
• Water vapour, methane, carbon dioxide, and ammonia released from the molten mass covered the surface of the Earth.
• Ultraviolet rays broke water into hydrogen and oxygen; lighter hydrogen escaped, while oxygen combined with ammonia and methane to form water, carbon dioxide, and other substances.
• The ozone layer formed later.
• As the Earth cooled, water vapour condensed as rain and filled depressions to form oceans.
• Life appeared about 500 million years after the formation of Earth, that is, almost 4 billion years ago.

Meaning of Life:

• Living matter shows characteristics such as responsiveness, growth, metabolism, energy transformation, and reproduction.
• The origin of life has remained an unsolved question despite advances in biochemistry, astrobiology, geography, and molecular biology.
• Many theories and hypotheses have been proposed to explain how life began.

Theory of Special Creation:

• This is the oldest theory.
• It is based on religious belief and has no scientific proof.
• It states that all living organisms were created by a supernatural power.
• It also states that plants and animals appeared on Earth in the same form in which they exist today.
• Since the process cannot be observed, it does not fall within scientific investigation.

Cosmozoic Theory or Panspermia:

• This theory states that life did not originate on Earth.
• It suggests that life came from other planets in the form of spores or microorganisms.
• Some thinkers believed that units of life were transferred from one planet to another.
• This theory is not accepted because it does not explain how life originated on other planets.

Theory of Spontaneous Generation:

• This theory is also called abiogenesis or autogenesis.
• It states that life originated spontaneously from non-living matter.
• It was believed in several ancient civilisations.
• Thinkers such as Anaximander, Epicurus, Aristotle, and Van Helmont supported ideas related to the spontaneous origin of life.
• This theory was later disproved by experiment.

Theory of Biogenesis:

• This theory states that living organisms are produced only from pre-existing living organisms through reproduction.
• It explains continuity of life, but it does not explain the origin of the first life on Earth.

Francesco Redi:

• In 1668, Francesco Redi cooked fish and placed it in three jars.
• One jar was left uncovered, one was covered with muslin cloth, and one was covered with parchment paper.
• Maggots appeared only in the uncovered jar.
• He concluded that maggots developed from eggs laid by flies, not from the meat itself.
• This disproved spontaneous generation of maggots.

A–C. Experiment of Francesco Redi to disprove the theory of spontaneous generation: A. jar uncovered; B. jar covered with muslin cloth; C. jar covered with parchment paper.

Lazzaro Spallanzani:

• In 1767, Lazzaro Spallanzani prepared nutritive broth of vegetables and meat and sealed it in flasks.
• He boiled the broth for four hours and kept it for several days.
• No organisms appeared in the sealed flasks.
• He concluded that microorganisms were not formed spontaneously but came from the air.
• When the neck of the flask was broken, and air entered, rich bacterial growth appeared within a few days.

A–B. Experiment of Lazzaro Spallanzani to show biogenesis of life: A. nutrient broth in a sealed flask, B. neck broken flask. Note that the broth boomed with rich bacterial growth.

Louis Pasteur:

• In 1861, Louis Pasteur used a flask half-filled with sugar and yeast syrup.
• He bent the neck of the flask into an S-shaped swan neck.
• After boiling and cooling, no growth of organisms appeared for several days.
• Air entered the flask, but airborne particles got trapped in the bend, so only pure air reached the liquid.
• When the neck was broken, microorganisms grew in the flask.
• This experiment conclusively disproved spontaneous generation.

Experiment of Louis Pasteur to refute the concept of spontaneous generation

• The origin of life is a unique event; Earth formed about 4.5 billion years ago, and life appeared about 4 billion years ago.
• Special Creation Theory: all organisms were created by a supernatural power; religion-based, no scientific proof.
• Cosmozoic Theory (Panspermia): life came from other planets as spores; not accepted, as it doesn't explain life's origin elsewhere.
• Spontaneous Generation (Abiogenesis): life arose from non-living matter; later disproved.
• Biogenesis: life arises only from pre-existing life; explains continuity but not the first origin.
• Redi (1668): maggots appeared only in the uncovered meat jar, showing they came from fly eggs.
• Spallanzani (1767): sealed boiled broth showed no growth, proving microbes come from air.
• Pasteur (1861): swan-neck flask trapped airborne microbes, conclusively disproving spontaneous generation.