Origin of life (Biology)

Origin of life (Biology):

The origin of life refers to how life appeared and began to exist on Earth. Many theories have been proposed by various biologists in several period of time in order to explain its origin. Some of the theories are listed as below:

Theory	Proposed by	Description
Theory of biogenesis	Louis Pasteur	· Supported the idea that life comes from pre-existing life.
Primordial Soup Hypothesis	Alexander Oparin	· Life originated from a soup of organic compounds in the early Earth’s atmosphere and oceans.
Miller-Urey Experiment	Stanley L. Miller	· Simple organic molecules could form from inorganic precursors under conditions similar to early Earth’s atmosphere.
Proteinoid Microspheres	Sidney W. Fox	· Suggested that protein-like molecules can self-organize into membrane-bound microspheres, resembling primitive cells.
RNA World Hypothesis	Leslie Orgel	· Supported the idea of RNA as the first genetic material and a key player in the origin of life.
Panspermia Theory	Christian de Duve	· Suggested life on Earth might have originated from extraterrestrial sources like comets or meteorites.
Iron-Sulfur World Hypothesis	Gunter Wachtershäuser	· Proposed that life might have originated around hydrothermal vents using iron-sulfur chemistry.
Clay Hypothesis	Graham Cairns-Smith	· Suggested that life’s building blocks assembled and evolved on the surfaces of clay minerals.
Deep Hot Biosphere Hypothesis	Thomas Gold	· Life might exists deep underground, independent of surface ecosystems.
Endosymbiotic Theory	Lynn Margulis	· Proposed that eukaryotic organelles, like mitochondria and chloroplasts, originated from symbiotic relationships between cells.
Theory of Evolution	Charles Darwin	· Explains how life evolves through natural selection and adaptation to environmental conditions.

Let us discuss some of the important theories in detail:

Theory of special creation:

The Theory of Special Creation states that all living organisms, including humans, were individually and separately created by a supernatural entity, typically a divine being.
It was proposed by Father Suerez as he was a strong believer in supernatural powers and believed in this theory. According to him, life on earth was created in six days.
Theory of Special Creation opposes the concept of evolution and common ancestry.
According to this theory each species were directly and uniquely created and have remained unchanged since their origin.
Theory of Special Creation is not supported by the scientific community as it does not provide testable explanations or evidence based on observable natural processes.

Theory of Abiogenesis

The Theory of Abiogenesis is also known as Spontaneous Generation.
This theory proposed that life can arise from non-living matter under certain conditions.
The Greek scientist and philosopher Aristotle (384-322 BCE) was one of the earliest scientists to describe a theory for abiogenesis,
According to him all life arose directly from different combinations of earth, wind, fire and water.
Although abiogenesis has never been observed in the lab, some mechanism involving it was likely involved in creating the simpler molecules such as lipids, carbohydrates, proteins, etc. that were part of the beginning of life on Earth.
These compounds could have formed through chemical reactions in the atmosphere, oceans, or hydrothermal vents.
This idea assumes that the transition from non-living matter to living entities occurred over a long period of time through gradual chemical and molecular processes.
This theory is opposite to theory of biogenesis, which states that living organisms can only arise from pre-existing living organisms.
Concept of abiogenesis has historical roots dating back to ancient times.
Modern scientific understanding of this process is based on the study of early Earth’s conditions.
Theory of abiogenesis is a fundamental theory in the study of life’s emergence on our planet.

Theory of Biogenesis

The term biogenesis comes from ‘bio’ meaning life, and genesis meaning beginning.
It is an important theory of biology and molecular genetics, which postulates the production of new living organisms from preexisting life.
In simple words, this theory explains how life originates from life.
This theory forms the basis of the cell theory, according to which cell is the basic unit of life and that all living organisms are composed of cells that arise from pre-existing cells.

A comparison between theory of abiogenesis and biogenesis

Aspect	Theory of Abiogenesis	Theory of Biogenesis
Theory	· Life arises from non-living matter under certain conditions.	· Life arises from pre-existing life
Proponent	· Alexander Oparin · J.B.S. Haldane and others.	· Rudolf Virchow · Louis Pasteur
Historical Roots	· Proposed by ancient thinkers	· Scientifically supported since the mid-19th century.
Experiment	· The Miller–Urey experiment was a chemistry experiment performed by Stanley in 1952. · Miller, supervised by Harold Urey at the University of Chicago, and published the following year. · This experiment used water (H₂O), methane (CH₄), ammonia (NH₃), hydrogen (H₂), and an electric arc. · CH_4, NH₃, H₂ were taken in the ratio 2:2:1. · This experiment supported Alexander Oparin‘s and J. B. S. Haldane‘s hypothesis · It is regarded as a groundbreaking experiment investigating abiogenesis.	· The concept of biogenesis was firstly demonstrated by Louis Pasteur by his experiment. · Pasteur use a swan-necked flasks filled with nutrient broth that were boiled to sterilize the contents. · The unique shape of the flasks prevented dust and microorganisms from entering while still allowing air to pass through. · As a result, no microbial growth occurred inside the flasks, even after long periods of time, effectively contradicting the idea of spontaneous generation.
Spontaneous Generation	· Initially associated with abiogenesis but later disproven by experiments.	· Replaced by the concept of biogenesis after Pasteur’s experiments.
Current Status	· Actively studied in modern abiogenesis research, specific mechanisms debated.	· Universally accepted and validated through numerous experiments.
Examples	· Hypotheses about the formation of primitive cells in hydrothermal vents.	· Understanding how a cell divides and produces new cells through mitosis.

The Hydrothermal Vent Hypothesis

According to this theory hydrothermal vents release water. This water is enriched with various minerals and chemical compounds, including hydrogen, methane, carbon dioxide, and metal ions.
Environmental conditions in porous hydrothermal vents created a gradient in positively charged protons that served as a “battery” to fuel the creation of organic molecules and proto-cells.
The high temperatures and mineral-rich nature of hydrothermal vent environments may have provided protection for early biomolecules against degradation and hydrolysis.

The Endosymbiotic Theory

· This theory was proposed by Lynn Margulis in the 1960s.

According to this theory certain organelles within eukaryotic cells were once free-living prokaryotic organisms that were engulfed by another cell in a symbiotic relationship.

A comparison of origin of mitochondria and chloroplast:

Aspect	Chloroplasts	Mitochondria
Proposed by	Lynn Margulis	Lynn Margulis
Origin	Originated from engulfed photosynthetic bacteria.	Originated from engulfed aerobic bacteria.
Prokaryotic Ancestry	Derived from ancient photosynthetic cyanobacteria.	Derived from ancient aerobic bacteria.
Energy Conversion	Chloroplasts convert light energy into chemical energy.	Mitochondria produce energy through aerobic respiration.
Replication	Chloroplasts replicate independently within host cells.	Mitochondria replicate independently within host cells.
Role in Eukaryotic Cells	Assist photosynthesis and produce glucose and oxygen.	Provide energy for the cell through ATP production.
Synergistic Relationship with Host	Mutualistic relationship benefiting both host and chloroplasts.	Mutualistic relationship benefiting both host and mitochondria.
Evidence	Genetic, structural, and functional similarities with cyanobacteria.	Genetic, structural, and functional similarities with bacteria.
Contribution to Eukaryotic Evolution	Enabled photosynthetic organisms and oxygen production.	Enabled complex life forms by providing energy efficiency.