1. The document discusses the origin of life on Earth, beginning with abiogenesis, the process by which life arises from non-living matter. 2. It describes the early Earth environment and the formation of simple organic molecules, followed by more complex polymers that aggregated to form early protocells like coacervates and microspheres. 3. Key events in the development of life included the evolution of protocells into prokaryotic cells, then eukaryotic cells, and later the rise of multicellular organisms.

1. Chapter: Origin of life
(How life originated on earth and how they developed)
by Ratnodip Saha
Contact: ratnodeep2016@duck.com

2. Contents:
1. What is life? What is life cycle?
2. Abiogenesis or Origin of life: chemogeny & biogeny
3. Steps of Abiogenesis
4. What is Hot dilute soup theory?
5. Protocells: coacervates & microspheres
6. What is evolution?
7. What is variation?
8. Differences between variation and Mutation

3. What is Life?
Life can be defined as a distinctive characteristic of a living organism from dead
organism or non-living thing, which can be specifically distinguished by the
capacity to grow, metabolize, respond (to stimuli), adapt, and reproduce.
Life cycle: Life cycle is the series of changes that the individuals of a species
undergo as they pass from the beginning of a given developmental stage
(generally an embryo) to the inception of that same developmental stage in a
subsequent generation.

4. Life cycle of human
The stages of human life cycle or
development include pre-birth, birth, early
childhood, middle childhood, late
childhood, adolescence, early adulthood,
midlife, mature adulthood, late adulthood,
and death.

5. Abiogenesis or Origin of life:
In biology, abiogenesis or the origin of life is the natural process by which life
has developed from non-living matter, such as simple organic compounds.
Abiogenesis consists of two phases:
1. Chemical evolution (chemogeny): Formation of complex organic molecules
from simpler inorganic molecules through chemical reactions in the oceans
during the early stages of the Earth; first step in the development.
2. Organic evolution (biogeny): Formation of protocell and subsequent
developments towards modern eukaryotic cell.

6. Steps of Origin of life:
CHEMOGENY
1. Presence of free atoms in environment: free atoms were present in the
environment. Like hydrogen, carbon, nitrogen etc.
2. Formation of simple inorganic molecules: Simple inorganic molecules were
formed from the free atoms. Like H2
O, NH3
etc. [These happened in prebiotic
oceans, i.e. hot dilute soup theory]
3. Formation of complex (monomer) organic molecules: Then complex organic
molecules (monomers) were formed. Like simple sugar, amino acids etc.
4. Formation of organic polymers: From the monomers, complex polymers of
organic molecules were formed. Like polysaccharides from simple sugar, proteins
from amino acids etc.

7. Steps of Origin of life:
BIOGENY
1. Formation of protocells: Complex organic polymers aggregated together to
form the earliest forms of cells. [Formation of coacervates or microspheres]
2. Evolution of protocells to Prokaryotic cells: Protocells were upgraded to
form prokaryotic cells (having no true nucleus and any membrane bound
organellea), the earliest form of living cells.
3. Formation of Eukaryotic cells: With the help of evolution, prokaryotic cells
evolved to advanced eukaryotic cells.
4. Rise of multicellular organisms: From the unicellular eukaryotic cells,
advanced multicellular organisms originated.

8. Hot dilute soup
The theory of Hot dilute soup or Primordial soup was proposed by Haldane to
describe the prebiotic (before life formation) oceans, where organic biomolecules
were formed from simplest inorganic compounds by the combination of carbon
dioxide, ammonia and ultraviolet radiation.
Oparin and Haldane thought that with the mix of gases in the atmosphere and the
energy from lightning strikes, amino acids could spontaneously form in the
oceans. This idea is now known as primordial soup.

9. Protocells or Protobionts:
Protocells or protobionts are self-organized, endogenously ordered, spherical
collection of lipids. Researchers assume that modern living cells were initially
originated from protocells.
According to Oparin-Haldane theory, simple molecules polymerized into complex
molecules and then these complex molecules formed aggregates, which were
known as coacervates and microspheres (two theories behind protocells).
Coacervates and microspheres are cell-like structures, and they resemble living
cells. But, they do not show all the properties of cells. They form spontaneously in
certain liquids. They are surrounded by a membrane.

10. Coacervate:
Coacervates are membrane-bound vesicles like microscopic structures, formed by
the aggregation of complex organic compounds mainly lipid compounds.
They resemble living cells. However, they do not contain heredity materials (like
DNA/RNA). Coacervates surround by a membrane-like a boundary. They are able
to take substances from their surroundings and grow in size. Once grow into a
certain limit, they divide and form new coacervates.
The word ‘coacervate’ was introduced by Oparin and according to him,
coacervate is a structure which consists of a collection of organic molecules
surrounded by a film of water molecules. He said coacervates are kind of
protocells.

11. Microsphere:
Microspheres are tiny droplet like structure made from the aggregation of organic
molecules especially from proteinoids (or amino acid molecules).
The term ‘Microsphere’ was introduced by Sidney Fox. According to him, the
microsphere is a non-living collection of organic macromolecules with double
layered (non-lipid) outer boundary.

12. Figure: Coacervates and Microsphere
Coacervates Microspheres

13. Coacervates vs Microspheres:
The key difference between
Coacervates and Microspheres is
that the coacervates have a single
membrane while the microspheres
have double membranes.
Furthermore, the coacervates are
aggregates of lipids while the
microspheres are aggregates of
proteinoids.

14. Prokaryotic cells vs Eukaryotic cells

15. What is Evolution?
Biological evolution is change in the
heritable characteristics of a
population over succeeding
generations, which and relies on the
process of natural selection.
Fig: Evolution
of horse

16. What is variation?
Variation refers to the differences or deviations from the recognized norm or
standard. It may be a modification in structure, form or function in an organism,
deviating from other organisms of the same species or group.
In genetics, variation refers to an individual that possesses characteristics different
from the others of the same kind. Genetic variation usually arises as a mutation
in a gene that encodes a protein or an RNA.
Example: A person's skin color, hair color, dimples, freckles, and blood type are
all examples of genetic variations that can occur in a human population.

17. Differences between Variation and Mutation:
Mutation is the natural and permanent change, causing
changes in the DNA or genetic sequence in any living
organisms. These changes can be small or large, which
may affect the entire genes or chromosomes.
On the other hand variation or genetic variation is seen
in groups or population and is observed in genes as
well as in alleles. It can be due to environmental factors
and intensify the process of natural selections. Variation
may bring evolutionary changes.