The origin of life

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Origin of life is the last phase of the Earth's evolution. It came after the Earth had formed, its layers had separated and its atmosphere and oceans had developed.
The early Earth could not support life.
Its first atmosphere had no free oxygen.
There was no ozone layer (a layer of ozone gas, O₃, high in the atmosphere that blocks harmful ultraviolet rays from the Sun). So strong UV rays reached the surface.
The surface was hot and was often hit by volcanic gases and lightning.

Chemical evolution means life arose step by step through chemical reactions, not all at once. Modern scientists see the origin of life this way.
The steps:
Simple molecules (water, methane, ammonia, hydrogen) reacted with each other.
These reactions made complex organic molecules (carbon-based molecules such as amino acids, the building blocks of proteins).
These molecules then became arranged so that they could copy themselves.
Result: lifeless (inanimate) matter became living substance.
When and where: life probably began about 3,800 million years ago. For a very long time it stayed only in the oceans. Seawater shielded early life from UV rays.

Oparin-Haldane "primordial soup" hypothesis: Oparin and Haldane put this idea forward in the 1920s. It says that simple molecules in the early oceans reacted and formed the first organic compounds. The oceans became a warm "soup" of these chemicals. [4]
Miller-Urey experiment (1953): Stanley Miller and Harold Urey tested this idea in a laboratory.
Set-up: a flask of water stood for the ocean. It was joined to a flask holding a reducing atmosphere (an atmosphere with no oxygen) of water vapour, methane, ammonia and hydrogen. [4]
Process: electric sparks were passed through the gases to copy lightning. [4]
Result: amino acids and other molecules needed for life formed from these simple compounds. [4]
Meaning: the experiment supports chemical evolution and the primordial soup idea.
Hydrothermal vents: openings on the deep ocean floor that pour out hot, mineral-rich water heated by magma below. They are a leading alternative setting for the start of life. Other proposed settings range from deep-sea hydrothermal vents to outer space. [5]

Idea	Setting	Source of energy	Key test or evidence
Primordial soup (Oparin-Haldane, 1920s)	Shallow early oceans	Lightning, UV rays	Miller-Urey experiment, 1953
Hydrothermal vent idea	Deep ocean floor	Heat and chemicals from the Earth's interior	Vents still host life today

Fossils are the remains or traces of past life kept in rocks. They are the main record of how life evolved.
Blue-green algae are cyanobacteria: tiny (microscopic) organisms that make their own food by photosynthesis (using sunlight, water and carbon dioxide to make food, which releases oxygen).
Structures very like modern blue-green algae are found in rocks older than about 3,000 million years.
Stromatolites: cyanobacteria grow in thin, sticky mats. These mats trap sediment, layer after layer, and harden into layered rock mounds called stromatolites.
Some are about 3.5 billion years old. They are among the oldest signs of life on Earth.
Living stromatolites still grow today at Shark Bay, Western Australia. They are thought to be some of the most ancient forms of life on Earth. [3]
India: the Stromatolite Fossil Park is at the Jhamarkotra rock phosphate deposit in Udaipur district, Rajasthan. The Geological Survey of India (GSI) declares such geo-heritage sites and protects them. [2]

Cyanobacteria added oxygen to the oceans and then to the air.
At first, oxygen made by photosynthesis built up in the seawater.
Rocks called the Mt. McRae Shale in Western Australia are 2.5 billion years old. They show that the oceans held a lot of oxygen, over a wide area and at great depth, before the air did. [3]
Great Oxidation Event (GOE): the time when oxygen first built up in the atmosphere, about 2.5–2.3 billion years ago. [3]
Life moved onto land late, only about 500–450 million years ago.
Oxygen in the air formed the ozone layer.
The ozone layer blocked UV rays.
The land became safe for living things.

Event	Approximate time (years ago)
Big Bang	13.7 billion
Earth formed	4.6 billion
Life began (in the oceans)	3,800 million
Oldest stromatolites	about 3.5 billion
Fossils like blue-green algae	older than 3,000 million
Great Oxidation Event	2.5–2.3 billion [3]
Life moves onto land	500–450 million

Life probably began about 3,800 million years ago, in the oceans and not on land.
Structures like blue-green algae are found in rocks older than about 3,000 million years. Stromatolites go back about 3.5 billion years.
Blue-green algae = cyanobacteria. They are bacteria, not true algae, and they carry out photosynthesis that releases oxygen.
Stromatolites are layered rocks built by cyanobacterial mats. Living examples grow at Shark Bay, Western Australia. [3]
Stromatolite Fossil Park is at Jhamarkotra, Udaipur district, Rajasthan, and is declared by the GSI. [2]
Miller-Urey (1953): sparks were passed through methane, ammonia, hydrogen and water vapour, and amino acids formed. There was no oxygen in the gas mix. [4]
Primordial soup idea: Oparin and Haldane, 1920s. [4]
Correct order: Big Bang → Earth forms → life in the oceans → Great Oxidation Event (2.5–2.3 billion years ago) → ozone layer → life on land (500–450 million years ago). [3]
Trap: the early atmosphere had no free oxygen. Oxygen in the air came from living things, not from volcanoes.

Life and the Earth changed each other. Cyanobacteria turned an atmosphere with no oxygen into one rich in oxygen. That oxygen formed the ozone layer, which made it possible for life to live on land. This is a GS-I example of how the Earth's spheres interact.
Chemical evolution can be tested. The Miller-Urey experiment (1953) made amino acids from early-atmosphere gases. Hydrothermal vents offer another possible setting. Together they explain how lifeless matter may have become living matter. [4][5]
Rocks as an archive: fossils and stromatolites date the start of life and of oxygen. Sites such as the Stromatolite Fossil Park in Udaipur show why India needs geo-heritage protection by the GSI, for science, education and geotourism (GS-I and GS-III, conservation). [2]