The origin of life and chemical evolution

How old is everything?

Start with the biggest question. Where did the Earth come from?

The usual answer is the Big Bang. The universe began as a tiny, extremely hot point and has been spreading out ever since. Stars and galaxies formed from it. NCERT says the universe is about 20 billion years old. The value accepted today is about 13.8 billion years, so you may meet both numbers.

The Earth formed about 4.5 billion years ago. At first it had no atmosphere. Gases such as water vapour, methane, carbon dioxide and ammonia came out of the molten rock. The ultraviolet rays of the Sun broke some water into hydrogen and oxygen. The light hydrogen escaped. The oxygen joined with ammonia and methane to make water, carbon dioxide and other things, and an ozone layer began to form. As the Earth cooled, the water vapour fell as rain and filled the hollows. These were the first oceans.

NCERT says that life appeared about 500 million years after the Earth formed, which is nearly 4 billion years ago.

Old ideas about where life came from

Several ideas have been given. You need to know three of them.

• Special creation. Each kind of living thing was made as it is, and it has not changed. This is a belief and not a result of experiments.
• Spontaneous generation. Life arises again and again from non-living matter. People once believed that maggots came from rotting meat and mice from heaps of grain.
• Panspermia. Units of life, called spores, travelled through space and reached the Earth and other planets. Some astronomers still like this idea. It does not say how life began. It only moves the question to another place.

Pasteur and the end of spontaneous generation

Louis Pasteur tested spontaneous generation with careful experiments. He boiled a broth in flasks to kill every living thing in it. In flasks that were closed or fitted with a long, curved neck, the broth stayed clear for as long as we like to wait. When air carrying dust and germs reached the broth, living things grew in it. NCERT describes his result in this way: life did not come from killed yeast in a sterilised flask, but new living things arose in a flask that was open to the air.

So life comes from pre-existing life. This is called biogenesis. It holds for the living things that we see today. It does not say how the very first living thing arose.

Chemical evolution: Oparin and Haldane

A Russian scientist, Oparin, and an English scientist, Haldane, proposed an idea about the first life. They said that the first form of life came from non-living organic molecules, which are the molecules of carbon that living cells are made of, such as RNA and protein. The making of life was preceded by chemical evolution: the slow formation of many kinds of organic molecules from inorganic ones.

They described the young Earth as a place with a high temperature, volcanic storms and a reducing atmosphere. A reducing atmosphere has no free oxygen. It holds gases rich in hydrogen, such as methane ($\text{CH}_4$) and ammonia ($\text{NH}_3$).

Miller's experiment

In 1953, S. L. Miller, an American scientist, tested the idea. He built a closed set of glass tubes and flasks.

• Inside went methane, hydrogen, ammonia and water vapour, with no oxygen. NCERT states a temperature of 800 °C.
• A spark, an electric discharge, passed through the gases. It stood for lightning.
• A condenser cooled the gases, and the liquid collected in a trap.

After some days the liquid held amino acids, the building blocks of proteins. Other workers made sugars, nitrogen bases, pigments and fats in similar experiments. Miller did not make a living cell. He showed that the molecules of life can form from simple gases and energy.

This part is not in the NEET syllabus, but A-Level and IB ask it. Scientists now think the early atmosphere had more carbon dioxide and nitrogen than Miller used. Hot vents on the sea floor are another possible place for chemical evolution.

From molecules to cells

NCERT gives the next steps.

• About 3 billion years ago, the first non-cellular forms of life appeared. They were probably giant molecules such as RNA, protein and polysaccharides, in capsules. They may have copied their own molecules.
• About 2 billion years ago, the first cellular form of life appeared. These were single-celled organisms, in water. Some of them could trap light and release oxygen, in a reaction like photosynthesis.
• After the cells arose, biological evolution went on by natural selection, which is the idea of the next lesson.

The dates are those printed in NCERT. Fossils now show cells older than 3 billion years. In an examination, use the order of events: chemical evolution, then non-cellular life, then cells.

Earth in one day

The time since the Earth formed is hard to feel. Squeeze 4.5 billion years into one day. One hour is 187.5 million years.

You should see that the first cells appear in the afternoon, the dinosaurs are gone in the last 21 minutes before midnight, and modern humans arrive in the last second or two.

Try it in Python

The program turns each date into a time on the 24 hour clock.

Practise with new numbers

Common mistakes

• Saying that Pasteur showed how life began. He showed that life today comes only from life. He did not explain the first life.
• Saying that Miller made a living cell. He made amino acids.
• Giving the early atmosphere free oxygen. It was a reducing atmosphere. Oxygen came later.
• Mixing the theory and the experiment. Oparin and Haldane gave the idea of chemical evolution. Miller tested it.
• Calling chemical evolution the same as Darwin's evolution. Chemical evolution made molecules. Biological evolution works on living things.

Remember these

• The Earth is about 4.5 billion years old. Life began in water, about 4 billion years ago by the NCERT date.
• Pasteur: life comes from pre-existing life (biogenesis).
• Oparin and Haldane: chemical evolution in a reducing atmosphere. Miller (1953): amino acids from methane, hydrogen, ammonia and water vapour with an electric discharge.
• The order: chemical evolution, non-cellular life, single-celled life, many-celled life.

Check yourself

Try each one on your own first. If you are stuck, open a hint. Then check with Show the answer.

1. Why did Pasteur boil the broth in his flasks before he began?

2. Which gases did Miller put in his flask, and what did he leave out on purpose? What did he find after some days?

3. What is a reducing atmosphere?

4. Put in order: first cells, chemical evolution, non-cellular forms of life, the ozone layer forming.

5. A rock event happened 1.8 billion years ago. At what time on the 24 hour Earth clock does it fall?

References

• NCERT (2023). Biology, Textbook for Class XII, chapter 6.
• Reece J. B. et al. (2017). Campbell Biology, 11th edition, Pearson, chapter 25.