Origin of Life

7.1 What is Evolution?

Evolutionary biology is the study of the history of life forms on earth. To understand the changes in flora and fauna that have occurred over millions of years, we must first understand the wider canvas — the origin of the universe, the formation of earth, and finally the emergence of life on our planet.

A Timeline of Cosmic Origins

The Big Bang theory proposes that the universe began approximately 13.8 billion years ago from a singular explosion. Following this event the universe expanded and cooled, leading to the formation of hydrogen and helium. Under gravity these gases condensed to form galaxies. Within the Milky Way galaxy, the earth is estimated to have formed about 4.5 billion years ago.

The early earth had no atmosphere as we know it today. Water vapour, methane (CH₄), ammonia (NH₃) and carbon dioxide (CO₂) — released from a molten interior — covered the surface. Ultraviolet (UV) radiation from the sun split water vapour into hydrogen (H₂) and oxygen (O₂); lighter hydrogen escaped while oxygen combined with ammonia and methane to form water, CO₂ and other compounds. The ozone (O₃) layer eventually formed in the upper atmosphere, shielding the surface from harmful UV. As the planet cooled, water vapour condensed and fell as rain, filling depressions to form the first oceans. Life appeared on earth approximately 4 billion years ago — about 500 million years after the planet itself formed.

7.2 Early Theories about the Origin of Life

Theory of Special Creation

Conventional religious literature describes the theory of special creation. Three claims define it: (i) all living organisms were created as they exist today, (ii) diversity has never changed and never will, and (iii) earth is about 4,000 years old. Nineteenth-century scientific evidence challenged all three claims.

Spontaneous Generation (Abiogenesis)

For a long time it was believed that life arose spontaneously from decaying or rotting matter — straw, mud, broth. This is the doctrine of spontaneous generation.

Louis Pasteur (1860s) demonstrated through careful experimentation that life arises only from pre-existing life. In pre-sterilised, sealed flasks containing killed yeast, no new life appeared. In flasks open to air, microbes appeared and grew on the killed yeast. The doctrine of spontaneous generation was finally dismissed. However, Pasteur's work did not answer how the first life form on earth had originated.

Panspermia

Panspermia proposes that life — or its molecular seeds — was transferred to earth from elsewhere in the cosmos, perhaps carried by meteorites or comets. Early Greek thinkers proposed that units of life called spores were transferred between planets. Panspermia remains a hypothesis favoured by some astronomers, but it merely shifts the question of life's origin from earth to another world.

7.3 Oparin–Haldane Hypothesis (Chemical Evolution)

The Russian biochemist A. I. Oparin and the British biologist J. B. S. Haldane independently proposed in the 1920s that the first form of life arose from pre-existing non-living organic molecules — RNA, proteins, fats, sugars — and that the formation of life was preceded by chemical evolution — the formation of diverse organic molecules from inorganic constituents.

7.4 Miller–Urey Experiment (1953)

In 1953, Stanley L. Miller, an American graduate student working with Harold Urey, designed a laboratory-scale apparatus to simulate the conditions of the early earth's atmosphere and ocean.

Apparatus and Method

Miller's apparatus consisted of:

1. A small flask of boiling water (representing the early ocean) — water vapour rose into a larger flask.
2. A large reaction flask containing the early "atmosphere": methane (CH₄), ammonia (NH₃), hydrogen (H₂) and water vapour, maintained at 800°C.
3. Two tungsten electrodes producing continuous electric sparks (simulating lightning).
4. A condenser that cooled the gases — droplets fell into a U-shaped trap.
5. The trap was sampled periodically for the analysis of any compounds formed.

Results of the Miller–Urey Experiment

After running the apparatus continuously for about one week, Miller analysed the contents of the trap and discovered:

• Amino acids — including glycine, alanine and aspartic acid — the building blocks of proteins.
• In follow-up experiments using slightly modified mixtures, sugars, nitrogenous bases, pigments and fats were also produced.

Analysis of meteorite content has subsequently revealed similar organic compounds — independent evidence that this kind of abiotic chemistry occurs elsewhere in space. With this evidence, the first part of the conjectured story — chemical evolution — became broadly accepted in the scientific community.

7.5 From Molecules to Cells (Biogenesis)

The story now picks up about 3 billion years ago. The first non-cellular forms of life were probably giant molecules — RNA, protein, polysaccharides — that could reproduce themselves to some extent. The first cellular forms of life did not appear until approximately 2,000 million years (2 billion years) ago. These were probably single cells, and all early life forms existed in water.

This view — that the first form of life arose slowly through evolutionary forces from non-living molecules — is called biogenesis in the modern scientific sense, and it is accepted by the majority of scientists today.

Comparing the Theories

Worked Examples