Origin of Life, Evidence and Darwin's Theory: Quick Revision

A brief note on Origin of Life, Evidence and Darwin's Theory. Read it before a test. Try each question at the end before you open its answer.

Origin of life

• Earth: about 4.5 billion years old. At first, no atmosphere. Then water vapour, methane, carbon dioxide and ammonia, and later the oceans.
• Old ideas: special creation, spontaneous generation, panspermia (spores from space).
• Pasteur: life comes from pre-existing life (biogenesis).
• Oparin and Haldane: chemical evolution of organic molecules in a reducing atmosphere with no free oxygen.
• Miller (1953): methane, hydrogen, ammonia and water vapour with an electric discharge gave amino acids.
• Order: chemical evolution, non-cellular life (about 3 billion years ago, NCERT), single-celled life (about 2 billion years ago, NCERT).

Evidence for evolution

• Fossils: deeper layers are older. The groups appear in the order invertebrates, fish, amphibians, reptiles, mammals.
• Homologous organs: same origin, different function. Forelimbs of whale, bat, cheetah and human. Thorn and tendril. Divergent evolution.
• Analogous organs: same function, different origin. Wings of butterfly and bird, eyes of octopus and mammals, flippers of penguin and dolphin, potato and sweet potato. Convergent evolution.
• Embryos: gill slits (Haeckel). Von Baer: no passing through adult stages.
• Molecules: similar proteins and genes mean a common ancestor.

Darwin's theory

• Lamarck: use and disuse. Not accepted.
• Darwin: branching descent and natural selection. More young than survive, limited resources, inherited variation, differential survival.
• Fitness: the number of offspring that survive and breed.
• De Vries: mutation theory (large, sudden changes).
• Peppered moth: pale form common before industry, dark form common by 1920.
• Adaptive radiation: one ancestor, many forms. Darwin's finches, Australian marsupials.
• Convergent evolution: different ancestors, similar forms. Tasmanian wolf and placental wolf.

A worked example

A bone has one eighth of the carbon-14 of a living bone. How many half-lives is that, and how old is it? The half-life is 5730 years.

1. One eighth is $\tfrac{1}{2^3}$, so three half-lives have passed.
2. Age: $3 \times 5730 = 17190$ years.

Practice with new numbers

A program to try

The program prints the carbon-14 left after some half-lives.

Mistakes to avoid

• Saying that Pasteur explained the first life. He showed that life today comes from life.
• Mixing up homologous and analogous. Same origin means homologous. Same function means analogous.
• Saying that individuals evolve. Populations evolve.
• Mixing up adaptive radiation and convergent evolution. One ancestor and many forms, against many ancestors and similar forms.

Check yourself

1. Which gas was left out of Miller's flask?

2. Are the forelimbs of a bat and a whale homologous or analogous?

3. Which process gave Darwin's finches?

4. What is fitness?

Sources

• NCERT (2023). Biology, Textbook for Class XII, chapter 6.
• Clark M. A., Douglas M. and Choi J. (2018). Biology 2e, OpenStax, chapter 18.