How Simple Chemicals on the Early Earth Could Have Formed Life's Building Blocks

Every living thing today shares the same basic chemistry, which points to a common origin. Scientists have tested how simple molecules on the early Earth could have formed the building blocks of life, and they read the story of later change in fossils, body structures and molecules.

This lesson covers the origin of life, the evidence for evolution, and adaptive radiation.

The Oparin-Haldane theory proposes that life arose on the early Earth from inorganic molecules through a long chemical evolution, and the Miller-Urey experiment showed that such conditions can produce organic molecules such as amino acids.

The Oparin-Haldane theory:

- The early atmosphere was reducing, with methane, ammonia, hydrogen and water vapour, and little or no free oxygen
- Energy from lightning, ultraviolet radiation and volcanic heat drove reactions between these gases
- Simple organic molecules such as sugars, amino acids and nitrogen bases formed and collected in the oceans
- These joined into larger molecules and eventually into membrane-bound droplets that could grow and divide

The Miller-Urey experiment:

- A closed glass apparatus held methane, ammonia, hydrogen and water vapour at about 800 °C
- Electric discharges imitated lightning, and a condenser cooled the gases so that liquid collected below
- The liquid was found to contain simple amino acids

Significance:

- Showed that organic building blocks can form from inorganic molecules without living cells
- Gave experimental support to the idea of chemical evolution
- Similar experiments later produced sugars, nitrogen bases, pigments and fats

An everyday example. Lightning during a monsoon storm releases the kind of energy the experiment imitated with electric sparks.

The substance. The experiment made building blocks, not life — how such molecules became self-copying cells is still an open question.

Evidence for evolution comes from fossils, comparative anatomy, embryology and molecular similarities; homologous organs share an origin but may differ in function, while analogous organs share a function but differ in origin.

Palaeontological evidence:

- Fossils are remains or traces of organisms preserved in rocks
- Rock layers formed at different times hold different life forms, showing change over time
- The ages of fossils are estimated by radioactive dating

Morphological and anatomical evidence:

- Homologous organs — similar basic structure and origin, different functions; forelimbs of whales, bats, cheetahs and humans; thorns of Bougainvillea and tendrils of Cucurbita — these show divergent evolution
- Analogous organs — similar function, different structure and origin; wings of butterflies and birds; eyes of the octopus and mammals; flippers of penguins and dolphins; sweet potato (a root) and potato (a stem) — these show convergent evolution

Embryological evidence. Embryos of all vertebrates, including humans, develop a row of vestigial gill slits behind the head, which are functional only in fish.

Molecular evidence. Similar proteins, genes and biochemical pathways in very different organisms point to common ancestry.

An everyday example. A potato and a sweet potato on a kitchen shelf both store food, but one is a stem and the other a root — a case of analogy.

The substance. Similar function does not mean shared ancestry — penguin and dolphin flippers look alike because of similar habitats, not common descent.

Adaptive radiation is the evolution of many different species from one ancestral form, each adapted to a different habitat or way of life, and the finches of the Galapagos Islands are a classic example.

Darwin's finches:

- An ancestral seed-eating finch reached the islands
- Over many generations, populations spread into habitats with different foods
- Their beaks evolved into different shapes and sizes suited to their diets
- The result includes seed-eating finches with thick, strong beaks, insect-eating finches with thin, pointed beaks, and vegetarian finches

Other examples:

- Australian marsupials — the kangaroo, koala, marsupial mole, sugar glider and Tasmanian wolf all arose from a common ancestor
- Placental mammals — a similar radiation produced wolves, flying squirrels, moles and anteaters

Convergent evolution between radiations. When separate radiations produce similar forms in similar habitats, such as the Tasmanian wolf and the placental wolf, it is convergent evolution.

An everyday example. The many dog breeds seen in Indian homes and streets show how one ancestral form can diverge into very different shapes — though here people did the selecting, not nature.

The substance. Adaptive radiation is divergent evolution on a large scale — like homologous organs, it begins with a shared ancestor and ends in varied forms.