How Fossils, Bones and Genes Show That Life Evolved

The living world is astonishingly varied, from bacteria to blue whales. Evolutionary biology asks how life first arose from non-living matter and how simple forms gave rise to today's diversity — and it answers with evidence from rocks, bodies, embryos and molecules.

This part covers theories of the origin of life, the sequence of life's diversification, the evidences for evolution, and adaptive radiation.

Panspermia proposed that life arrived from outer space, spontaneous generation claimed that life arose directly from decaying matter until Pasteur disproved it, and the Oparin-Haldane hypothesis proposed chemical evolution of organic molecules from simple inorganic ones, supported when Miller produced amino acids from simple gases with electric sparks.

Panspermia. Units of life, called spores, were thought to have been transferred to Earth from other planets.

Spontaneous generation. Life was believed to arise from decaying and rotting matter such as straw and mud. Pasteur showed that life did not appear in pre-sterilised flasks of killed yeast, while flasks open to air developed new organisms — so life comes only from pre-existing life.

Oparin-Haldane hypothesis:

- The first life arose from non-living organic molecules such as RNA and proteins
- It was preceded by chemical evolution — the formation of organic molecules from inorganic ones
- Conditions on early Earth included high temperature, volcanic storms and a reducing atmosphere of methane, ammonia, hydrogen and water vapour

Miller's experiment:

- An electric discharge was passed through a closed flask of **CH${}_4$, H${}_2$, NH${}_3$ and water vapour at $800^{\circ }$C
- Amino acids formed

An everyday example. Food in a sealed, sterilised can does not grow mould until it is opened, echoing Pasteur's finding that life does not arise on its own.

The substance. Miller showed that organic molecules could form, not that life itself could** — the step from molecules to cells remains unexplained.

The first non-cellular forms of life, probably self-replicating molecules, gave rise to single-celled organisms in water, then to multicellular life, which diversified from invertebrates to fish, amphibians, reptiles, birds and mammals, while plants progressed from algae through bryophytes and pteridophytes to seed plants.

The broad sequence of animals:

- Non-cellular forms — giant molecules such as RNA and proteins, perhaps enclosed by lipid membranes
- Single-celled organisms, which arose in water
- Multicellular invertebrates
- Jawless fish, then lobefinned fish with stout, strong fins that could move on land and in water
- Amphibians, which evolved from lobefins and still depend on water to breed
- Reptiles, laying thick-shelled eggs that do not dry out on land, and diversifying widely, including dinosaurs
- Birds and mammals, which arose from reptile ancestors

Plants:

- Seaweeds and a few plants in water
- Bryophytes and pteridophytes
- Seed ferns and gymnosperms, and finally angiosperms

An everyday example. Frogs laying eggs in monsoon puddles show how amphibians still need water to reproduce, unlike reptiles with shelled eggs.

The substance. The sequence is a branching tree, not a ladder — groups did not simply replace one another in a single line.

Fossils in rock layers show that different life forms existed at different times; homologous organs reveal common ancestry through divergent evolution, while analogous organs show convergent evolution; vertebrate embryos share features such as gill slits; and similarities in proteins and genes point to common descent.

Palaeontological evidence:

- Fossils are remains of the hard parts of organisms preserved in rocks
- Sedimentary rocks form layers, and each layer holds different life forms
- Deeper layers generally hold older forms, including extinct ones such as dinosaurs

Comparative anatomy:

- Homologous organs — similar structure and origin but different functions; the forelimbs of whales, bats, cheetahs and humans share the same bones, showing divergent evolution
- Analogous organs — similar function but different structure and origin; the wings of butterflies and birds, the eyes of octopuses and mammals, and the sweet potato (a root) and potato (a stem), showing convergent evolution

Embryological evidence. Vertebrate embryos, including human embryos, develop a row of vestigial gill slits behind the head, though these function only in fish.

Molecular evidence. Similar proteins and genes in different organisms indicate shared ancestry.

An everyday example. Your arm and a bat's wing contain the same set of bones arranged differently — striking everyday proof of shared ancestry.

The substance. Analogous organs do not show close relationship — similar needs shaped unrelated organs in similar ways.

Adaptive radiation is the evolution of many different species from a common ancestor within one geographical area, each adapting to a different way of life; Darwin's finches and Australian marsupials are classic examples, and when separate radiations produce similar-looking forms, the result is convergent evolution.

Darwin's finches of the Galapagos Islands:

- Many finch species live on the islands
- They evolved from seed-eating ancestors
- Their beaks changed to suit different diets, producing insect-eating and other forms

Australian marsupials:

- Many different marsupials evolved from a common ancestral stock within Australia
- Examples include the kangaroo, koala, wombat, marsupial mole, flying phalanger, Tasmanian wolf and banded anteater

Convergent evolution:

- Placental mammals in other continents underwent their own adaptive radiation
- Some placental forms closely resemble marsupials, such as the placental wolf and the Tasmanian wolf, or the mole and the marsupial mole

Telling them apart:

- Adaptive radiation — one ancestor, many different descendants
- Convergent evolution — different ancestors, similar-looking descendants

An everyday example. Birds in an Indian garden — sunbirds sipping nectar, kingfishers catching fish and parakeets cracking seeds — show how beak shape matches diet, the same link seen among the finches.

The substance. Isolated islands and continents favour adaptive radiation, because they offer many unoccupied ways of life.