Evidence Evolution

7.6 Evidences for Evolution

That evolution of life forms has taken place on earth is supported by evidence from many independent lines of investigation. We examine the five major categories below — paleontological, embryological, anatomical, molecular and biogeographical evidence — each of which converges on the same conclusion: living organisms share common ancestry and have diversified gradually over deep time.

7.6.1 Paleontological Evidence (Fossils)

Fossils are the remains of hard parts of life-forms preserved in sedimentary rocks. Rocks form sediments layer by layer; a vertical cross-section of the earth's crust shows older sediments at the bottom and younger ones above. Different-aged rock layers contain different fossils, indicating that life forms have changed over geological time.

Some fossils resemble organisms that exist today; others represent extinct groups such as dinosaurs and trilobites. Study of fossils in different sedimentary layers tells us the geological period in which each form lived. The ages of fossils are estimated using radioactive dating — measuring the decay of radioisotopes in rock layers.

7.6.2 Embryological Evidence

Ernst Haeckel proposed that the embryos of all vertebrates show certain features in common during their early development that are absent in adults. For example, embryos of all vertebrates — including humans — develop a row of vestigial gill slits just behind the head, although these are functional only in adult fishes.

Haeckel's strong claim — "ontogeny recapitulates phylogeny" — was later corrected by Karl Ernst von Baer, who showed that embryos never pass through the adult stages of other animals; they pass through early embryonic forms common to the group. Even in its corrected form, embryological similarity remains good evidence for shared ancestry.

7.6.3 Anatomical Evidence: Homology & Analogy

Comparative anatomy reveals two contrasting patterns: homologous structures (similar internal anatomy, different function) and analogous structures (similar function, different internal anatomy). They point to two different evolutionary processes.

Homologous Organs (Divergent Evolution)

Homologous organs share the same basic structural plan inherited from a common ancestor but have been modified for different functions. The classic example is the forelimbs of mammals:

• Whale flipper — swimming
• Bat wing — flying
• Cheetah forelimb — running
• Human arm — grasping

All four contain the same bones in the same arrangement: humerus, radius, ulna, carpals, metacarpals, phalanges. The same structure has developed in different directions due to adaptation — this is divergent evolution. Other examples: vertebrate hearts and brains.

In plants, the thorn of Bougainvillea and the tendril of Cucurbita are homologous — both are modifications of the stem at the same nodal position but serve different functions (protection vs. climbing).

Analogous Organs (Convergent Evolution)

Analogous organs perform similar functions and may look alike, but have different internal anatomy. They evolved independently in response to similar environmental selection pressures — this is convergent evolution. Examples:

• Wings of butterfly and bird — both fly, but butterfly wings are membranous folds, bird wings are modified forelimbs.
• Eye of octopus and mammal — both image-forming, but built differently in detail.
• Flippers of penguin and dolphin — both for aquatic propulsion; penguin's is a modified wing, dolphin's is a modified forelimb.
• Sweet potato (root) and potato (stem) — both are food-storing underground organs, but one is a root and the other is a stem.

7.6.4 Molecular Evidence

The most powerful modern evidence comes from biochemical and molecular similarity. Genes and proteins performing the same function across diverse organisms are remarkably similar in sequence — pointing to shared ancestry. For example, the cytochrome c protein involved in cellular respiration differs by only 1 amino acid between humans and chimpanzees, but by 13 amino acids between humans and dogs. The greater the similarity, the more recent the common ancestor.

7.6.5 Evidence from Artificial Selection

Humans have bred plants and animals for agriculture, sport and ornament for millennia — creating dramatically different breeds of dogs, cattle, pigeons and crops within just a few centuries. If humans can produce such diversity in a few hundred years, nature has had millions of years to do the same. Artificial selection is therefore a powerful analogue for natural selection.

7.6.6 Industrial Melanism (Peppered Moth)

In England before industrialisation (1850s), most moths in Biston betularia collections were white-winged. After industrialisation (1920), in the same areas, most were dark-winged (melanised). The proportions had reversed.

7.7 Biogeographical Evidence: Darwin's Finches

During his voyage on HMS Beagle, Charles Darwin visited the Galapagos Islands and observed an amazing diversity of small black birds — later called Darwin's finches. All varieties on the islands had evolved from a single seed-eating ancestor that arrived from the South American mainland.

From the original seed-eating form, descendants with altered beaks arose, enabling them to exploit different food sources: insect-eating, vegetarian, cactus-eating. This is the classic example of adaptive radiation — the evolution of multiple species from a common ancestor adapted to different ecological niches in a geographical area.

Australian marsupials are another example: many different marsupials (kangaroo, koala, Tasmanian wolf) evolved from a single ancestral stock within the isolated Australian continent. When more than one adaptive radiation occurs in different geographical areas to produce similar-looking organisms (e.g., placental wolf in North America and marsupial Tasmanian wolf in Australia), this is also called convergent evolution.

7.8 Summary of Evolutionary Evidence

Worked Examples