Evidence from living things

Homologous structures

Comparative anatomy compares the body structures of different species. The forelimbs of a human, cat, horse, dolphin, bat and bird do very different jobs: grasping, walking, galloping, steering, flying. Yet each is built from the same set of bones in the same order: one upper arm bone (humerus), two forearm bones (radius and ulna), a group of wrist bones (carpals), hand bones (metacarpals) and finger bones (phalanges). This shared plan with five digits is the pentadactyl limb.

Features that share the same basic structure because they were inherited from a common ancestor are homologous structures, even if their functions now differ. An engineer designing a wing, a flipper and a hand from scratch would not use the same bones for all three. Evolution explains it: all these animals descend from a four-limbed ancestor, and natural selection modified its limb in different ways in different environments. This is divergent evolution.

Analogous structures and convergent evolution

A butterfly's wing and a bird's wing both work for flight, but they are built completely differently: the insect wing is a sheet of chitin with no bones, growing from the body wall. Structures with the same function but different structure and origin are analogous structures. They show that natural selection can arrive at similar solutions to the same problem in unrelated groups. This is convergent evolution.

Australia is full of examples, because marsupials evolved in isolation and came to fill the same roles as placental mammals on other continents:

DNA shows that a sugar glider is far more closely related to a kangaroo than to a flying squirrel, despite looking so similar. Similar features are only good evidence of a close relationship when they are homologous.

Vestigial structures

A vestigial structure is a reduced feature that has lost most or all of its original function, but is still there because it was inherited from ancestors that used it:

• Whales and some snakes, such as pythons, have small pelvis bones; pythons even have tiny claw-like spurs where hind legs once were.
• Emus and cassowaries have small wings with feathers but cannot fly.
• Humans have a tailbone (coccyx) made of fused tail vertebrae, tiny muscles that raise goosebumps (useful for fluffing up fur, not for skin with little hair), and muscles that can wiggle the ears.
• The human appendix is a much smaller version of the caecum that plant-eating mammals use to digest cellulose. It is not useless: it contains immune tissue and may store helpful gut bacteria. But it no longer digests plant material, and people live normally without it.

Vestigial structures make sense only if species have changed over time.

Evidence from embryos

Comparing embryos (embryology) shows features that adults have lost. Early embryos of fish, birds, reptiles and mammals, including humans, all have a tail and a series of folds in the neck region called pharyngeal arches. In fish these develop into gills; in mammals the same arches become parts of the jaw, the middle ear bones and the larynx. Dolphin embryos grow hind-limb buds, and baleen whale embryos grow tooth buds that are reabsorbed before birth.

A word of caution. In 1874 the German biologist Ernst Haeckel published famous drawings of embryos that exaggerated how alike they are, and you may still see them in older books. Modern photographs show that early embryos differ more than Haeckel drew. But the shared features, such as the tail and pharyngeal arches, are real, and modern developmental genetics shows that the same genes build them in all vertebrates.

Evidence from DNA and proteins

All living things use DNA, the same four bases, the same genetic code and the same 20 amino acids to build proteins. This shared biochemistry points strongly to a single common origin of life. Differences in DNA build up over time, so the more closely related two species are, the fewer differences in their DNA and proteins.

Cytochrome c is a protein of 104 amino acids, used in respiration by almost every living thing. The table shows how many amino acids differ between pairs of species, from one published set of comparisons (different studies give slightly different counts).

DNA comparisons are even more powerful, because they can compare billions of bases. Humans and chimpanzees differ at about 1.2% of the DNA bases that can be lined up; counting pieces of DNA that have been added or lost, the overall difference is closer to 4%. Humans and chimpanzees also share scars in the same places in their DNA: copies of ancient viruses that inserted themselves into the genome of a common ancestor. The chance of the same virus landing in the same place independently is effectively zero.

Because some genes change at a fairly steady rate, the number of differences can be used as a molecular clock to estimate when two lineages split, checked against dated fossils. When DNA was first used to build family trees in the 1970s and 1980s, it could have contradicted the fossil and anatomical evidence. Instead, it broadly confirmed it, while also sorting out puzzles: for example, showing that whales are most closely related to hippos.