Evidence of evolution: fossils, comparative anatomy and embryology

1A bird with teeth

In 1861, two years after Darwin published On the Origin of Species, quarry workers in Bavaria split a slab of fine limestone and found the skeleton of an animal about the size of a crow, surrounded by clear impressions of feathers. It had wings, but also teeth in its jaws, three clawed fingers on each wing and a long bony tail, features of the small dinosaurs, not of modern birds. Archaeopteryx, about 150 million years old, was exactly the kind of in-between form that Darwin's theory predicted should have existed. Fossils are one of several independent lines of evidence for evolution. This lesson examines three of them: fossils, comparative anatomy and comparative embryology.

2Fossils

Most organisms decay completely when they die. Fossils form only in special conditions, usually when remains are buried quickly in fine sediment where there is little oxygen, so decomposers cannot work. Over time, minerals in the water seeping through the sediment replace the hard parts, or the remains dissolve and leave a mould that may fill to form a cast. Organisms can also be preserved in amber, ice, tar or acid peat, and their activity can leave trace fossils such as footprints and burrows.

Dating fossils. In undisturbed sedimentary rock, lower layers are older than the layers above them, which gives the relative order of fossils. Absolute ages come from radiometric dating: radioactive isotopes decay at a constant rate, so the proportion remaining shows how much time has passed.

• Carbon-14 (half-life about 5 730 years) is taken up by living things and starts to decay when they die; it dates remains up to about 50 000 years old.
• Potassium-40, which decays to argon-40 with a half-life of about 1.25 billion years, dates volcanic rock layers above and below fossils, up to billions of years old.

What fossils show. The fossil record shows that life has changed over time, that simple organisms appear before complex ones, and that groups appear in an order that fits evolution: fish before amphibians, amphibians before reptiles, reptiles before birds and mammals. It also contains transitional fossils:

• Archaeopteryx (about 150 million years ago): feathers and wings like a bird; teeth, clawed fingers and a bony tail like a small dinosaur.
• Tiktaalik (about 375 million years ago): a fish with scales and gills, but also a flat head, a neck and fins containing the bones of a wrist, a step towards four-limbed land animals.
• Early whales: a series of fossils from about 50 to 40 million years ago shows four-legged land mammals, then animals that lived in water and on land, then fully aquatic whales with tiny hind limbs.

Limits of the record. The record is incomplete: soft-bodied organisms rarely fossilise, many rocks have been eroded or melted, and only a tiny fraction of fossils have been found. Gaps are expected; what matters is that no fossil has ever been found in a place that contradicts the order evolution predicts.

3Comparative anatomy

Homologous structures. The forelimbs of a human, a bat, a whale and a horse do very different jobs, yet all are built from the same set of bones in the same order: one upper-arm bone, two forearm bones, a group of wrist bones, and five digits (or fewer, reduced from five). This pentadactyl limb makes sense if all these mammals inherited it from a common ancestor and natural selection then modified it for grasping, flying, swimming or running.

Analogous structures tell a different story. The wings of a bird, a bat and a butterfly all allow flight, but the insect wing is a fold of cuticle with no bones at all, and even the bird and bat wings are built differently (feathers on the arm and fingers in a bird; skin stretched between long fingers in a bat). Similar environments and ways of life select for similar solutions in unrelated groups: convergent evolution. The streamlined shapes of sharks (fish) and dolphins (mammals) are another example.

Vestigial structures are hard to explain except as inheritance from ancestors in which they worked: the small pelvic bones buried in the bodies of whales; the tail bones (coccyx) of humans and apes; the tiny, useless eyes of some cave animals; and the remains of hind-limb bones in pythons.

4Comparative embryology

Early embryos of vertebrates are strikingly alike. At some stage, the embryos of fish, frogs, chickens and humans all have a notochord, a dorsal hollow nerve cord, a tail that extends beyond the anus, and a series of pharyngeal arches separated by pouches in the neck region. These features develop very differently later:

Shared developmental features like these are explained by descent from a common ancestor in which they first evolved. In the nineteenth century, Ernst Haeckel claimed that embryos pass through the adult forms of their ancestors and drew embryos that looked more alike than they really are. That claim is wrong: a human embryo never has working gills. The correct point is that related animals share early stages of development because they share the genes that control it.