The evidence for evolution: fossils, anatomy, DNA and Gondwana

Fossils and what they can show

• A fossil is the remains or trace of an organism preserved in rock; most form when a dead organism is buried quickly in sediment, and its hard parts are slowly replaced by minerals.
• Soft-bodied organisms, land animals and anything in eroding uplands rarely fossilise, so the record is incomplete and biased towards shelled sea creatures.
• Within a sequence of rock layers, lower layers are older (superposition), so fossils show the order in which forms appeared and changed.
• Transitional fossils show mixtures of ancestral and descendant features, such as Tiktaalik (a fish with limb-like fins) and Archaeopteryx (a feathered dinosaur with teeth and wing claws).
• New Zealand has some of the world's oldest penguin fossils: Waimanu, about 60 million years old, from near Waipara, was already flightless and swam with its wings, but its wing bones were less flattened than a modern penguin's flipper and it had a long, slender beak.

Dating rocks and fossils

• Relative dating puts layers and fossils in order (older below, younger above) and uses index fossils, species that lived briefly but widely, to match layers between places.
• Absolute (radiometric) dating uses radioactive isotopes, which decay at a fixed rate: the half-life is the time for half the parent atoms in a sample to decay.
• Carbon-14 (half-life 5730 years) dates once-living material up to about 50 000 years old, because after that too little carbon-14 is left to measure.
• Potassium-40 decays to argon-40 with a half-life of 1.25 billion years, so it dates volcanic rock; a fossil between two dated ash layers must be between their ages.
• In New Zealand, volcanic ash layers (tephras) from known eruptions give dated markers in lake and swamp sediments.

Try it: half-lives

Homologous structures

• Homologous structures have the same basic structure and position because they were inherited from a common ancestor, even if they now do different jobs.
• The forelimbs of humans, bats and whales all have one upper arm bone, two forearm bones, wrist bones and five digits, used for grasping, flying and swimming.
• No engineer would build a wing and a flipper from the same set of parts; shared ancestry followed by divergent evolution explains it.
• Vestigial structures are reduced homologous structures with little or no function, such as the tiny wings of kiwi, the hip bones inside whales and the human tailbone; they make sense only as leftovers from ancestors that used them.

Analogous structures

• Analogous structures do the same job but have different structures and origins, because they evolved separately by convergent evolution.
• Insect wings are sheets of chitin with no bones, while bird wings are modified forelimbs with feathers; both fly, but they share no common winged ancestor.
• The eyes of octopuses and humans are both camera-type eyes, but in the octopus the nerve fibres run behind the retina, so it has no blind spot; the eyes were built separately.
• Analogous structures are evidence for natural selection (similar pressures give similar solutions), but they do not show close relationship, so they must not be used to group species.

Try it: homologous, analogous or vestigial?

Molecular evidence: DNA and proteins

• All living things use DNA, the same four bases and almost the same genetic code, which points to a single common origin of life.
• The more closely two species are related, the fewer differences there are in their DNA base sequences and in the amino acid sequences of shared proteins such as cytochrome c and haemoglobin.
• Humans and chimpanzees share about 98.8% of their DNA, more than either shares with gorillas, which matches the order in which their lineages split.
• If differences build up at a roughly steady rate, they act as a molecular clock: the number of differences, divided by the rate, estimates the time since two lineages split, calibrated with dated fossils.
• Molecular evidence can test anatomy, because DNA is not fooled by convergence: it shows that kiwi are not close relatives of moa despite both being flightless New Zealand birds.

Biogeography and Gondwana

• Biogeography is the study of where species live; related species tend to be found near each other, and islands have species related to those on the nearest mainland.
• About 80 million years ago Zealandia began to split from Gondwana, the southern supercontinent that also held Australia, Antarctica, South America, Africa and India.
• Groups that were on Gondwana before it broke up, such as southern beech (Nothofagus) and podocarps, are now spread across the southern continents, as are fossils of the older Glossopteris flora; this splitting of a range by moving land is called vicariance.
• But much of New Zealand's life arrived later across the sea: about 25 million years ago much of Zealandia was under water, and many plants, birds and insects show DNA links to Australia that are far younger than 80 million years.
• So biogeography shows both ancient Gondwanan groups and later dispersal, and DNA dating is what separates the two.

What Achieved, Merit and Excellence look like

• Achieved describes the evidence; Merit explains how it shows evolution or relationship; Excellence weighs two or more kinds of evidence together, including their limits.