1Fossils and how they are dated

A fossil is the preserved remains or trace of an organism that lived long ago: bone, shell, a leaf impression, a footprint. Fossils show organisms that no longer exist, and a sequence of fossils through time shows forms changing.

To use fossils as evidence of change, you need their ages. The CED names three ways.

1. The age of the rocks. Sedimentary rock forms in layers, each laid down on top of the last, so in undisturbed rock, deeper layers are older. A fossil in a lower layer is older than one in a higher layer. This gives relative ages (older or younger), not a number of years.

2. Radioactive decay. Some elements have unstable isotopes that decay into other elements at a fixed rate. The half-life is the time taken for half of the radioactive atoms in a sample to decay. It is not changed by temperature or pressure, so it works as a clock.

• Carbon-14 has a half-life of about 5,730 years. Living things take in carbon-14 while they are alive; when they die, intake stops and the carbon-14 already in their tissues decays. Measuring how much remains, compared with the starting level, gives the time since death. After about ten half-lives too little is left to measure, so carbon-14 dates material up to roughly 50,000 years old.
• For older fossils, scientists date volcanic rock layers above and below the fossil using isotopes with much longer half-lives, such as potassium-40 (half-life about 1.3 billion years).

3. Geographical data. Where a fossil is found matters. The positions of continents have changed, and rocks in different places can be matched by their layers and the fossils they contain. Finding the same fossil species in rock layers on two continents now far apart is evidence that those rocks are of similar age and that the land was once joined.

Worked example (invented). A piece of charcoal from an ancient hearth has one-eighth of the carbon-14 found in living wood.

In undisturbed rock, deeper layers are older; radioactive decay turns "older" into a number of years.

2Homologous structures

Homologous structures have the same basic structure, in the same arrangement, because they were inherited from a common ancestor, even if they now do different jobs. The forelimbs of mammals are the standard case: a human arm, a bat's wing, a whale's flipper and a cat's front leg all contain one upper arm bone, two forearm bones, a group of wrist bones and five digits. Grasping, flying, swimming and walking do not need the same skeleton. The shared plan makes sense only if all four inherited it.

Do not confuse these with analogous structures, which do the same job but are built differently and evolved separately: a bird's wing (bone and feathers) and a butterfly's wing (a sheet of cuticle, no bones). Analogous structures are evidence of similar selective pressures (topic 7.10, convergent evolution), not of close common ancestry.

3Vestigial structures

A vestigial structure is a reduced or non-functional version of a structure that was functional in the organism's ancestors. Its presence is hard to explain except by descent with modification.

• Whales have small pelvic bones buried in the body wall, not attached to the spine and supporting no legs. Their ancestors walked on land.
• Humans have a coccyx, a few fused vertebrae at the base of the spine, where other primates have a tail.
• Some fish that live in permanently dark caves have small, non-functional eyes, often covered by skin. Their relatives in surface waters see.

Vestigial does not have to mean useless: the human coccyx anchors some muscles. It means much reduced from its function in ancestors.

4Biochemical evidence: DNA and protein sequences

All living things store genetic information in DNA, copy it into RNA, and translate it into protein using nearly the same genetic code. That shared machinery is itself evidence of a single common origin.

Sequences go further: they measure how closely related two species are. Mutations change DNA sequences at a fairly steady overall rate, and some of those changes alter the amino acid sequence of the protein. After two lineages split, each builds up its own changes. So the longer ago two species shared an ancestor, the more differences there are between their sequences.

An invented data set: the number of amino acid differences in one 120-amino-acid protein, between a species of fox and five other mammals.

Percent identity is the share of positions that are the same:

Describing these data, as the exam asks:

• A data point: the fox and the horse differ at 15 of 120 positions.
• A pattern: the wolf is most similar (2 differences), the bat least (18).
• A relationship: the fewer the differences, the more recently the two species shared a common ancestor, so the fox is most closely related to the wolf.

DNA comparisons usually give more detail than protein comparisons, because some DNA changes do not change the amino acid (the code has several codons for most amino acids), and some DNA does not code for protein at all.

5Other kinds of evidence

• Geographical (biogeography). Species on islands most resemble species on the nearest mainland, not species on distant islands with similar climates. That is what you would expect if colonists arrived from the mainland and then changed.
• Physical. Measurements of organisms, living and fossil (bone lengths, shell shapes, body sizes), let changes be traced and compared quantitatively.
• Mathematical. Allele frequency calculations (topic 7.5), rates of sequence change used as a molecular clock (topic 7.9), and models of selection make quantitative predictions that data can test.