Evidence for Evolution

Rocks in Pakistan and Egypt hold fossil whales with leg bones. Living whales still carry small hip bones deep inside their bodies. Closer to home, Virginia State Parks reports fossil shark teeth along the Potomac shore at Westmoreland State Park, and Maryland’s Calvert Cliffs on the Chesapeake Bay expose fossils of whales and sharks. So how can rocks and bones tell us what life was like millions of years ago, and how could anyone show that whales are related to animals that walked?

Explore: stack rock layers and count DNA differences

First you stack rock layers that each hold a fossil, then put them in order from oldest to youngest. Next you line up a short stretch of DNA from five species, base by base, and count the differences. Your counts build a similarity table.

Try this.

1. Before you stack the layers, predict which fossil is the oldest. Then build the stack and check your prediction.
2. Tilt or flip part of the stack. Predict whether the bottom layer is still the oldest, then test it.
3. Predict which species will have the fewest DNA differences from humans. Count the differences and see.
4. Compare a longer DNA stretch instead of a short one. Predict whether your ranking of relatives will change.

Explain: what rocks and fossils record

A fossil is the preserved remains or traces of a living thing from the past, such as a bone, shell, tooth or footprint. The fossil record is all the fossils found so far, arranged by age. It shows that kinds of life appeared, changed and often went extinct over time.

The record is incomplete. Most dead plants and animals rot or are eaten before they can fossilize. Fossils form more often when a body is buried quickly in mud or sand under water, so sea animals are better recorded than forest animals. Life has existed for about 3.5 billion years or more, and most of it left no fossils.

Relative dating puts rocks and fossils in order from older to younger, without an age in years. Sedimentary rock builds up in layers, like a stack of mail on a table. In an undisturbed stack, the bottom layer was laid down first. So a fossil in a lower layer is older than one higher up. The analogy breaks where layers have been folded, tilted or broken by Earth movements, so geologists check for that first.

Radioactive dating uses radioactive atoms in certain minerals to estimate a rock’s age in years. These atoms change into other atoms at a steady rate. Measuring how much starting atom remains and how much product has built up gives the time since the mineral formed. Often dated minerals sit in volcanic layers above and below a fossil, so the fossil’s age falls between the two dates.

Different atoms suit different spans of time. Carbon-14 works only for remains up to about 50,000 years old. Fossil whales millions of years old need other atoms.

Explain: what DNA comparisons show

DNA is passed from parents to offspring. When it is copied, small changes sometimes happen, and some are passed on. Comparative genomics is the comparison of the whole DNA, or genome, of different species. A common ancestor is an ancestral population from which two or more species descended. Two species that split from a common ancestor recently have had less time to collect differences. So, for matching stretches of DNA, more similar sequences usually mean a more recent common ancestor.

In 2005, a team compared the genome of a chimpanzee named Clint with the human genome. Across about 2.4 billion lined-up bases, 1.23 of every 100 differed.

percent matching = 100% − percent different = 100% − 1.23% = 98.77%, or about 98.8%

That number counts single-base differences in matching regions only. The same study also found about five million places where short pieces of DNA had been added or lost, which the 1.23% leaves out. A 2025 study of complete ape genomes compared regions that earlier work had skipped. So a “percent similar” depends on what was compared.

DNA also tested where whales belong. In 1999, a team compared pieces of DNA that had been inserted at particular spots in the genomes of whales and hoofed mammals. Species sharing an insertion at the same spot likely inherited it from one ancestor. The pattern placed whales among the even-toed hoofed mammals, grouped with hippos. This does not mean hippos were whale ancestors. It means whales and hippos share a common ancestor.

Humans did not come from chimpanzees; both descend from a common ape ancestor.

Explain: many fields, one history

Each field below could have contradicted the others. For whales, they agree.

A homologous structure is a body part in different species that comes from the same structure in a common ancestor. A whale flipper, a human arm and a bat wing hold the same set of bones, arranged differently.

A vestigial structure is a reduced body part that no longer does the job it did in ancestors. Vestigial does not mean useless. A 2014 study measured hip bones from 130 whales, dolphins and porpoises of 29 species. In males, these bones anchor muscles used in mating.

Embryology is the study of how an organism develops before birth or hatching. Pantropical spotted dolphin embryos grow hind-limb buds that stop growing and break down around the fifth week. The embryo is not replaying its ancestors; it starts a limb and then stops.

A theory in science is a well-tested explanation supported by many lines of evidence. The theory of evolution explains fossils, anatomy, embryos and DNA with one history of common descent. For the mechanism that drives that change, see Natural Selection and New Species.

Elaborate: read DNA comparisons and fossil ages

The table mixes two kinds of evidence. Read the third column before you compare any numbers.

Sources: the chimpanzee row compares one chimpanzee, Clint, with the human reference genome. The mouse, chicken and fly values are published summaries from the National Human Genome Research Institute (NHGRI), not one study. The first three fossil ages are rounded teaching ages from a Burke Museum classroom activity, so they are rounded values for a group, not measurements of one skeleton.

1. Put the five fossils in order from oldest to youngest. How many million years separate Pakicetus and WH 10001?
2. The mouse value counts matching bases, and the chicken value counts genes. Can you use them to rank mouse and chicken as human relatives? Why or why not?
3. Does the fossil order prove that each whale was the parent species of the next one?

Design problem

Some people say a whale’s small hip bones are leftovers with no job. Plan a study, using museum skeletons, to test whether the hip bones are linked to mating in male whales.

Evaluate: check your thinking

Fossil whales with legs sit in rock layers dated tens of millions of years old, and living whales still carry small hip bones. Dolphin embryos start hind limbs and then lose them, and DNA groups whales with hippos. Several independent fields point to one answer: whales descend from ancestors that walked on land.

Science practices you used: ordering rock layers and naming a model’s limits, calculating a percent and a difference from a data table, checking whether two numbers measure the same thing, and making and judging a claim from several lines of evidence.