Life Science · Unit 8: Change over time
Evidence for Evolution
How do scientists know that living things have changed over time and are related? You will compare fossils, arm and leg bones, and the same protein in humans and other animals. Afterwards you can use these three kinds of evidence to make a claim about which living things are close relatives.
Where this lesson sits
- Virginia SOL LS.11 b
- Practices LS.1 a, LS.1 c, LS.1 d, LS.1 f
Essential question: How do scientists know that living things have changed over time and are related?
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Life Science (grade 7) LS.11 b You are here
This lesson
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Biology
Preview it at high school level
In Biology you will learn how scientists find the age of a fossil. Rock layers give an order: in a stack that has not been disturbed, lower layers are older. Radioactive dating gives an age in years. Biology also gives names to kinds of body parts. For example, body parts that come from the same part in a common ancestor are called homologous structures. You will look at small leftover parts, like the hip bones inside a whale. You will add two more kinds of evidence: how embryos develop, and where species live on Earth. You will also use data to build family trees of life.
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Biology II Biology II: Advanced Survey of Biology Topics, BIOII.5 a and BIOII.7 b; Genetics, GEN.2 e
Preview where it goes next
In Biology II: Advanced Survey of Biology Topics, BIOII.5 a asks how evidence from fossils, body comparisons, DNA, where species live, and direct observation of changing populations all point to one history. BIOII.7 b compares body structures across animals with and without backbones in more detail. In Biology II: Genetics, GEN.2 e uses the genetic code itself as evidence. Almost all living things read their genes with the same code, which is what you would expect if they share a common ancestor.
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College
Watch first Three Kinds of Evidence Agree · 2:45
Virginia’s state fossil is a scallop shell called Chesapecten jeffersonius. It is millions of years old, yet it looks a lot like a scallop shell on a beach today. These shells come from old sea floor deposits that show along Virginia rivers such as the James. That sea was here long before the Chesapeake Bay. The Bay began to fill as sea level rose after the last ice age, about 10,000 years ago. So what can old shells, bones and DNA tell us about how living things change?
What do you think? Decide before you read.
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Scientists disagree: In everyday talk, a theory is a hunch. In science, a theory is a well-tested explanation backed by a lot of evidence. It also makes predictions. For example, it predicts that close relatives will have similar DNA, and when scientists check, they do.
What scientists think
Scientists disagree: In everyday talk, a theory is a hunch. In science, a theory is a well-tested explanation backed by a lot of evidence. It also makes predictions. For example, it predicts that close relatives will have similar DNA, and when scientists check, they do.
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Scientists disagree: Humans and chimpanzees share a common ancestor that lived millions of years ago. After the split, each group changed along its own branch. That is why both are alive today. Their DNA is very alike, which fits a shared ancestor.
What scientists think
Scientists disagree: Humans and chimpanzees share a common ancestor that lived millions of years ago. After the split, each group changed along its own branch. That is why both are alive today. Their DNA is very alike, which fits a shared ancestor.
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Scientists disagree: Most dead plants and animals rot or get eaten. Fossils form only in special conditions, such as quick burial in mud or sand. So the record has gaps. Even so, the fossils we have show clear patterns of change.
What scientists think
Scientists disagree: Most dead plants and animals rot or get eaten. Fossils form only in special conditions, such as quick burial in mud or sand. So the record has gaps. Even so, the fossils we have show clear patterns of change.
Explore: build an evidence board
Pick two living things, such as a human and a bat. Then choose one kind of evidence: fossils, arm and leg bones, or DNA. Each choice shows you something different, and you can place the pair on a family tree.
- 1 upper arm bone
- 2 two lower arm bones
- 3 wrist bones
- 4 finger bones
Made up in this sketch: the bone sizes, the shape of the tree (cat, whale and bat share one branch point because the lesson does not put them in order; fish is drawn outermost), the limit of four pairs, and which table row each animal uses (human, chimpanzee, cat, whale and bat use mammal relatives; chicken uses bird-like dinosaurs; fish uses fish). The order of the other joins is not from the lesson. The fossil ages and the protein counts are the lesson’s table values.
Keep this rule: scientists check fossils, bones and DNA together, and when they agree they are strong signs of a common ancestor. A join nearer the tips means a more recent common ancestor. This is a sketch of that logic, not a real tree.
Try this.
- Predict whether a whale’s flipper has the same bones as your arm. Choose whale and human, pick bones, and check.
- Predict which is the closer relative of humans, a chicken or a chimpanzee. Use the DNA bars to test it.
- Predict where a fish will join the family tree. Look at the fossil timeline, then place it on the tree and see.
Explain: what fossils record
- evidence
- fossil
- fossil record
- extinct
- common ancestor
- DNA sequence
- hypothesis
- theory
Evidence is information from observations or measurements that supports or argues against an idea. A fossil is the preserved remains or traces of a living thing from the past. Shells, bones, teeth and footprints can all be fossils.
Fossils are rare. Hard parts like shells and bones last longer than soft parts. Quick burial in mud or sand helps too. So the fossil record, all the fossils found so far, has gaps.
Even with gaps, the fossil record shows clear patterns:
- Existence and diversity. Many kinds of life lived that are not alive now.
- Extinction. A group is extinct when no living members are left. Trilobites, sea animals with hard shells, are found in rocks in western Virginia. They are extinct.
- Change. Old forms look like, but are not the same as, living ones. The state fossil scallop is one example.
- Environments. Sea shells in Virginia’s coastal plain show that a shallow sea once covered that land.
Groups also show up at different times. The table lists one well-known early fossil for each group.
| Group | Example fossil and place | About how old (millions of years) |
|---|---|---|
| Fish | Haikouichthys, China | about 518 |
| Four-legged animals | Ichthyostega, Greenland | about 365 |
| Mammal relatives | Morganucodon teeth | about 205 |
| Bird-like dinosaurs | Archaeopteryx | about 150 |
These are not the oldest fossils of each group, and they are not the day each group began. New finds can push dates back.
Explain: same bones, similar DNA
Hold out your arm. It has one upper arm bone, two lower arm bones, wrist bones and finger bones. A cat’s front leg, a whale’s flipper and a bat’s wing have the same set. The bones differ in shape and size, and the limbs do different jobs. A common ancestor is an early population that two or more kinds of living things came from. A shared ancestor with these bones explains the match best.
- upper arm bone
- two lower arm bones
- wrist bones
- finger bones
- upper arm bone
- blank
- blank
- finger bones
Word bank: two lower arm bones, wrist bones
- blank
- blank
- blank
- blank
Check your labels
- upper arm bone
- two lower arm bones
- wrist bones
- finger bones
Doing the same job is not enough. A butterfly wing and a bird wing both fly, but only the bird wing has bones. So scientists compare how parts are built, not what they do.
DNA gives a second check. A DNA sequence is the order of the bases A, T, C and G along a piece of DNA. Many genes carry the code for a protein, as you saw in DNA: The Code of Life. Small changes in DNA happen and get passed on. Species that split from a common ancestor more recently have had less time to collect changes. So closer relatives usually have more similar DNA. DNA is also used to sort living things into groups, as in Domains, Kingdoms and DNA.
Explain: why scientists call it a theory
A hypothesis is a possible explanation that a test can support or reject. A theory is a well-tested explanation backed by many lines of evidence. A theory also makes predictions that can be checked.
The theory of evolution says that populations change over time and share common ancestors. Natural selection drives that change, as you saw in Natural Selection in Action. The theory predicts that fossils, bones and DNA will tell the same family story. Each one could have told a different story. Instead, they agree.
Elaborate: read real protein data
Cytochrome c is a small protein that helps your cells release energy from sugar. Humans, monkeys, dogs and chickens all make it. The table counts how many of 104 positions in the protein differ from the human one. Each protein difference comes from a difference in the gene’s DNA.
| Animal | Differences from human (out of 104 positions) |
|---|---|
| Human | 0 |
| Chimpanzee | 0 |
| Rhesus monkey | 1 |
| Mouse | 9 |
| Dog | 11 |
| Horse | 12 |
| Chicken | 13 |
| Eastern diamondback rattlesnake | 15 |
Source: one reviewed cytochrome c sequence per animal from the UniProt protein database, retrieved in October 2026 and compared position by position for this lesson.
- Not counting humans, which two animals differ from humans the least? Which differs the most?
- How many positions match between the human and horse proteins?
- Chimpanzees show 0 differences here. Does that mean their whole DNA is the same as ours?
- The animal with the fewest differences is blank, so it is probably blank.
- This fits the bone evidence because blank.
- One limit of these data is blank.
Claim, evidence, reasoning
Make a claim about which animals in the table are the closest relatives of humans. Use the protein data and one other kind of evidence from this lesson.
Start with: The closest relatives of humans in this table are blank.
Start with: The blank protein differs from ours at blank of 104 positions, while blank differs at blank. The arm bones of blank also blank.
Start with: Fewer differences mean blank, because blank. The bones agree because blank.
Compare with a model answer
Chimpanzees and rhesus monkeys are the closest relatives of humans in this table. Their cytochrome c differs from ours at 0 and 1 of 104 positions. The rattlesnake differs at 15 and the chicken at 13. Changes in DNA build up over time, so species that split more recently share more of the same sequence. Monkeys and apes also have arm bones like ours. One limit: this is one small protein from one animal of each kind, so other genes should be checked too.
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Design problem
Fossils form more often when remains are buried fast and when they are hard. Plan a model test: does quick burial in sand slow the breakdown of soft remains, compared with hard ones?
One way to do it
- Question: Do apple slices (soft remains) lose less mass when buried in damp sand than when left uncovered? Do shells change at all?
- Independent variable: buried under 3 centimetres of damp sand, or left uncovered.
- Dependent variable: mass in grams after 7 days, measured with a balance. Uncovered slices also dry out, so look at how the slices look and smell as well.
- Constants: the same slice size, the same cups, the same amount of water, the same room.
- Control: uncovered cups. Add clean shells to both groups as a hard-part check.
- Trials: five cups of each kind; find the mean mass change.
- Safety: use clear covered cups, wear gloves, never taste anything, and let the teacher throw out anything mouldy. Wash your hands.
If buried slices lose less mass and also look and smell less rotten, burial slowed decay in this model. Shells should barely change. This is a model, not real fossil formation, which takes thousands of years or more.
Evaluate: check your thinking
The state fossil scallop looks a lot like living scallops because scallops share common ancestors and have changed over time. Fossils show which kinds of life lived, changed and went extinct. Matching bones and similar DNA show which living things are close relatives.
Science practices you used: asking a testable question, planning a fair test with a control, telling a hypothesis from a theory, and making a claim from evidence and reasoning.
How we measure it
Comparing protein sequences
Scientists line up the same protein from two species, position by position, and count the places where the building blocks differ. Fewer differences in the same protein usually point to closer relatives.
Comparing bones
Scientists lay the bones of living and fossil animals side by side and match each bone by its place and shape, not by its job. Matching sets of bones are evidence of a shared ancestor.
Collecting and describing fossils
Scientists record where in the rock each fossil was found and describe its parts. Comparing fossils with living things shows which kinds of life changed and which went extinct.
Finish line
Test yourself
8 questions
Practice and MCAT-style, with explained answers.
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