Fossil Evidence

Essential Question

What can fossils and rock layers tell us about the history of life?

Fossils are the preserved remains and traces of organisms that lived long ago. Arranged by age, they form a record of how life has changed. In this lesson, you’ll see how fossils form, how scientists use rock layers and half-lives to date them, and how fossils that show a mix of traits give evidence for common ancestry.

How Fossils Form

• Fossils are preserved remains or traces of organisms
• Quick burial in sediment helps preservation
• Minerals slowly replace hard parts
• Bones, teeth, and shells fossilize best

A fossil is the preserved remains or trace of an organism that lived long ago. Most organisms decay or are eaten and never become fossils. A fossil is most likely to form when an organism is buried quickly in mud or sand, which protects it from scavengers and decay. Over millions of years, minerals dissolved in water replace the hard parts, such as bone, teeth, and shell, and the sediment hardens into rock. Erosion can later expose the fossil. Because soft tissues rarely fossilize, the fossil record is incomplete.

Reading Rock Layers

• Sediment settles in layers, called strata
• Older layers lie below younger layers
• Fossils in a layer lived at about the same time
• Layers show the order of events

Sedimentary rock forms in layers, with each new layer settling on top of the older ones. This is the law of superposition: in rock that has not been disturbed, the oldest layers are at the bottom and the youngest are at the top. Fossils found in the same layer lived at about the same time, so layers let scientists put fossils in order from oldest to youngest. Putting fossils in order this way, without giving an age in years, is called relative dating. In the diagram, fossil D is found only in the bottom layer, so it is the oldest of the four. Fossil A appears only in the top two layers, so it is the youngest.

Radiometric Dating

• Radioactive isotopes decay at a fixed rate
• Half-life: time for half of an isotope to decay
• Carbon-14 half-life: 5,730 years
• Older rocks use isotopes with longer half-lives

Layers show the order of fossils but not their age in years. For that, scientists use radiometric dating. Some atoms are radioactive and change into other atoms at a steady rate. The half-life is the time it takes for half of the atoms in a sample to change. After one half-life, 50% of the original isotope remains, after two half-lives 25% remains, and so on. Carbon-14 has a half-life of 5,730 years and is used for remains up to about 50,000 years old. For older rocks, scientists measure isotopes with longer half-lives, such as uranium-238, in volcanic rock.

Transitional Fossils

• Transitional fossils show traits of two groups
• They fit between ancestors and descendants
• Predictions from evolution can be tested

Fossil	Age (millions of years)	Traits	Links these groups
Tiktaalik	about 375	Gills and scales like a fish, plus a neck and wrist-like bones in its fins	Fish and four-legged animals
Archaeopteryx	about 150	Feathers and wings like a bird, plus teeth, claws, and a long bony tail like a dinosaur	Dinosaurs and birds
Ambulocetus	about 49	Four legs for walking like a land mammal, plus whale-like ear bones and skull	Land mammals and whales

A transitional fossil shows features of an older group and a newer group, as expected if one group descended from the other. Evolution predicts that such fossils should exist in rocks of the right age. Scientists predicted that a fish with limb-like fins would be found in rocks about 375 million years old, and Tiktaalik was found there. Fossils like these are evidence that new groups of organisms arise from existing ones.

The Order of Life

• The fossil record shows life changing over time
• Simple organisms appear before complex ones
• New groups appear in a predictable order

The fossil record shows that groups appear in order. Fish are found in much older rocks than four-legged animals, and mammals and birds are found only in much younger rocks. We do not find a mammal fossil in rocks that are 500 million years old. This pattern is what we would expect if later groups descended from earlier ones. The graph shows approximate dates for the first appearance of some major groups.

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Worked Examples

Try each problem on your own first, then open the solution to check your work.

Example 1: How Old Is the Bone?

A bone fragment contains 12.5% of its original carbon-14. The half-life of carbon-14 is 5,730 years. How old is the bone?

Show the solution

1. Find the number of half-lives: 100% → 50% → 25% → 12.5%. The amount was cut in half 3 times, so 3 half-lives have passed.
2. Calculate the age: 3 × 5,730 years = 17,190 years
3. Check: (1/2)³ = 1/8 = 12.5%, which matches the sample.
4. Answer: About 17,190 years old.

Example 2: Dating a Fossil Between Two Layers

Fossil Q is found in a limestone layer between two layers of volcanic ash. Radiometric dating shows the upper ash is 150 million years old and the lower ash is 160 million years old. (a) What is the age of Fossil Q? (b) Why can’t carbon-14 be used to date Fossil Q?

Show the solution

1. (a) Use the layers: Layer 3 lies below the 150-million-year ash, so it is older than 150 million years. It lies above the 160-million-year ash, so it is younger than 160 million years. Fossil Q is between 150 and 160 million years old.
2. (b) Carbon-14: 150 million years ÷ 5,730 years per half-life ≈ 26,000 half-lives. After that many half-lives, no measurable carbon-14 is left. Carbon-14 works only for remains up to about 50,000 years old.
3. Answer: (a) Between 150 and 160 million years old. (b) Too old: no measurable carbon-14 would remain.

Example 3: Horse Ancestors

The graphs show approximate data for four horse ancestors. (a) How many toes did Hyracotherium have on each front foot? (b) How much did the shoulder height increase from Hyracotherium to Equus? (c) What does the pattern show?

Show the solution

1. (a) Toes: Hyracotherium had 4 toes on each front foot.
2. (b) Height: 140 cm − 40 cm = 100 cm. Equus is 3.5 times as tall as Hyracotherium (140 ÷ 40 = 3.5).
3. (c) Pattern: over about 55 million years, horse ancestors became taller and the number of toes decreased from 4 to 1. Fossils in order of age show gradual change in one line of descent.
4. Answer: (a) 4 toes. (b) 100 cm. (c) Horses became taller and lost toes over time.

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Practice

Read a rock layer diagram, complete a half-life table, calculate the age of a sample, and use fossil data as evidence. Try it on your own first, then check your work with the answer key.

• Fossil Evidence Practice Worksheet (PDF)
• Fossil Evidence Practice Answer Key (PDF)

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Lesson Slides

• 4.2 Fossil Evidence Slides (PDF)

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Big Ideas

• Fossils form when hard parts are buried and mineralized
• Older rock layers lie below younger ones
• Half-life: time for half an isotope to decay
• Transitional fossils link groups
• The fossil record shows groups in order of appearance

Back to the essential question: fossils preserve the history of life. Rock layers show the order of fossils, and radiometric dating gives their ages in years. Transitional fossils and the order in which groups appear in the record are what we would expect if life has changed over time through common descent. Next, you’ll compare the body structures and DNA of living organisms for more evidence.

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