Evidence for Evolution
1. Fossil Evidence
Learning outcomes
- I can explain how fossils provide evidence for evolution.
- I can describe how fossils are formed.
- I can interpret fossil sequences.
- I can identify transitional fossils.
- I can explain limitations of the fossil record.
What Are Fossils?
Fossils are preserved remains, impressions, traces, or other evidence of organisms that lived in the past.
Fossils can include:
- Bones.
- Teeth.
- Shells.
- Leaves.
- Wood.
- Pollen.
- Footprints.
- Burrows.
- Eggs.
- Feces.
- Impressions of organisms.
Fossils provide scientists with a record of organisms that lived millions or even billions of years ago.
By comparing fossils from different periods, scientists can investigate how organisms and populations have changed through Earth's history.
Fossils as Evidence for Evolution
The fossil record shows that life on Earth has changed over time.
Fossils reveal that:
- Many organisms that lived in the past are now extinct.
- Organisms found in older rocks often differ from organisms found in younger rocks.
- Groups of organisms have changed through geological time.
- Some fossils have combinations of characteristics that help connect major groups.
- Modern organisms share characteristics with organisms found in the fossil record.
Together with genetic, anatomical, and other evidence, fossils provide strong evidence for evolution.
The Fossil Record
The fossil record is the collection of fossils and their positions within rock layers and geological time.
It provides information about:
- Which organisms lived in the past.
- When organisms appeared.
- When groups became extinct.
- How organisms changed through time.
- Past environments.
- Relationships between groups of organisms.
The fossil record extends across billions of years of Earth's history.
Fossils and Rock Layers
Many fossils are found in sedimentary rock.
Sedimentary rocks form when sediments such as:
- Sand.
- Mud.
- Silt.
- Clay.
are deposited in layers.
Over time, these sediments can become compacted and cemented into rock.
Fossils trapped within these layers can provide information about the organisms living when the sediment was deposited.
Relative Age of Fossils
In an undisturbed sequence of sedimentary rocks, lower layers are generally older than layers above them.
A simplified sequence might be:
Youngest layer
Layer A
Layer B
Layer C
Layer D
Oldest layer
If a fossil is found in Layer D and another in Layer A, the fossil in Layer D is generally older, assuming the layers have not been disturbed.
This principle is called the law of superposition.
Rock Layers Can Be Disturbed
Scientists must be careful when interpreting rock layers.
Geological processes can:
- Fold rocks.
- Tilt layers.
- Break rocks along faults.
- Erode layers.
- Move sections of rock.
Therefore, scientists use multiple forms of geological evidence when determining the ages and relationships of fossils.
How Fossils Form
Fossilization is relatively rare.
Most organisms that die:
- Decompose.
- Are eaten.
- Are broken apart.
- Are destroyed by weathering.
For fossilization to occur, special conditions are usually required.
A common fossilization sequence is:
Organism dies
→ rapid burial
→ decomposition of soft tissues
→ sediment accumulates
→ minerals preserve or replace hard structures
→ sediment becomes rock
→ erosion may eventually expose the fossil
Step 1: Death
An organism dies.
If the organism remains exposed at the surface, it is unlikely to become fossilized.
Scavengers, microorganisms, weather, and physical damage may quickly destroy the remains.
Step 2: Rapid Burial
Rapid burial greatly increases the chance of fossilization.
An organism might be buried by:
- Mud.
- Sand.
- Volcanic ash.
- Sediment at the bottom of a lake or ocean.
Burial protects the remains from some scavengers and slows physical destruction.
Aquatic environments are therefore particularly important for fossil formation.
Step 3: Decomposition
Soft tissues usually decompose.
Hard structures are more likely to remain, including:
- Bones.
- Teeth.
- Shells.
- Woody tissues.
This is one reason organisms with hard body parts are better represented in the fossil record.
Step 4: Mineralization
Groundwater containing dissolved minerals can move through buried remains.
Minerals may:
- Fill spaces in bones.
- Replace original biological material.
- Preserve microscopic details.
Over long periods, the remains can become mineralized.
Step 5: Rock Formation
Additional sediment accumulates above the remains.
Pressure increases.
The surrounding sediments can eventually become sedimentary rock.
The fossil becomes preserved within the rock.
Step 6: Exposure
Millions of years later, geological processes may raise the rock toward Earth's surface.
Weathering and erosion can expose the fossil.
Scientists may then discover and study it.
Different Types of Fossils
Fossils can form in several ways.
Body Fossils
These preserve parts of an organism's body.
Examples include:
- Bones.
- Teeth.
- Shells.
- Wood.
Trace Fossils
These preserve evidence of an organism's activity.
Examples include:
- Footprints.
- Burrows.
- Bite marks.
- Nests.
- Fossilized feces.
Trace fossils can reveal how organisms behaved.
Molds and Casts
Sometimes an organism buried in sediment decomposes completely but leaves an impression.
This creates a mold.
If minerals or sediment later fill the mold, they can form a cast.
A cast reproduces the shape of the original organism or body part.
Exceptional Preservation
Under unusual conditions, more complete organisms can be preserved.
Examples include organisms preserved in:
- Amber.
- Ice.
- Very dry environments.
- Peat bogs.
These fossils may preserve structures that normally disappear during decomposition.
Dating Fossils
Scientists use several techniques to determine the ages of fossils and rocks.
Two important approaches are:
- Relative dating.
- Radiometric dating.
These methods answer slightly different questions.
Relative Dating
Relative dating determines whether something is older or younger than something else.
It does not necessarily provide an exact numerical age.
Rock layers are particularly useful.
For example:
If Fossil X occurs below Fossil Y in an undisturbed sequence:
Fossil X is generally older than Fossil Y.
Scientists can compare fossils from different layers to establish sequences through time.
Radiometric Dating
Radiometric dating uses the predictable radioactive decay of certain isotopes to estimate numerical ages.
Scientists often date:
- Volcanic rocks.
- Minerals.
- Rock layers associated with fossils.
The appropriate radioactive isotope depends partly on the age and type of material being studied.
Radiometric dating allows scientists to place many fossils within an approximate numerical timescale.
Fossil Sequences
A fossil sequence is a series of fossils from different ages that reveals patterns of change through time.
Imagine several rock layers containing fossils of related organisms.
Older layers might contain organisms with one set of characteristics.
Younger layers may contain organisms with gradually different combinations of characteristics.
Scientists can compare:
- Body size.
- Limb structure.
- Teeth.
- Skulls.
- Shells.
- Body proportions.
Patterns in these characteristics can provide evidence of evolutionary change.
Interpreting a Simple Fossil Sequence
Imagine three fossils found in successive rock layers.
Oldest fossil
Small body, short limbs, simple teeth.
Intermediate fossil
Larger body, longer limbs, modified teeth.
Youngest fossil
Large body, long limbs, highly specialized teeth.
This sequence suggests that characteristics within the lineage changed over time.
However, scientists would need additional evidence before determining the exact evolutionary relationships among these organisms.
Fossils Do Not Form a Perfect Ladder
Evolution should not be imagined as:
Species A → Species B → Species C → Species D
in a simple straight line.
Evolution usually produces branching patterns.
One ancestral population may give rise to several lineages.
Some lineages continue.
Others become extinct.
Fossils therefore represent parts of a complex evolutionary tree.
Transitional Fossils
A transitional fossil contains a combination of characteristics that helps scientists understand evolutionary changes between major groups or stages within a lineage.
A transitional fossil is not necessarily a direct ancestor of a modern species.
Instead, it may show a combination of:
- Ancestral characteristics.
- More recently evolved characteristics.
These combinations can help scientists reconstruct evolutionary relationships.
Archaeopteryx
Archaeopteryx is a famous fossil from approximately 150 million years ago.
It possessed a mixture of characteristics associated with non-avian dinosaurs and birds.
Characteristics included:
- Feathers.
- Wings.
- Teeth.
- Clawed fingers.
- A long bony tail.
This combination of characteristics makes Archaeopteryx important evidence for understanding the evolution of birds from theropod dinosaurs.
Tiktaalik
Tiktaalik is a fossil organism that lived approximately 375 million years ago.
It had a combination of characteristics associated with aquatic vertebrates and early tetrapods.
Features included:
- Fins.
- Scales.
- A flattened skull.
- A movable neck.
- Strong internal bones within the fins.
Its anatomy helps scientists understand evolutionary changes associated with the transition from primarily aquatic vertebrates toward early four-limbed vertebrates.
Whale Evolution
The fossil record also provides extensive evidence for the evolution of whales.
Early relatives of modern whales lived primarily on land.
Later fossils show increasingly aquatic characteristics.
Changes include:
- Modification of limbs.
- Reduction of hind limbs.
- Changes in the vertebral column.
- Changes in skull structure.
- Movement of the nostrils toward the top of the skull.
These fossils help scientists reconstruct the evolutionary transition from terrestrial ancestors to fully aquatic whales.
Horse Evolution
Horse fossils provide another well-studied example of evolutionary change.
Different fossil horse lineages show changes in characteristics such as:
- Body size.
- Tooth structure.
- Number of toes.
- Limb length.
These changes are associated with changing environments and diets.
However, horse evolution was a branching evolutionary history rather than a simple straight progression from one species to another.
Fossils and Extinction
The fossil record also provides evidence of extinction.
Many organisms found as fossils have no living representatives.
Examples include:
- Non-avian dinosaurs.
- Trilobites.
- Ammonites.
- Many prehistoric mammals.
The disappearance of particular fossils from later rock layers can indicate that a species or group became extinct.
Mass Extinctions
The fossil record shows several periods when unusually large numbers of species disappeared over relatively short geological intervals.
These are called mass extinctions.
After mass extinction events, surviving groups may diversify into ecological opportunities left by extinct organisms.
The fossil record therefore provides evidence not only of evolutionary change but also of major disruptions in the history of life.
Fossils and Changing Environments
Fossils can also reveal information about past environments.
For example, finding marine fossils high in mountains indicates that the rocks containing them originally formed in a marine environment.
Plant fossils can provide information about:
- Past climate.
- Rainfall.
- Vegetation.
- Temperature.
Fossils therefore help scientists reconstruct both evolutionary and environmental history.
Fossils and Common Ancestry
Similar structures found in fossils and living organisms can provide evidence of evolutionary relationships.
For example, the forelimbs of many vertebrates contain corresponding bones.
These similarities can be studied in:
- Fossils.
- Living animals.
- Embryos.
Scientists combine this anatomical evidence with genetic evidence to reconstruct evolutionary relationships.
Fossils and DNA Evidence
DNA usually does not survive for the vast periods represented by most fossils.
However, scientists can compare DNA from living species.
If fossil anatomy suggests two groups are closely related, DNA evidence from living representatives may provide additional evidence.
Modern evolutionary research therefore combines:
- Fossils.
- Comparative anatomy.
- DNA.
- Developmental biology.
- Biogeography.
- Geological evidence.
No single source of evidence is used in isolation.
Why Is the Fossil Record Incomplete?
The fossil record contains enormous amounts of information, but it is incomplete.
This is because fossilization is rare.
Most organisms never become fossils.
For fossilization to occur, several favorable conditions may be required.
Soft-Bodied Organisms Are Less Likely to Fossilize
Organisms with hard structures such as:
- Bones.
- Teeth.
- Shells.
are more likely to leave fossils.
Soft tissues usually decompose rapidly.
Therefore, organisms such as jellyfish and worms are generally less likely to fossilize than organisms with shells or bones.
Exceptional soft-tissue fossils do exist, but they are much rarer.
Habitat Affects Fossilization
Organisms living in environments where sediment accumulates rapidly are more likely to become fossilized.
For example:
- Lakes.
- River deltas.
- Ocean floors.
may provide good conditions for burial.
Organisms living in:
- Mountains.
- Forest floors.
- Dry exposed environments.
may be less likely to become fossilized.
This creates sampling bias in the fossil record.
Geological Processes Destroy Fossils
Even when fossils form, they may later be destroyed.
Rocks can be:
- Eroded.
- Melted.
- Metamorphosed.
- Crushed.
- Buried deeply.
- Altered by tectonic activity.
Millions of fossils may therefore have formed but no longer exist.
Many Fossils Have Not Been Discovered
Some fossils remain:
- Deep underground.
- Beneath oceans.
- Under ice.
- In inaccessible locations.
Others may simply not have been found yet.
The known fossil record is therefore only a portion of the fossils that currently exist.
Missing Fossils Do Not Mean Evolution Stopped
Because fossilization is rare, scientists do not expect to find fossils representing every generation or every evolutionary change.
Imagine a population existing for one million years.
Perhaps only a tiny number of individuals become fossilized.
The fossil record may therefore contain large time gaps.
This does not mean organisms suddenly disappeared and reappeared.
It reflects the incomplete nature of fossil preservation and discovery.
Preservation Bias
The fossil record is biased toward organisms that:
- Had hard body parts.
- Were abundant.
- Lived for long periods.
- Lived in environments favorable to burial.
- Lived relatively recently.
- Occurred in locations where rocks remain accessible.
Scientists account for these biases when interpreting fossil evidence.
Worked Example: Reading Rock Layers
Imagine four undisturbed sedimentary layers.
Layer A – top
Fossil species D
Layer B
Fossil species C and D
Layer C
Fossil species B and C
Layer D – bottom
Fossil species A and B
From this information we can infer:
- Layer D is the oldest.
- Layer A is the youngest.
- Species A occurs earlier in the sequence than Species D.
- Species B overlaps with Species A and Species C.
- Species C overlaps with Species B and Species D.
We should not automatically conclude that A evolved directly into B, B into C, and C into D.
More evidence would be required.
Worked Example: A Transitional Fossil
Suppose scientists discover a fossil with:
- Scales.
- Fish-like fins.
- Strong limb-like bones inside the fins.
- A movable neck.
Older fossils mainly show fish-like characteristics.
Younger fossils include early tetrapods with limbs.
The newly discovered fossil contains a combination of characteristics.
Scientists could investigate whether it represents a form close to the evolutionary transition between these groups.
The combination of ancestral and derived characteristics is what makes transitional fossils informative.
Fossil Evidence and Natural Selection
Fossils usually do not directly show natural selection occurring.
Instead, they document patterns of biological change through time.
Scientists can combine fossil evidence with information about:
- Environmental change.
- Anatomy.
- Genetics.
- Natural selection.
- Geological history.
to develop explanations for evolutionary patterns.
For example:
Environment changes
→ selection pressures change
→ populations evolve
→ descendants differ from earlier populations
→ fossils preserve some stages of that history.
Interpreting Fossils Carefully
Scientists must distinguish between:
Observation
and
Interpretation.
For example:
Observation: A fossil has both feathers and teeth.
Interpretation: The combination may provide information about evolutionary relationships between groups possessing these characteristics.
Scientific interpretations are strengthened when multiple independent sources of evidence support the same explanation.
Fossils Can Change Scientific Understanding
New fossil discoveries sometimes change our understanding of evolutionary history.
A newly discovered fossil may:
- Fill part of a previously unknown sequence.
- Reveal unexpected characteristics.
- Change estimated dates.
- Suggest a different relationship between groups.
- Reveal previously unknown biodiversity.
Science therefore continually updates explanations as new evidence becomes available.
Fossil Evidence and Evolutionary Trees
Scientists use fossils to help construct phylogenetic trees.
A phylogenetic tree represents hypotheses about evolutionary relationships.
Branches represent evolutionary lineages.
Branching points represent common ancestors or lineage divergence.
Fossils can help scientists determine when particular characteristics appeared and how different groups may be related.
Transitional Does Not Mean "Half-Finished"
A transitional organism was a complete, functioning organism adapted to its own environment.
It was not an incomplete version of a modern organism.
For example, Tiktaalik was not simply "half fish and half amphibian."
It was an organism with its own combination of characteristics.
The term transitional describes its importance for understanding evolutionary relationships.
Evolution Is Branching, Not a Ladder
A common misconception is that transitional fossils should form a perfect sequence:
fish → amphibian → reptile → mammal
Real evolutionary history is much more complex.
Evolution produces branching lineages.
Some branches:
- Diversify.
- Continue to the present.
- Become extinct.
Modern organisms are the surviving tips of this branching evolutionary history.
Limitations of Fossil Evidence
Important limitations include:
- Fossilization is rare.
- Soft-bodied organisms are underrepresented.
- Some environments preserve fossils better than others.
- Geological processes destroy fossils.
- Many fossils remain undiscovered.
- Fossils are often incomplete.
- Dating involves measurement and uncertainty.
- Fossil relationships must be interpreted rather than directly observed.
These limitations do not make fossils useless.
Instead, scientists account for them when evaluating evidence.
Why the Fossil Record Is Still Powerful Evidence
Despite its incompleteness, the fossil record contains consistent patterns.
Scientists can observe:
- Ordered sequences through geological time.
- Extinctions.
- Appearance of new groups.
- Changes within lineages.
- Transitional combinations of characteristics.
- Connections between organisms and changing environments.
These patterns can be compared with predictions from evolutionary theory.
The fossil record is therefore one of several major independent sources of evidence supporting evolution.
Common Mistakes
Thinking Every Dead Organism Becomes a Fossil
Fossilization is rare and requires favorable conditions.
Thinking Fossils Are Only Bones
Fossils can include footprints, burrows, shells, impressions, pollen, and many other forms of evidence.
Assuming Lower Rock Layers Are Always Older
This is generally true for undisturbed sedimentary sequences, but geological processes can overturn or disturb layers.
Thinking Transitional Fossils Are Direct Ancestors
A transitional fossil may be close to an evolutionary transition without being the direct ancestor of a later species.
Thinking Transitional Organisms Were Incomplete
Every fossil organism was a functioning organism in its own environment.
Expecting Fossils From Every Generation
Fossilization is far too rare to produce a continuous record of every organism or generation.
Thinking Evolution Is a Straight Line
Evolution produces branching patterns rather than a simple ladder.
Assuming a Gap in the Fossil Record Means Nothing Happened
Gaps are expected because fossilization, preservation, discovery, and sampling are incomplete.
Assuming Fossils Alone Tell the Entire Evolutionary Story
Scientists combine fossils with genetics, anatomy, geology, biogeography, and other evidence.
Check Your Understanding
1. Define a fossil.
2. What is the fossil record?
3. Why are sedimentary rocks particularly important for fossils?
4. Describe the major stages involved in fossil formation.
5. Why does rapid burial increase the chance of fossilization?
6. Why are bones and shells more likely to fossilize than soft tissues?
7. Distinguish between a body fossil and a trace fossil.
8. What is the law of superposition?
9. Distinguish between relative dating and radiometric dating.
10. How can a fossil sequence provide evidence for evolution?
11. What is a transitional fossil?
12. Why is Archaeopteryx considered important evolutionary evidence?
13. What characteristics make Tiktaalik useful for studying vertebrate evolution?
14. Explain how whale fossils provide evidence for evolutionary change.
15. Why is the fossil record incomplete?
16. Give three factors that create bias in the fossil record.
17. Why should evolution be represented as a branching tree rather than a ladder?
18. Why should scientists be cautious about claiming that one fossil species was the direct ancestor of another?
19. Explain why gaps in the fossil record are expected.
20. Explain how fossil evidence, geological evidence, and genetic evidence can work together to support evolutionary explanations.
Key Terms
- Fossil – preserved remains, impression, trace, or other evidence of an organism from the past.
- Fossil record – collection of known fossils and their positions through geological time.
- Fossilization – processes through which evidence of organisms becomes preserved.
- Sedimentary rock – rock formed through deposition, compaction, and cementation of sediments.
- Mineralization – preservation process involving minerals filling or replacing biological material.
- Body fossil – fossilized remains of part of an organism's body.
- Trace fossil – preserved evidence of an organism's activity.
- Mold – impression left after an organism or body part disappears.
- Cast – three-dimensional copy formed when a mold fills with material.
- Law of superposition – principle that lower layers in an undisturbed sedimentary sequence are generally older than layers above them.
- Relative dating – determining whether something is older or younger than something else.
- Radiometric dating – estimating age using predictable radioactive decay.
- Transitional fossil – fossil containing a combination of characteristics informative about evolutionary transitions.
- Common ancestor – ancestral population from which different evolutionary lineages descended.
- Phylogenetic tree – branching representation of hypotheses about evolutionary relationships.
- Extinction – permanent disappearance of a species.
- Preservation bias – unequal likelihood that different organisms or environments will be represented in the fossil record.
Key Takeaways
- Fossils are preserved remains, traces, impressions, or other evidence of organisms from the past.
- The fossil record provides evidence that life has changed through geological time.
- Fossils are commonly preserved in sedimentary rocks.
- Rapid burial increases the chance of fossilization.
- Hard structures such as bones, teeth, and shells are more likely to fossilize than soft tissues.
- Trace fossils such as footprints and burrows provide evidence about organism behavior.
- In undisturbed sedimentary sequences, deeper layers are generally older than layers above them.
- Relative dating establishes sequences, while radiometric dating can provide numerical age estimates.
- Fossil sequences allow scientists to investigate changes in organisms over time.
- Transitional fossils contain combinations of ancestral and more recently evolved characteristics.
- Archaeopteryx, Tiktaalik, and fossils documenting whale evolution are important examples.
- Evolutionary history is branching rather than a simple ladder.
- Fossils also provide evidence of extinction and environmental change.
- The fossil record is incomplete because fossilization is rare.
- Soft-bodied organisms and organisms from poorly preserving environments are underrepresented.
- Geological processes can destroy fossils.
- Many fossils remain undiscovered.
- Gaps in the fossil record are therefore expected.
- Scientists interpret fossils alongside evidence from genetics, anatomy, geology, and biogeography.
- Despite its limitations, the fossil record provides powerful evidence for evolutionary change over Earth's history.