Table of Contents
The fossil record is one of the central lines of evidence for evolution. In this chapter, the focus is not on individual spectacular fossil finds, but on the general patterns that appear when many fossils from different places and times are compared.
Incomplete but Informative: Nature of the Fossil Record
The fossil record is:
- Fragmentary – only a tiny fraction of all organisms that ever lived have been fossilized.
- Biased – hard parts (bones, shells), aquatic environments, and large, common organisms are preserved more often than soft-bodied, land-dwelling, or small organisms.
- Patchy in time and space – some geological periods and regions have rich fossil beds, others very few.
Despite this, when all fossils are considered together, clear and repeatable large‑scale patterns emerge. These patterns match the expectations of descent with modification and contradict a static, unchanging view of life.
Succession of Life Forms Over Geological Time
One of the most important general findings is the ordered succession of fossil forms through geological layers:
- Oldest rocks with fossils contain only very simple life (e.g. bacterial microfossils, stromatolites).
- Later rocks show unicellular eukaryotes, followed by simple multicellular algae and soft-bodied animals.
- Still younger layers contain diverse marine invertebrates, then the first land plants and land animals.
- Vertebrates appear first in water (jawless fishes, then jawed fishes), later on land (amphibians, reptiles), and later still birds and mammals.
Two key points:
- Complex groups appear later than simpler ones. For example, mammals never appear in the oldest fossil-bearing rocks.
- Organisms appear, change, and often disappear. Many forms that once existed have no living representatives.
This ordered succession is global: the same general pattern appears on different continents, even when the local species differ.
Law of Faunal (and Floral) Succession
The pattern of succession is so regular that it can be used as a tool:
- Certain combinations of fossils – especially of quickly evolving, widespread organisms (index fossils) – are characteristic of particular geological periods.
- Sedimentary layers in different regions can be matched (correlated) by comparing their fossil content.
This regularity leads to the law of faunal succession:
Fossil species succeed one another in a definite, recognizable order through geological strata.
Implications:
- Once a fossil species disappears from the record, it does not reappear in younger layers.
- The same “sequence of appearances and disappearances” is found worldwide, supporting a common history of life rather than independent, local creations.
Extinction as a Common and Natural Process
The fossil record shows that:
- Many groups of organisms once abundant and diverse no longer exist.
- Extinction has occurred continually, not just in rare, catastrophic events.
- Some times in Earth history show mass extinctions, where many unrelated groups die out in a geologically short time.
General findings include:
- Background extinction: a low, continuous rate of species loss.
- Mass extinctions: abrupt peaks in extinction rate (e.g. at the boundary between major geological periods), followed by partial recovery and diversification.
Thus, extinction is not an exceptional abnormality; it is a core feature of the history of life.
Trends and Directional Changes in Lineages
When long fossil series from particular groups are examined, directional trends often become visible. These trends are not universal laws but repeated patterns in many lineages. Examples of general kinds of trends:
- Increase or decrease in body size over time in certain lineages.
- Changes in proportions of body parts (e.g. limb length, skull shape).
- Shift in number or form of skeletal elements, such as the reduction or fusion of bones.
Several important points about these trends:
- They occur within lineages, through many small changes over long times.
- They are often gradual, with intermediate forms preserved.
- They are not always linear – trends can reverse, stop, or proceed in different directions in different branches.
These observations are consistent with populations evolving through many small modifications, rather than sudden appearance of final, “perfect” forms.
Appearance of New Major Groups in Sequence
The fossil record reveals that major groups (for example, large categories like fishes, amphibians, reptiles, birds, mammals, flowering plants) appear:
- In a temporal sequence (older to younger layers),
- with earlier forms showing partial sets of features typical of the new group and of earlier groups.
General findings include:
- New major groups are first represented by relatively simple or generalized forms.
- Later, these groups diversify into many specialized subgroups.
- There is overlap: when a new major group appears, the older group often still exists alongside it.
This pattern reflects branching evolution, not complete replacement at a single moment.
Transitional Morphologies and “Mosaics” in the Record
On a broad scale, the fossil record contains many organisms that combine features typical of different major groups. Without focusing on particular named fossils, some general patterns are:
- Skeletal structures that look intermediate between those characteristic of earlier and later groups.
- “Mosaic” organisms that mix traits: some features resemble one major group, others another.
Important general observations:
- Transitional forms do not need to be direct ancestors; they can be close relatives that share many intermediate characteristics.
- They show that large structural innovations (e.g. new modes of locomotion, new body plans) are built through stepwise modifications of existing structures.
These fossils contradict the idea that major groups appear suddenly with fully formed, unique structures.
Parallel Histories in Different Groups
When the histories of different groups are compared, several parallel patterns emerge:
- Radiations after extinctions: Following mass extinctions, multiple unrelated groups show increased diversification – new species and forms appear relatively rapidly in the fossil record.
- Repeated solutions: Different lineages independently evolve similar adaptations (for example, for swimming, flying, or burrowing). This is visible as recurrent body shapes and structures appearing in different periods and groups.
These parallels show that:
- Evolution is constrained by physical and ecological principles; similar problems (e.g. moving efficiently in water) often lead to similar structural solutions.
- Yet the details of these solutions differ among groups, revealing their different histories.
Shifts in Biodiversity Through Time
The fossil record not only reveals individual species but also changes in overall diversity and community composition:
- In older layers, most fossils are of marine microorganisms and simple invertebrates.
- Over time, there is an increase in the number of species, complexity of ecological communities, and especially diversification on land (plants, insects, vertebrates).
- Certain times show explosive increases in the number and disparity of forms (radiations) and other times show sharp declines (mass extinctions).
General findings:
- Biodiversity has fluctuated strongly over geological time.
- Major environmental shifts (climate, sea level, tectonics, impacts, volcanism) are often associated with these changes in diversity.
This supports the idea that evolution is both creative and constrained by environmental changes.
Geographic Patterns in the Fossil Record
The distribution of fossils in space and time shows consistent patterns:
- Closely related fossil species are usually found in geographically neighboring regions or in regions that were connected in the past.
- Similar types of organisms appear independently in different areas when similar environments develop, but their detailed structures differ, reflecting separate evolutionary histories.
- When continents that are now separate were once connected, they often share related fossil faunas and floras in rocks from those times.
These findings link evolutionary change with Earth’s geological history, including the movement of continents and changes in climate and sea levels.
General Conclusions from the Fossil Record
Putting these observations together, the fossil record as a whole shows:
- Temporal order: Life changes through time in a consistent, global sequence from simple to complex forms.
- Descent with modification: Groups appear gradually, diversify, and often show intermediate forms between earlier and later morphologies.
- Extinction and replacement: Many once-successful forms vanish; their ecological roles are later filled by new, related or unrelated groups.
- Branching history: There is no repeated re‑creation of identical forms; instead, we see divergent lineages that share common structural patterns.
Although incomplete, the fossil record is coherent and patterned. Its general findings are exactly what would be expected if all living and extinct organisms are parts of a single, branching evolutionary history.