Fossil Evidence for Common Descent
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You should understand the basic mechanism of natural selection and the concept of descent with modification. Familiarity with basic geological time scales (eons, eras, periods) is assumed.
Why this matters
The fossil record provides the temporal dimension to evolutionary theory, transforming it from a static classification system into a dynamic history of life on Earth.
While molecular biology and comparative anatomy reveal the patterns of relatedness among living organisms, the fossil record offers the only direct physical evidence of organisms that lived in the past. It allows us to observe the gradual accumulation of changes over millions of years, providing a chronological framework that links the present diversity of life to its ancient origins. Without fossil evidence, the theory of evolution would lack the historical depth required to explain the origin of major body plans and the timing of key adaptive radiations. This lecture examines how specific fossil sequences, such as the transition from aquatic to terrestrial life and the evolution of horses, provide concrete support for the hypothesis of common descent.
The Logic of Fossil Succession
Imagine a library where books are not arranged alphabetically but by the date they were published. If you walk through the stacks from the oldest section to the newest, you would observe a clear progression in the complexity of the stories, the sophistication of the language, and the themes explored. The fossil record functions similarly, but instead of books, we are examining the physical remains of organisms preserved in sedimentary rock layers. The fundamental principle here is the law of superposition: in an undisturbed sequence of sedimentary rocks, the oldest layers are at the bottom and the youngest are at the top. Therefore, fossils found in deeper strata are older than those found in shallower strata.
When we examine this "library" of life, we do not find a random assortment of creatures. Instead, we observe a consistent pattern of succession. Simple, single-celled organisms appear in the oldest rocks. As we move up through the geological time scale, we see the emergence of complex multicellular life, followed by the diversification of invertebrates, the appearance of fish, then amphibians, reptiles, and finally mammals and birds. This sequence is not merely a list of appearances; it reflects a branching tree of life. Crucially, we often find "transitional" forms—organisms that possess a mix of ancestral and derived traits. For example, we find creatures that look like fish but have limb-like fins, or creatures that look like reptiles but have feathers. These intermediates are the physical links that connect the major groups of animals. They demonstrate that new forms did not appear suddenly and fully formed, but evolved gradually from pre-existing forms. This gradualism is the hallmark of natural selection acting over long periods. The fossil record thus provides a narrative of change, showing that the diversity of life today is the result of a long, continuous process of modification and branching from common ancestors.
Geological Time and Stratigraphy
To interpret fossil evidence, we must first understand the framework of geological time. The Earth's history is divided into eons, eras, periods, and epochs. The Phanerozoic Eon, which spans roughly the last 539 million years, is the period where complex life is most abundant in the fossil record. It is divided into three eras: the Paleozoic, Mesozoic, and Cenozoic. Each era is characterised by distinct biological assemblages. The Paleozoic Era is known for the explosion of marine invertebrate life and the eventual colonization of land by plants and animals. The Mesozoic Era is dominated by dinosaurs and the rise of flowering plants. The Cenozoic Era is the age of mammals, following the mass extinction that wiped out the non-avian dinosaurs.
The preservation of fossils is a rare event. For an organism to become a fossil, it must be buried rapidly in sediment to prevent decay and scavenging. Over time, mineralization replaces organic material with rock, preserving the shape and sometimes even the chemical composition of the organism. Because of this bias, the fossil record is incomplete. We do not have fossils for every species that has ever existed. However, the gaps in the record do not invalidate the evidence; rather, they highlight the rarity of preservation. Despite these gaps, the overall pattern of succession remains robust. We consistently find that older rocks contain simpler organisms, while younger rocks contain more complex ones. The same order of first appearances is found on every continent and in every type of sedimentary sequence, which supports a single, branching history of life, rather than appearing randomly or independently in different locations.
Transitional Forms and Morphological Continuity
One of the strongest pieces of evidence for common descent is the existence of transitional fossils. These are organisms that exhibit a mixture of characteristics found in two distinct groups, bridging the morphological gap between them. A classic example is Tiktaalik roseae, discovered in rocks dating to the late Devonian period, approximately 375 million years ago. Tiktaalik possesses features of both fish and tetrapods (four-limbed vertebrates). It has gills and scales like a fish, but it also has a mobile neck, a robust ribcage, and limb-like fins with distinct bones corresponding to the arm and hand of a tetrapod. This organism provides a physical link between the aquatic ancestors of amphibians and the terrestrial descendants. It demonstrates that the transition from water to land was not an abrupt event but a gradual process where organisms adapted to shallow water environments before fully colonizing land.
Another well-known example is the evolution of the horse. In the Eocene epoch, the earliest horse ancestors, such as Hyracotherium (often called Eohippus), were small, dog-sized animals with multiple toes. As we move through the geological record to the modern horse, Equus, we observe a trend towards larger body size and a reduction in the number of toes, culminating in a single hoof. This sequence is not a straight line of direct descent, but a branching tree where side branches died out. However, the main lineage shows a clear trend of adaptation to open grassland environments, where speed and efficient locomotion were advantageous. The preservation of these intermediate forms in the North American fossil record provides a detailed snapshot of how natural selection can shape morphology over millions of years, responding to changing environmental pressures.
Biogeography and Fossil Distribution
The geographic distribution of fossils also provides evidence for common descent. If species were created independently in different locations, we would expect to see similar forms appearing in different places without a clear historical connection. However, the fossil record shows that related species are often found in close geographic proximity, reflecting their shared ancestry. For example, the fossil record of marsupials in Australia shows a unique radiation of forms that are not found elsewhere. This pattern is consistent with the idea that marsupials colonised Australia when it was part of the supercontinent Gondwana and then evolved in isolation, filling ecological niches that were occupied by placental mammals in other parts of the world.
Similarly, the distribution of fossil plants and animals across different continents supports the theory of continental drift. When we reconstruct the positions of continents in the past, we find that fossil species that are now separated by oceans were once part of a single landmass. For instance, the fossil plant Glossopteris is found in rocks of similar age in South America, Africa, India, Australia, and Antarctica. This distribution is best explained by the fact that these continents were once joined together as part of Gondwana, allowing the plant to spread across the landmass. As the continents drifted apart, the populations of Glossopteris became isolated and eventually went extinct in different places at different times. This biogeographic evidence, combined with the morphological evidence from transitional forms, creates a coherent picture of life's history that is consistent with the theory of evolution by natural selection.
The concept
The evidence for common descent
Homology, fossils and molecules point to shared ancestry.
A common misconception is that a transitional fossil must be the direct ancestor of later forms. Tiktaalik is better read as a close relative of the ancestors, showing which combinations of traits existed. Indeed, trackways from Zachełmie in Poland, about 395 million years old, suggest that tetrapods already existed before Tiktaalik lived; that does not undo the transition, it shows that Tiktaalik is a late-surviving member of a lineage close to it. Gaps and side branches are expected in a sampled record. C
Worked example
Problem. A sedimentary rock layer contains fossils of Tiktaalik and is dated to 375 million years ago. A deeper layer contains fossils of early fish, dated to 420 million years ago. A shallower layer contains fossils of early amphibians, dated to 350 million years ago. Calculate the time interval between the appearance of the fish and the amphibians, and determine how far through this interval (as a percentage, measured from the fish) Tiktaalik lies.
- Identify the ages of the key fossils: Fish = 420 Ma, Tiktaalik = 375 Ma, Amphibians = 350 Ma.
- Calculate the total time interval between the fish and the amphibians: \( 420 - 350 = 70 \) million years.
- Calculate the time interval between the fish and Tiktaalik: \( 420 - 375 = 45 \) million years.
- Calculate the percentage of the total interval that Tiktaalik represents relative to the fish: \( \frac{45}{70} \times 100 \).
- Perform the division: \( 45 \div 70 \approx 0.6428 \).
- Convert to percentage: \( 0.6428 \times 100 = 64.28\% \).
Units check. The result is a dimensionless percentage, which is appropriate for a ratio of time intervals.
Three quick tests
- What is the law of superposition?
Answer
In undisturbed sedimentary rock, the oldest layers are at the bottom and the youngest are at the top. - Why is the fossil record considered incomplete?
Answer
Fossilization is a rare event requiring rapid burial and specific conditions, so most organisms do not leave a fossil record. - How does the distribution of Glossopteris support common descent?
Answer
Its presence on multiple continents that were once joined supports the idea that these landmasses were connected, so one population of the plant could spread across them before they drifted apart. This is evidence about where lineages lived and how they spread, consistent with common descent rather than separate origins on each continent.
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Every lecture and every concept in this unit, with prerequisites and threads.
Lecture notes checked line by line by the editor; every number in the worked example was recomputed.
Concepts cited: T-020 · Threads: evolution