Problem 6 - Campbell Biology

Question

Which of the following could be considered the most recent common ancestor of living tetrapods? (A) a sturdy-finned, shallow-water lobe-fin whose append- ages had skeletal supports similar to those of terrestrial vertebrates (B) an armored, jawed placoderm with two pairs of appendages (C) an early ray-finned fish that developed bony skeletal supports in its paired fins (D) a salamander that had legs supported by a bony skeleton but moved with the side-to-side bending typical of fishes

Step-by-Step Solution

Answer

Option (A) is the most recent common ancestor of living tetrapods.

Step 1 - Understanding the question

The question asks for the most recent common ancestor of living tetrapods from the given options. Tetrapods are four-limbed animals, including amphibians, reptiles, birds, and mammals.

Step 2 - Evaluating Option (A)

Option (A) describes a sturdy-finned, shallow-water lobe-fin fish whose appendages had skeletal supports similar to those of terrestrial vertebrates. This option suggests a transition towards limbs suitable for land.

Step 3 - Evaluating Option (B)

Option (B) describes an armored, jawed placoderm with two pairs of appendages. Placoderms are an extinct class of armored prehistoric fish. They do not closely resemble the typical tetrapod ancestor.

Step 4 - Evaluating Option (C)

Option (C) describes an early ray-finned fish that developed bony skeletal supports in its paired fins. Ray-finned fishes are distinct from the lobe-finned fishes, from which tetrapods evolved.

Step 5 - Evaluating Option (D)

Option (D) describes a salamander that had legs supported by a bony skeleton but moved with the side-to-side bending typical of fishes. Salamanders are modern amphibians and not ancestral forms.

Step 6 - Conclusion

After evaluation, Option (A) is the most likely candidate as the most recent common ancestor of tetrapods. The description fits early tetrapodomorph fishes, such as Tiktaalik, which had both aquatic and terrestrial features.

Key Concepts

Common Ancestors

The concept of common ancestors is crucial in understanding evolutionary biology. A common ancestor is an ancient organism from which different species have evolved. In the case of tetrapods, these are four-limbed vertebrates that include amphibians, reptiles, birds, and mammals. Identifying the most recent common ancestor provides insights into how these different groups are related. This common ancestor would possess traits that are shared across subsequent species.

In the context of tetrapod evolution, recognising the right common ancestor helps scientists understand the transition from aquatic to terrestrial life forms. In this case, a shallow-water lobe-finned fish, described in option (A), is the best candidate. It had robust fins with skeletal structures resembling limbs, thus marking a significant evolutionary step toward life on land.

Lobe-finned Fish

Lobe-finned fish play a critical role in the story of vertebrate evolution. These fish possess fleshy, lobed, paired fins, which are supported by a single bone. These anatomical structures are different from those of ray-finned fish, which have fins supported by spines.

The sturdy-finned, shallow-water lobe-fin described in option (A) is particularly significant. Lobe-finned fishes such as *Eusthenopteron* and *Tiktaalik* exhibit many features that are considered precursors to the limbs of terrestrial tetrapods. The skeletal supports in their fins eventually evolved into the limbs of early land animals. This makes them key figures in understanding how vertebrates transitioned from water to land.

Transitional Fossils

Transitional fossils are essential for piecing together the puzzle of evolutionary history. These fossils display traits common to both an ancestral group and its derived descendant group, demonstrating how evolutionary transitions occurred.

Transitional fossils like *Tiktaalik* roseae are often highlighted in the study of tetrapod evolution. *Tiktaalik* exhibits both fish-like and tetrapod-like features. It had gills and scales, like a fish, but also had a flattened head, a neck, and robust ribs, features more akin to early tetrapods. These characteristics support the idea that tetrapods evolved from lobe-finned fishes with adaptations allowing them to venture onto land. Such fossils offer critical evidence for the gradual evolutionary bridge between major anatomical changes.

Vertebrate Evolution

Vertebrates are animals with a backbone and include categories such as fish, amphibians, reptiles, birds, and mammals. The evolution of vertebrates from simple aquatic forms to diversified land dwellers is a cornerstone of evolutionary biology.

The transition from water to land necessitated significant adaptations. Early vertebrates, like the lobe-finned fish mentioned in the solution, adapted their fin structures to support their weight in shallow waters and eventually on land. Over time, these early tetrapods developed lungs, changes in limb structures, and other features to better survive in terrestrial environments.

This evolution has been traced back through various transitional fossils, helping scientists map out the gradual progression from simple fish to complex land animals. Understanding vertebrate evolution not only sheds light on the origins of humans but also provides perspective on biodiversity and the interconnectedness of all life forms.