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Isn't it strange that there is a group of air-breathing mammals that lives in an environment that may result in them suffocating? Other than humans in space, the only other creatures that chose to leave their comfort zones are cetaceans (whales and porpoises).
The explanation for humans is simple: adventure and/or stupidity. But what about whales? |
"Today comparative anatomy has reached such a point of perfection that, after inspecting a single bone, one can often determine the class, and sometimes even the genus of the animal to which it belonged. This is because the number, direction, and shape of the bones that compose each part of an animal's body are always in a necessary relation to all the other parts, in such a way that one can infer the whole from any one of them."
Georges Cuvier, 'Tableau élémentaire de l'histoire naturelle des animaux' (1798)
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Comparative Anatomy is the study of the similarities and differences in the anatomy of difference species. It has long served as one of the main evidences for evolution, due to the fact that it is very concrete, and does not require extensive technology.
Scientists will go more in depth than just looking at the outer appearences of organisms. Many times, scientists use bones to determine similarities and differences in comparative anatomy. There are two types of structures you will now research. These have both been used to determine whale evolution. The first are Homologous Structures. These are parts of the body in organisms that have similar structure to other organsisms' comparative parts. One example of this is the bone structure in limbs of different mammals, shown below. |
Skeletons of mammalian hands: orangutan, dog, swine, cattle, tapir, horse
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You can see that even though some bones are slightly different lengths and shapes, all limbs above share the same bone structure. This points to the fact that even though each animal shown has a specialized limb used for different things, all of these mammals have descended from one organism.
The second type of structures are Analogous or Vestigial Structures. These are structures that, over time, have lost much of their ancestral function. One example in humans is the appendix. The appendix was once an enlarged part of the digestive system, used to digest leaves in primates who were herbivores. As evolution occured and the appendix was no longer needed, it shrank in size and function over a very long time. By looking at the reduced appendix in humans, and its similarity to a functional appendix in some primates, scientists can infer that there was a common ancestor between the two structures.
The second type of structures are Analogous or Vestigial Structures. These are structures that, over time, have lost much of their ancestral function. One example in humans is the appendix. The appendix was once an enlarged part of the digestive system, used to digest leaves in primates who were herbivores. As evolution occured and the appendix was no longer needed, it shrank in size and function over a very long time. By looking at the reduced appendix in humans, and its similarity to a functional appendix in some primates, scientists can infer that there was a common ancestor between the two structures.
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Comparative Anatomy of Whales
We can first look at the homologous structures in whales. One major homolgous structure is the fin of a whale. If you look at the skeleton of a whale's fin, notice that all of the bones match up to comparative bones in other mammals. This is evidence that whales, as mammals, share a common ancestor with other mammals. Another example of a homologous structure in whales is their inner ear bones. The inner ear bones of a whale are extremely similar to land mammals, but the one difference is that they are fused together. Instead of several, detatched earbones, the ear bones of whales are attached, which helps in hearing under water. There is another mammal with fused ear bones as well, the hippo. The vestigial features of whales tell us two things. They tell us that whales, like so many other organisms, have features that make no sense from a design perspective - they have no current function, they require energy to produce and maintain, and they may be deleterious to the organism. They also tell us that whales carry a piece of their evolutionary past with them, highlighting a history of a terrestrial ancestry. Modern whales often retain rod-like vestiges of pelvic bones, femora, and tibiae, all embedded within the musculature of their body walls. These bones are more pronounced in earlier species and less pronounced in later species. As the example of Basilosaurus shows, whales of intermediate age have intermediate-sized vestigial pelves and rear limb bones. Whales also retain a number of vestigial structures in their organs of sensation. Modern whales have only vestigial olfactory nerves. Furthermore, in modern whales the auditory meatus (the exterior opening of the ear canal) is closed. In many, it is merely the size of a thin piece of string, about 1 mm in diameter, and often pinched off about midway. All whales have a number of small muscles devoted to nonexistent external ears, which are apparently a vestige of a time when they were able to move their ears - a behavior typically used by land animals for directional hearing. |
Cetacean evolution
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Finally, several occurrences of captured sperm whales with visible, protruding hind limbs. Similarly, dolphins have been spotted with tiny pelvic fins, although they probably were not supported by limb bones as in those rare sperm whales. And some whales, such as belugas, possess rudimentary ear pinnae - a feature that can serve no purpose in an animal with no external ear and that can reduce the animal's swimming efficiency by increasing hydrodynamic drag while swimming.
Although this list is by no means exhaustive, it is nonetheless clear that the whales have a wealth of vestigial features left over from their terrestrial ancestors.
Although this list is by no means exhaustive, it is nonetheless clear that the whales have a wealth of vestigial features left over from their terrestrial ancestors.
Ponder this
How often do creatures evolve to leave one environment for another? Are there other examples?
How and why did the pelvic bone and hind legs of whales detached itself from its spine? What are the evolutionary advantages of it?
Why did cetaceans adapt for a fully aquatic environment, while ancient semi-aquatic animals such as crocodiles and alligators remain unchanged?
Discuss
Pick three random animals, they can be of any genus or species, live in different ecosystems, and dependent on different food sources. Identify the anatomical features of the selected animals, and compare and contrast these features to that of the human anatomy. List these similarities and differences, and rank them in the order of similarities. Explain the differences and the reasons thereof to that of the human anatomy.
Further readings
Paleoanthropology and Comparative Anatomy, from the University College London's Anthropology department.
The Evolution of Whales, for a detailed evogram of cetacean evolution.
