Free Essay Example: Evolution of Whales

Whales, porpoises, and dolphins comprise the Cetaceans. All contemporary cetaceans reside in water and are unable to survive outside the water habitat. However, cetaceans are mammals, similar to other animals, and different from other vertebrates. Whales take care of their babies, possess three ear bones implicated in the transmission of sound, and their lower jaws comprise of a dentary. Cetaceans consist of the largest animal species ever, the blue whale, and also very small representatives such as the vaquita. Despite the differences in body size, all contemporary cetaceans are reasonably identical in shape; they have horizontal tail flukes employed in swimming; their forelimbs are flippers, absence of external hind limbs, short necks, and streamlined bodies. Whales are believed to have evolved from land mammals; evolution may entail all facets of life, including natural selection, mutation, genetic drift, behavior, and technology. Charles Darwin is recognized for his contribution to the theory of evolution through natural selection (Parvinen, 743). Natural selection is a process that leads to the adaption of an organism to its habitat by way of selectively reproducing changes in its genetic constitution or genotype. Charles Darwin postulated that all species originated from other species and ultimately had a mutual ancestor. Hence, whales are mammals with mammalian land ancestors, thus whale descendants must have also dwelled on land. The paper aims to investigate the connection between mammalial whales and the ancestral land mammals using Charles Darwin's theory.

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Background Study

The land origin of whales continued to be a contentious topic for scientists since all fossils of whales across the 19th century demonstrated wholly aquatic characteristics of animals that may well never exist on land. Even in Darwin's era, it was of common knowledge that whales had land descendants; however, the fossil that recorded shift from land to water was a mystery; and all fossils resembled similar features to modern whales (Lucas). Creationists demanded these fossil proofs and made a mockery of the concept that whales are in a way related to cows and other even-toed animals, and further labeled the idea as an "udder failure." However, all that altered in the early 1990s and 2000s when an outstanding sequence of fossils was unearthed. Conferring to, in 1990, paleontologists found the first of a series of cetacean fossils, mainly in Pakistan and India. The relics recorded the change from land to see in the Eocene Era, which was about 34 to 54 million years ago. Currently, the cetacean origin is one of the most prominent examples of macroevolution recorded in the fossil evidence. The fossil evidence has made it possible to study in detail evolutionary changes enabling unparalleled knowledge of adaptations that made it possible to shift from land to water.

Evolutionary Transformations To Whale Adaptation Transition From Land To Water

Evolutionary changes took place across the body. The limbs became vestigial and could not support the weight and instead transformed into organs for propulsion in the novel water habitat. Also, the ears of cetacean transformed ears due to the difference in the sound of air and water. Similarly, the nose of whales adjusted and positioned itself on top of the forehead to facilitate easier breathing when under the water. The kidney also transformed due to the absence of fresh water to drink in ocean living cetaceans. The changes amassed in short successions, and within eight million years, cetacean descendants transformed from land animals to aquatic swimmers. Massive experimentations typified the early stage of evolution of cetacean. For instance, some whales' resembled seals, crocodiles, otter, and every body part was tried and went nonexistent until only a single type of body was achieved. The body type that remained is seen in approximately all ninety new species of cetaceans. The body is streamlined with neck absent, ends in a horizontally situated triangular fluke, external hind limbs missing with forelimbs that are designed like paddles, skin lacking hair, and a nose entrance on the forehead that forms the blowhole.

However, despite these evolutionary changes, some cetacean species have retained some hairs on their face, and in other species, whiskers are present in the fetus. Hence, these are strong evidence of their mammalian ancestry. Also, traces of cetacean land heritage are still evident in certain embryos. For example, current cetaceans do not have hind limbs; however, their embryos show the beginning of hind limbs. The figure 1 below shows four embryos organized from young to old. In the earlier days, the hind limbs are observable in the embryos but progressively disappear with embryonic development.

Phylogeny

The moment scientists discovered that cetaceans had land ancestry; they started to research on the closest relatives cetaceans. According to Dominic, through molecular biology, scientists managed to identify genetic similarities between whales and artiodactyls. Current artiodactyls representatives include camels, hippos, pigs, cattle, deer, giraffe, and sheep, and of all these, hippos are perceived as the closest living cetaceans relatives (Dominic, 103). The oldest whale fossils were about fifty million years, and it was impossible to find the closest living relative of whales until scientists discovered artiodactyl. Artiodactyl was found in 2007 when raoellids skeletons were found in the Himalayas and were proven to be the closest relatives to whales. Phylogenetic inference is the research on how organisms are related to each other. The hypothesis on phylogeny is documented using a cladogram. A cladogram is a branching diagram that associated more-and-more closely linked groups as closer-and-closer branches. Figure 2 below shows the link between cetaceans and their land relatives and taxa that have a lot of braches on the cladogram that are more closely linked.

Raoellidae: Closest Cetacean Relative

It is one of the artiodactyls families that have small groups of species that are identified from teeth and jaws. Only skulls and skeletons for one raoellid, Indohyus, are known. Raoellid is known from India and Pakistan and is confined to the lower and middle Eocene. The Indoyus was a cat size, an animal; however, comparably resembled a deer devoid of antlers. Currently, the Indoyus could have been identical to the Hyemoschus aquaticus. Hyemoschus feeds on leaves and fruits on the floor of the forest and lives close to streams. When Hyemoschus sense danger, they escape into the streams, completely submerged when hiding. It is likely Indohyus lived similarly, and the predator evasion was the initial aquatic behavior of cetacean ancestors. More evidence of the marine environment for Indohyus is the chemical composition of its teeth, which was mainly calcium phosphate. Also, the chemistry of Indohyus teeth showed it was herbivorous, and its heavy bones imply that they worked as ballast, enabling the Hyemoschus to remain submerged. Besides, tooth wear of Indohyus resembles that of meat-eating whales, a trait that aided its descendants to turn out to be meat-eating whales.

Pakicetidae: the First Cetaceans

Pakicetus was identified in India and Pakistan. As states by Geisler, although Pakicetus is the earliest whale, they never resembled the current whales and instead resembled a wolf or a big dog (Geisler, 2036). Pakicetus had elongated skulls with big carnivorous dentition. Nonetheless, their skull, especially in the region of the ear, is covered by a bony wall, which is strongly identical to those of modern cetaceans. Pakicetus was probably waders in streams since their fossil was located in rocks that made shallow waters and not in the ocean. The position of the ears and eyes is a trait linked with animals that have submerged bodies such as crocodiles. The eyes of whales are used for terrestrial spying. Scientists hypothesize pakicetus were ambush predators that attacked animals that came to drink.

Ambulocetidae: the First Marine Cetaceans

The first cetacean known to have moved towards the ocean approximately 48 million years ago is the Ambulocetus. Ambulocetus has been identified only in Pakistan, and just a single skeleton has ever been found. Conferring to Geisler, Ambulocetus looks like a crocodile and more sprawling, unlike pakicetus, which could stand on its limbs. Ambulocetus has short limbs, a muscular tail, and a long snout. Regardless of the short limbs, it has large feet and was likely the locomotors of Ambulocetus (Geisler, 2036). Although marine shells lined with rocks that Ambulocetus was discovered, the presence of freshwater nearby was evident. These cetaceans were likely coastal, taking advantage of prey that came to water.

Remingtonocetidae: Long-Snouted Cetaceans

According to Geisler, Remingtonocetid is also only known for India and Pakistan. The transition towards aquatic life progresses; they have shorter limbs compared to earlier whales and had a long, powerful tail. The nature of the remingtonocetids vertebrae shows that it lacks a fluke; however, the vertebrae of the tails are a bit plane, implying that the tail was plane in the horizontal angle. Remingtonocetids probably swung their tail through the water in an up and down motion similar to modern cetaceans when propelling themselves using their triangular flukes. Other feature suggestive of aquatic life includes small eyes implying they were less significant for preying. Besides, the location of the eye is better suited for hunting aquatic prey. The section of the skull that accommodates the ears is big, implying that remingtonocetids' hearing was excellent. There is a high probability that remingtonocetids used their ears to detect prey, a trait seen modern toothed whales.

Protocetids: the Cetaceans that Conquered the Oceans

Protocetid existed simultaneously with remingtonocetids, however, in separate habitats. Apart from South Asia, protocetids also dominated oceans in Africa, North, and South America. Different from other cetaceans, it shows that protocetids could swim through expansive waters and therefore excelled at swimming (Geisler. Protocetids are a broad family with a lot of morphological variation. Some protocetids had tails that resembled those of remingtonocetids and ambulocetids, while some had flukes (Thewissen, 242). Protocertids are located in open areas with clear waters, and had big eyes, unlike remingtonocetids. Besides, protocetids are the initial whales that had nasal entrance located on their skull and not on the snout. However, it is not considered a blowhole which is present in modern whales.

Basilosauridae: the First Fully Aquatic Cetaceans

Basilosaurids are the earliest entirely aquatic whales and a family from which current whales have originated from. Basilosaurids are spread across the globe. Basilosaurids have an identical feature to modern whales, they have a streamlined body, they are equipped with flukes, and they use their forelimb as locomotor (Thewissen, 242). Different from modern cetaceans, basilosaurids had outer hind limbs that were vestigial, since they were too small to support its body weight.