Whales are warm blooded mammals (vertebrate) whose females give birth to live young ones. Due to the nature of the environment they thrive in they have high metabolism which helps in producing heat and regulate their body temperatures. The adult whales temporarily acquire some body hair and sensory bristles in their mouth which is characteristic unmistakably mammalian.
The whales have some unique features that are not found in other mammals, many of these are not even found in other marine mammals such as pinnipends and manatees. Examples of these features include streamlined thick and rounded bodies. This feature is different from the fish bodies which have slim and elongated bodies. The whales also have a tail with horizontal fluke which it uses for its movement in water. The neck of the whale has vertebrae which is shortened and fused partially into its skeletal mass. The bones behind the neck are similar. The skull of the whale has jaws extended forward which gives them the feature of a long head. The whales rip cage is very mobile as compared to other mammals. The whales have no olfactory sense, glands on the skin nor the tear gland. These animals have a sharp sense of hearing although they don’t have an external ear opening. These are the key features that are distinct with the whale which shows its adaptations to the marine life. There are many features which show that the whale shares a common ancestor with the terrestrial mammals (McGraw-Hill Yearbook of Science &Technology, 2004).
Classification of the whales
Whales are classified in the class cetacean. In this class there are two general species of whales that exist on earth today, the whales with teeth known as, odontoceti and the toothless whale mystgiceti. The first group has simple uniform teeth found in the lower and the upper jaw. The mysticetes have a kelitinous tissue called the baleen on their jaws. The other group of whale is long extinct it called the Archaeocete which is only known from the fossils found to have existed 53 to 35million years ago. They had complex teeth formulae with molars with double roots, a feature found in the terrestrial mammals. The feature of this group of whales and the modern whales forms the basis that is used to explain the evolution of whales (McGraw-Hill Yearbook of Science &Technology, 2004).
Different arguments for evolution of whales
Theories have been developed that have convincing evidence that the ancestors of the whale were covered with fur and seldom got into water. Whereas the current whales are full aquatic and inhabit the rivers lakes and the ocean. These theories shows the whale ancestors had four limbs with hooves to walk on land. Whereas when we observe the whale today has modified fore limbs as flipper and they don’t possess the hind limb and they move by propelling themselves through the water using the powerful adapted tail. The evolution of whales is said to be a backward kind of macro evolutionally transition because they moved from the terrestrial life to aquatic life as compared to evaluation of other vertebrate evolution from aquatic life to terrestrial life.
The whales have long been known to be the ancestor of land mammal because of their physical characterizes, they have flippers which are similar to the forelimbs found in mammals and the movement of its spines. There some genetic relationship that has been identified between the whales and the terrestrial mammals. Due to various studies the scientist has been able to unearth the origin of whales. In the year 1693 John Ray brought to attention that whale were not fish but mammals. Century or so later William Henry flower discovered that whales had some vestigial features of a terrestrial mammal. This brought to attention that the whales might have originated from the terrestrial mammals rather than the earlier prediction that they were the ancestors of the modern terrestrial mammals(Raymond, n.d.).
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From this time various scientist had come up with different hypotheses of the origin of whales. In the mid 18th century Charles Darwin suggested that the modern whales had originated from the bear. A point he withdrew later. In 1966 and 1969 Leigh van vale and F.S. Szaley respectively, independently noticed that the whales’ teeth resembled those of extinct carnivore’s ungulates known as mesonychids. This was the beginning of the modern evolution theory of whales. To be specific the earliest whale was said to look like a wolf and was of the genus sinoyx.
The morphological evidence was developed in the 1990s to give a better understanding to the development of whales. This theory shifted attention from van vale hypothesis to the group of mammals known as Artiodactyls which contains a wide variety of mammals including hippos, dog, and cows. Another twist in the evolution developed in the 2000 when Gingerish Philip discovered some fossils of an extinct whale sub-order which was linked with artiodactyls (Gingerich, 2002). Gentrich has been working in the field for many years and has contributed a lot to the evolution theory of whales. Most recently Hans Thewisssen found a 48 million year old fossil of an indohyus which is also linked with the evolution of whales.
In this paper I will analyze various theories that have been developed to support the Evolution of whales by various scientists in different disciplines.
The evidence of whale’s evolution
The theory of the whale evolution is supported by different evidences that have been found and documented they includes.
This is the evidence that comes from the study of fossils which have been found in various sedimentary rocks in the earth crust. Through the sequence of fossils found they have shown different transformation that has occurred bridging the evolution of whales from the terrestrial mammals to the current aquatic whales.
Sinonyx is said is said to be the earliest ancestor of the whales who existed 60 millions years ago. It had 44 teeth which is the number found in a primitive mammal. Their teeth were differentiated like the heterodoth, the teeth of the modern mammal. They had elongated mouth and elongated jaw. Sinonyx had narrow molars for tearing fresh and the lower jaw had many cups. The elongation of the mouth was seen as an adaption for hunting fish; this feature is also found in the fish hunting whales which have elongated muzzle. Dolphins also have an elongated muzzle. These features are found in mesonychids which shows that the whales had started to develop features which are found in the whales today (Gingerich, 2002).
This is the second oldest fossil found of cetacean and was formerly known as archaeocete. It originates from Eocene found in Pakistan 53 million years ago. Pakicetus is identified from a fragmented skull remains which shows that it was an intermediary between the Sinonyx and afterward whale’s. Pakicetus had many cusps in its upper and lower molars which were similar to those of the Sinonyx but the premolars had developed small triangular teeth with single cusp with serrated edges. The whales found in later stages of evolution shows molars with more serrated triangles which resemble those of carnivorous sharks. These features show that Pakicetus was also adapted to hunting fish from water. The well preserved skull fossil shows that Pakicetus was a cetacean which had a narrow brain cage and a narrow lamboidal crown. It is said that its sense of hearing was poor in water. Its other features show that it was not adapted to go into high depth in water. This evidence is similar to geochemical evidence of evolution of whales.
Like the Pakicetus the Indohyus had an inner ear structure which is unique to cetaceans and a thick strong tail. It also had a thick layer of bones like the ones found in mammals like hippopotamus. According to the teeth of the Indohyus it was herbivorous. It said that Indohyus is not necessarily an ancestor of the whales but it may have diverged from the whale’s evolution (Thewissen, 1998).
Ambulocetus was found in the same area with Pakicetus but was in the sedentary rock 120 meters above. Ambulocetus is said to have existed 50 million year’s ago, it could walk on land and also had some ability to move in water. The Ambulocetus could walk on land using the hind limbs being supported by the large strong femur; however the limbs lacked strong muscles, so it was not efficient walker on land. According to how the limbs were set they could interfere with each other during walking.
The fore limbs also shows a transition from a land moving mammal to aquatic life, the ulna and the radius were strong enough to enable an animal to walk on land, the elbow was leaning backward which could enable the Ambulocetus to swim in water. Their wrists were flexible unlike those of the present whales which are stiff. The anatomy of its vertebrae shows that the spinal cord of Ambulocetus swayed up during swimming and it used the back feet to propel its body forwarding water. The neck of the Ambulocetus was flexible and it had long cervical vertebrae as compared to the modern whales.
The skull of Ambulocetus was that of cetacean, the muzzle was long with uniform teeth like archaeocete. The mouth had triangular arch and bulla that supported the ear drum that was poorly connected to the skull. Ambulocetus had no blowhole but all the other skull features resembles a modern whale. The features of the Ambulocetus shows that it was not totally disconnected with its terrestrial ancestors.
Rodhocetus existed 46 millions years ago and showed many changes but still had many features of the terrestrial primitive mammals. It is one of the earliest archaeocete which the thoracic, lumbar, and the vertebrae features had been preserved. The lumbar vertebrae had many spines than those of an Ambulocetus. The fossils show that it had extended top vertebrae, which show that it had a well developed swimming tail.
The vertebrae of Rodhocetus shows than it had long spine with four sacral vertebrae that were not fused together ,this feature made the spine more flexible which enabled it to be a better swimmer than the earlier whales. The fossil shows that it had short cervical vertebrae and strong proximal tail vertebrae. These features are closely associated with the modern whales. The fossil shows that Rodhocetus must have been a good swimmer.
The pelvic girdle of the Rodhocetus was smaller as compared to the earlier whales but still it had shortened sacral vertebrae. This feature shows that Rodhocetus could still walk on land but with difficulties. The femur was short and less masculine which made the hip weak during walking. The skull of Rodhocetus was larger as compared to the whole skeleton the muzzle was more extended than those of its predecessors. This elongated its skull making look more a cetacean. The molars were simple with extended crowns, the teeth were more uniform. The Rodhocetus had a large bulla which was made of a dense bone but they did not contain sinus of the modern whale. The overall feature of Rodhocetus showed general improvement from its predecessors, by virtue of having more enhanced swimming ability like expanded strong tail and increased vertebrae flexibility.
This fossils show another grade of evolution, it is estimated to have lived 35 to 45 millions years ago. Basilosaurus was along thin snakelike animal, the fossil was initially thought to be the remains of a sea serpent. The body length is estimated to be of about 15 meters which is a distinct features in the modern whales its vertebrae was so long as compared to the its predecessors. These features show that the modern whales originated from Basilosaurus.
There are some features which are interesting about this whale, these are its distinctive anatomy of its hind limbs, the small limbs looks ineffective for swimming and were less than 60 centimeters from its body, the pelvic girdle was disconnected from the spine so that it could not withstand its body weight. The body features represented a good transition in the whale evolution. According to the fossils found Basilosaurus represents whales without tail flukes. Basilosaurus spent most of its time in water and it had advanced large single nostril and an extended snout which are characteristics of aquatic mammals.
Dorudon seems to have originated from Basilosaurus in 40 million years ago and represents recent ancestors of the modem whales. Dorudon was smaller and shorter than Basilosaurus which a short vertebrae. The skull was similar to that of Basilosaurus. Dorudon lived in water and it had no features that could help it walk on land. They had very small hind limbs which could not support them to walk. Both Basilosaurus and Dorudon were fully aquatic whales and they had no features that could enable them to walk on land. They are the most recent ancestors of the modern whales.
The molecular biology has also been able to explain the relationship between the terrestrial mammals and the whales, studies have been able to show that these two share a common ancestor. According to this study the whales should have more similarities in their molecular make up with their more recent ancestors than the animals they share the same aquatic environment.
The molecular study shows that that the whales have a close relationship with ungulates than other mammals. This is consistent with the evolution expectations. The researches that were conducted shows that the whales were closely related to artiodactyls. This close molecular relationship of whales and the ungulates support strongly the evolution theory. They support a common ancestry between the whales and the ungulates.
The morphological studies of the characters found in the fossils of whales and ungulates have been able to make the evolution theory more clear. The anatomy of the Basilosaurus foot has a close relationship with artiodactyls foot. Another example is the ear of anatomy of Pakicetus and the artiodactyls; they are oriented at the same angle in the side and front. This morphological feature of the intermediate whales’ ancestors shows a common ancestry origin of the whale and other ungulates (Sheen, 2002).
The vestigial feature in modern whales tells us that the whales have an evolutionary past and also they make little sense on the makeup perspective. These features have no use in the modern whales but it still need energy to support their survival.
Some of the vestigial features includes rod shaped pelvic bones and tibiae which are attached to the wall of its masculine body. These features were more pronounced in the intermediate whales than in the modern whales. Like in Basilosaurus the pelvis bones were more enhanced as compared to the modern whales (Hoelzel, 2002).
The other feature is that the whale retains the sensory organs which are no longer in use. The modern whales have vestigial olfactory nerves. They also have small muscles of non existence ears which in other terrestrial mammals are enhanced to move the ears. This shows that these features in the ancestors of the whales were useful, a thing that support the evolution of the whales. The thoracic cage of the whale is vestigial and has very few muscles. The modern whale uses the outward movement of rips to force air into the lungs.
The embryological feature shows that a whale embryo develops some features which at later stages are abandoned to take the final form. Like in the vestigial evidence it makes no sense for some features to develop and then be abandoned for no biological reason. It seems unreasonable for the embryo to take such a long process rather than directly to the form of the adult whale. During the embryo formation some features which are found in these terrestrial mammals are formed and later discarded in the later stages. An embryo of the whale on the womb develops body hair while there is no adult whale that retains the hair. The fact that the whales have genes that produces body hair shows that the ancestor of the whales possessed the body hair (Berta et al, 2006).
During embryo development it has the hind limb which further disappears as the whale grows bigger. The position of the nostril in early development is placed the same position with the position of other mammals but the position changes later. Some whale’s embryos develop teeth while in the womb which later disappears before birth.
All these features found in the embryo development that are similar to those of terrestrial mammals show that the whale and other terrestrial mammals share a common ancestors. They support the theory of evolution.
These involve viewing fossils in the taxa based history; the big picture is in viewing the fossils in the larger environmental framework. The succession of whale’s fossils and the different evolution changes should have some similarities with the fossils of other mammals, which existed the same time in similar environment. When this evidence is analyzed it should show consistence with the evolution of the whales.
Sinonyx according to evolution it was completely terrestrial, in the paleoenviromnet evidence we find that its fossils were found together with fossils of other terrestrial animals. Pakicetus which may have spent most of its time in water were found together with fossils of snails, bats, and other rodents. There were also some aquatic animals found in the sediment associated with Pakicetus. The paleoinviromental evidence shows that Pakicetus lived in shallow terrestrial environment, sometimes getting into fresh waters to hunt for food. The evidence shows that Pakicetus give birth on land adjacent to the waters.
The Ambulocetus fossils contained some leaves fossils and snails’ fossils in the same place. This shows that this fossil was once found on the shallow waters and the leaves were carried by water to the sea to fossilize. Rodhocetus fossil was found deposited in the outer part of the continental crust. Rodhocetus must have lived in the deep seas because it was found together with the fossil of the planktons and other micro fossils which show it was in deep waters. Basilosaurus and Dorudon have been found in many types of sedimentary rocks which show they were capable of living in both deep and shallow waters.
This evidence is analyzed by checking the chemical content differences in the teeth of the fossils found. The earlier whales lived in fresh waters had the ratios of heavy oxygen to light oxygen lower as compared to the modern whales who live in the salty waters. This evidence shows that the modern whales were initially found in the terrestrial waters and later moved on the salty waters.
The geographical evidence of distribution of fossils is also consistent with the evolution of the whales. This evidence argues that terrestrial organisms are expected to be restricted within some geographical zones than the marine species which are only limited by the geographical expansion of the oceans. The earlier whales that were terrestrial like Sinoyx were restricted to small geographical location as compared to the later version. Sinoyx fossils were only found in central Asia while Basilosaurus and Dorudon were found in many areas something that shows they were adapted to the aquatic life and occupied a much wider area in the ocean (Gingerich, 1977).
This paper has been able to explore different areas of the evolution of the whales. First we realize that the modern whales have some similar characters that are found in the terrestrial mammals. Some of these features are no longer functional and they are considered as vestigial organs. The study shows that there are two sub-orders of the cetacean class that are inexistence today and the third sub-order is extinct and helps in forming the basis for the evolution of the whales.
Looking back we realize many scholars and scientist had developed various hypotheses trying to explain the evolution of whales but the concrete theories have been developed in the late19th century when the two scientist Leigh and F.S. szaley noticed the similarity in the whales teeth and that of an extinct carnivore ungulates known as mesonychids that give the evolution of whales a new direction. In his field work, Genrich Philip discovered some fossils in Pakistan these fossils have been linked with some intermediate ancestors of whale evolution. Genrich field work has contributed a lot in the advancement of the evolution theory of whales.
There are many evidences that have been highlighted in this paper that supports the evolution theory. When we analyze all of them, then it becomes clear that the whales shared a common ancestor with the terrestrial ungulate. Looking at the Paleontological evidence we realize that it gives strongest evidence that is convincing, the other evidence given seems to strengthen this evidence. The Paleontological evidence gives the fossils evidence of the different evolutionary stages of the whales. It shows the earliest fossil of Sinonyx which is considered to be the earliest form of evolution of whale from the terrestrial mammals. When the scientist analyses this fossil they concluded that Sinonyx was the first terrestrial mammal to develop whale features. Evidence then shows fossils of later creatures that had developed enhanced whale features. Following this evidence we realize that it is able to give into account the various changes that the whale underwent in different evolution stages. The evidence is strong and convincing because it is able to explain different adaptations possessed by different creatures at different stages of evolution.
The molecular evidence also gives the evolution theory a big support i.e. it shows a close relationship between the whales and other terrestrial ungulates. It is able to show that the whales have a close relationship with the terrestrial ungulates than the other animals with whom they share the aquatic environment. The embryological evidence is based on many assumption and prepositions but it also drives its argument to support the evolution theory. Paleoenvironmental evidence is closely linked with Pale ontological evidence and they supplement each other on explaining the characteristics fossils found in a given sedimentary rock. Both theories combined are able to give insights on why certain fossils were found at given geographical area.
The other theory which gives support to the evolution theory is the vestigial theory which tries to explain the vestigial features found in the whale. The various explanation of these features are convincing that at one time they were functional and were used by a terrestrial ancestor of the whale. The geochemical theory has also been able to confirm the different habitat of different creatures that were intermediate in the evolution of the whales.
Analyzing the various evidence given I can conclude that the Paleontological evidence forms the basis for the evolution theory of whales. It has been able to unearth very strong evidence that support the evolution claims. This evidence should be explored further and more evidence lies in the field. The scientists should put more efforts to further this evidence by getting more fossils that will back the already documented evidence. The scientists should also look at the possibility of any other creature that may have undergone the reverse evolution to be able to support the evolution of whale’s theory.
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