Cetaceans Evolution

Until recently, the evolutionary origin of cetaceans has been a mystery. Cetaceans are aquatically specialized mammals that exhibit body streamlining, have no hair or hind limbs, but adapted flippers and flukes for underwater movement. This group includes porpoises, dolphins, and whales (Thewissen et al., 2001). Despite having features so specifically adapted for aquatic life, cetaceans, according to scientists, have actually evolved from land animals. Some scientists even argue that cetaceans evolved from artiodactyls, which are land-dwelling ungulates (Rose., 2001). Artiodactyls include mammals such as cattle, deer, hippopotami, and pigs (Spaulding et al., 2009). In fact, some scientists go as far as to claim the hippopotamids are a sister group to cetaceans (Rose, 2001). Other scientists claim cetaceans actually evolved from mesonychians, which are an extinct group of carnivorous ungulates. To make their claims, scientists use morphological data from recent cetacean fossil discoveries, such as pakicetids, as well as genetic and molecular studies. Morphological evidence tends to support mesonychian origin, while molecular studies indicate cetaceans have evolved from artiodactyls
(Thewissen et al., 2001).
Body
The evolution of cetacean from land to sea dates back to the Eocene epoch, approximately 50 million years ago (Rose, 2001). Formerly, Ambulocetus natans was considered among scientists as the most ancient cetacean, having features adapted for an amphibious lifestyle. Though Ambulocetus natans does not closely resemble strictly aquatic cetaceans, it also does not closely resemble known artiodactyls or mesonychians. As a result, it has not provided enough information to indicate cetaceans’ closest land-dwelling relative (Thewissen et al., 2001). According to Thewissen et al. (2001), discovery of pakicetids, which are considered more ancient cetaceans and maintain more terrestrial characteristics than Ambulocetus, have allowed scientists to fill in the gap between cetaceans’ transition from terrestrial to aquatic. By studying pakicetids, scientists have a closer link to cetaceans’ terrestrial relatives.
The skull bones of pakicetids indicate cetaceans are more closely related to artiodactyls.
In addition, study of the pakicetid ankle indicate a morphology that was specifically seen in artiodactyls (Thewissen et al., 2001). According to Rose (2001), it is highly unlikely that such an adaptation is a result of convergent evolution because cetaceans have evolved to be more equipped for an aquatic lifestyle. Rather, it is a remnant feature of life on land (Rose, 2001). Despite this evidence, other scientists argue that tooth morphology of ancient cetaceans is not consistent with artiodactyls. Instead, tooth and some skull features are more similar to the carnivorous mesonychians. However, this morphology is argued by opponents to be most likely a result of convergent evolution (Rose,
2001). In order to determine whale ancestry, scientists have been heavily reliant on cladistic study. Cladistic study has provided more evidence of a closer relationship between mesonychians and cetaceans, according to Spaulding et al. (2009). However, even cladistic study has not been quite consistent. This lies in the fact that scientists are not certain of whether mesonychians should be positioned within the artiodactylan clade along with cetaceans. Because of the uncertainty revolving around mesonychian placement in cladistic analysis, it has proven to be inconsistent in showing a closer relationship between cetaceans and mesonychians (Spaulding et al., 2009).
Conclusion
Overall, though some morphological and molecular evidence have tipped the scales in favor of a closer relationship between cetaceans and artiodactyls, there is still disagreement among scientists who believe more morphological evidence in skull and dental structure and cladistic analysis supports a closer relationship with mesonychians. However, only time and discovery of more intermediate cetaceans will give the scientific community a more complete picture of the transition of whales from land to sea.