Evolution of cetaceans

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A phylogeny showing the relationships among cetacean families.[1]

The Cetaceans (whales, dolphins and porpoises) are mammalian marine descendants of land mammals. Their terrestrial origins are indicated by:

  • Their need to breathe air from the surface;
  • The bones of their fins, which resemble the limbs of land mammals
  • The vertical movement of their spines, characteristic more of a running mammal than of the horizontal movement of fish.

The question of how land animals evolved into ocean-going leviathans was a mystery until recent discoveries in Pakistan revealed several stages in the transition of cetaceans from land to sea.

DNA sequence analysis[change | change source]

After many years when palaeontologists thought whales had evolved from mesonychids, DNA sequence analysis showed the closest relationship was with artiodactyls, the even-toed ungulates. A new clade was created to include Cetacea and their nearest relatives, the hippopotamus family. This clade is called the Cetartiodactyla.

Fossil record[change | change source]

Possible relationships between cetaceans and other ungulate groups.[1] [2]
Reconstruction of Indohyus

Hippo fossils are not found until the Miocene, but whale ancestors have been found from the Eocene. This leaves a gap of nearly 30 million years where no hippo ancestors are found. The most recent hypothesis is that hippos and whales shared a common semi-aquatic ancestor which branched off from other Artiodactyls around 60 million years ago (mya).[3][4] This ancestral group probably split into two branches around 54 mya.[5] One branch evolved into cetaceans, possibly beginning with the proto-whale Pakicetus from 52 mya. [6] These early cetaceans gradually became adapted to life in the sea. They became the completely aquatic cetaceans.[7]

Indohyus[change | change source]

Some modern whales have traces of their land-dwelling ancestors. The skeleton of a Bowhead whale shows its hind limb and pelvic bone structure (circled in red). This bone structure stays inside its body its entire life: it is a vestigial structure.

Indohyus is a small deer-like creature, which lived about 48 million years ago in Kashmir. It belongs to the artiodactyls family Raoellidae, and is believed to be the closest sister group of Cetacea.[1]

About the size of a raccoon or domestic cat, this herbivorous creature shared some of the traits of whales.[1] It also showed signs of adaptations to aquatic life, including a thick and heavy outer bone coating. This is similar to the bones of modern creatures such as the hippopotamus,[8][9] and reduces buoyancy so that they can stay underwater. This suggests a similar survival strategy to the African mousedeer or water chevrotain which, when threatened by a bird of prey, dives into water and hides beneath the surface for up to four minutes.[10][11][12]

References[change | change source]

  1. 1.0 1.1 1.2 1.3 Thewissen J.G M. & Williams E.M. (2002). "The early radiations of Cetacea (Mammalia): evolutionary pattern and developmental correlations". Annual Review of Ecology and Systematics 33 (1): 73–90. doi:10.1146/annurev.ecolsys.33.020602.095426. 
  2. Thewissen J.G.M. et al (2007). "Whales originated from aquatic artiodactyls in the Eocene epoch of India". Nature 450 (7173): 1190–1194. doi:10.1038/nature06343. PMID 18097400. 
  3. "Scientists find missing link between the dolphin, whale and its closest relative, the hippo". Science News Daily. 2005-01-25. Retrieved 2007-06-18.
  4. Gatesy J. (1997). "More DNA support for a Cetacea/Hippopotamidae clade: the blood-clotting protein gene gamma-fibrinogen". Molecular Biology and Evolution 14 (5): 537–543. PMID 9159931. http://mbe.oxfordjournals.org/cgi/content/abstract/14/5/537. 
  5. Ursing B.M. & Arnason U. (1998). "Analyses of mitochondrial genomes strongly support a hippopotamus-whale clade". Proceedings of the Royal Society 265 (1412): 2251–5. doi:10.1098/rspb.1998.0567. PMC 1689531. PMID 9881471. 
  6. Pakicetus and other early whale ancestors are collectively known as the Archaeoceti.
  7. Boisserie J.R; Lihoreau F. & Brunet M. (2005). "The position of Hippopotamidae within Cetartiodactyla". Proceedings of the National Academy of Sciences 102 (5): 1537–1541. doi:10.1073/pnas.0409518102. PMC 547867. PMID 15677331. 
  8. University Of California, Berkeley (2005). "UC Berkeley: French scientists find missing link between the whale and its closest relative, the hippo". ScienceDaily. Retrieved 2007-12-21.
  9. University Of Michigan (2001). "New fossils suggest whales and hippos are close kin". ScienceDaily. Retrieved 2007-12-21.
  10. Carl Zimmer (2007). "The loom: whales: from so humble a beginning". ScienceBlogs. Retrieved 2007-12-21.
  11. Ian Sample (2007). "Whales may be descended from a small deer-like animal". Guardian Unlimited. Retrieved 2007-12-21.
  12. Myers P.Z. (2007). "Pharyngula: Indohyus". Pharyngula. ScienceBlogs. Retrieved 2007-12-21.