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]

Size[change | change source]

Scientists from the University of Plymouth studied many animals that dive and hold their breath, from insects to whales, and they found that larger animals can hold their breath longer than smaller animals because they can store more oxygen for their size, and this difference was much bigger for warm-blooded animals than for cold-blooded animals. They said this may be why modern whales and extinct diving animals like plesiosaurs became so large.[13][14]

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. Bibcode:2007Natur.450.1190T. 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.
  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. |access-date= requires |url= (help)
  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.
  13. Eurekalert.org (May 26, 2020). "Scientists reveal new fundamental principles governing diving in animals". Press release. https://www.eurekalert.org/pub_releases/2020-05/uop-srn052620.php. Retrieved June 1, 2020. 
  14. Wilco C. E. P. Verberk; Piero Calosi; François Brischoux; John I. Spicer; Theodore Garland; David T. Bilton (May 27, 2020). "Universal metabolic constraints shape the evolutionary ecology of diving in animals". Proceedings of the Royal Society B. doi:10.1098/rspb.2020.0488. Retrieved June 1, 2020. Cite journal requires |journal= (help)