Evolution of cetaceans
- 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.
DNA sequence analysis [change]
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]
Hippo fossils are not observed until the Miocene, but whale ancestors have been dated to 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). This hypothesized ancestral group probably split into two branches around 54 mya. One branch evolved into cetaceans, possibly beginning with the proto-whale Pakicetus from 52 mya.  These early cetaceans eventually became adapted to life in the sea. They became the completely aquatic cetaceans.
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.
About the size of a raccoon or domestic cat, this herbivorous creature shared some of the traits of whales. 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, 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.
- 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.
- "Scientists find missing link between the dolphin, whale and its closest relative, the hippo". Science News Daily. 2005-01-25. http://www.sciencenewsdaily.org/story-2806.html. Retrieved 2007-06-18.
- 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.
- Ursing B.M.; U. Arnason (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.
- Pakicetus and other early whale ancestors are collectively known as the Archaeoceti.
- Boisserie, Jean-Renaud; Fabrice Lihoreau and Michel Brunet (February 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.
- Thewissen, J.G.M. et al. (20 December 2007). "Whales originated from aquatic artiodactyls in the Eocene epoch of India". Nature 450 (7173): 1190–1194. doi:10.1038/nature06343. PMID 18097400.
- University Of California, Berkeley (2005, February 7). "UC Berkeley: French scientists find missing link between the whale and its closest relative, the hippo". ScienceDaily. http://www.sciencedaily.com/releases/2005/02/050205103109.htm. Retrieved 2007-12-21.
- University Of Michigan (2001, September 20). "New fossils suggest whales and hippos are close kin". ScienceDaily. http://www.sciencedaily.com/releases/2001/09/010920072245.htm. Retrieved 2007-12-21.
- Carl Zimmer (2007-12-19). "The loom: whales: from so humble a beginning.". ScienceBlogs. http://scienceblogs.com/loom/2007/12/19/whales_from_so_humble_a_beginn.php. Retrieved 2007-12-21.
- Ian Sample (2007-12-19). "Whales may be descended from a small deer-like animal". Guardian Unlimited. http://www.guardian.co.uk/science/2007/dec/19/whale.deer?gusrc=rss&feed=networkfront. Retrieved 2007-12-21.
- PZ Myers (2007-12-19). "Pharyngula: Indohyus". Pharyngula. ScienceBlogs. http://scienceblogs.com/pharyngula/2007/12/indohyus.php. Retrieved 2007-12-21.