Among the major discoveries were that among extant animals today, whales are most closely related to hippos. Previously, it was thought that whales were descended from mesonychids, a group best described as carnivorous artiodactyls (even toed ungulates). It now appears that whales are descended from herbivorous artiodactyls (hippos are herbivorous artiodactyls). Also, whales evolved relatively quickly - within 8 million years. It is also possible to trace cetacean evolution from the beginning, just after cetacean ancestors began dipping their toes into the water, to today's massive, completely marine creatures.
1. Indohyus
Indohyus (adjacent image by Carl Buell, who is a fantastic natural history artist, and blogs under the name Olduvai George) is a member of an order called Raoellidae, closely related to the artiodactyls, that lived about 48 million years ago. Hans Thewissen published a paper at the end of 2007 advancing the theory that cetaceans and raoellids descend from a creature very much like Indohyus (but which lived at least 6 million years earlier). There are multiple strands of evidence - its heavy bones, which are an adaptation common to marine mammals, and basically provide ballast; isotopic analysis of the fossils also prove that Indohyus was semi-aquatic; and best of all, Indohyus has an involucrum, which is a thickened piece of bone in the inner ear that whales have (it aids hearing underwater). No other mammal group has an involucrum.2. Pakicetus
Pakicetus, which lived 53m years ago (adjacent image again by Carl Buell), was discovered by in Pakistan in 1981. It is classified under its own family, Pakicetidae. Hans Thewissen found complete skeletons in 2001, with adaptations of the inner ear that are today unique to modern day whales. They were the earliest cetaceans, but they were not aquatic. Pakicetus lived much like Indohyus must have, wading along swampy shores looking for fish and small animals. Note that Pakicetus was already carnivorous. It was between 1 and 2 metres long, and looked very much like a large dog.3. Ambulocetus
Ambulocetus, which lived 50m years ago, (adjacent image is a screen capture from the BBC natural history series Walking with Beasts) was the type species of its family, Ambulocetidae. It was up 5m long, and was for most purposes a large, furry crocodile. It hunted like a crocodile too, as an ambush predator, and could also walk on land, although not very well. It swam, unlike crocodiles, with an up and down motion of the spinal cord (as dolphins do today), and had vestigial hooves on its very large hind feet. It is hypothesized that they walked somewhat like sealions, pointing their hind feet laterally and waddling forward.4. Kutchicetus
Kutchicetus, which lived 46m years ago (adjacent image again by Carl Buell, who has made something of a hobby of drawing ancient cetaceans), was a member of the family Remingtonocetidae. This family of cetaceans was characterised by their long snouts and broad tails, which made them slightly better adapted for marine life compared to the ambulocetids. Kutchicetus was small, about the same size as an otter, and probably had a similar lifestyle.5. Rodhocetus
Rodhocetus (adjacent image from Science magazine) lived about 46m years ago too. It was a member of the family Protocetidae. This family displayed body plan features in common with today's whales, a long streamlined body, and some species may have had a tail fluke. However, they still retained large feet, which may have been webbed to aid swimming. Rodhocetus was about the same size as a sealion, and much the same shape. This family is heterogeneous, however, and is mainly characterised by large eyes set deep under the supraorbital bone. Remingtonocetids and ambulocetids had smaller eyes.6. Dorudon
Dorudon belongs to the family Dorudontidae, from which the modern day whale families Mysticetes (baleen whales) and Odontocetes (toothed whales, including dolphins) descend. It lived about 38m years ago, and was about 5 metres long. It was fully aquatic (as can be inferred from where the fossils were found - not only in South Asia but also in Egypt and the US, so it had a wide range in the oceans of the day), although it retained small hind legs that were probably just stubs on the torso. Dorudon was toothed (baleen evolved later), and did not display any signs of echolocation apparatus, as modern day cetaceans do. The adjacent image is from Walking with Beasts.7. Basilosaurus
Basilosaurus lived slightly later than Dorudon, from about 37m years ago. It was discovered in 1840, and was initially mistaken for a reptile, hence the dinosaur-sounding name. It was far larger than Dorudon, up to 18m in length (due to extremely elongated vertebra), and was fully aquatic as well. It also had small vestigial hind legs. Basilosaurus had a blowhole, which was located about halfway up its snout, which is further evidence of a fully aquatic lifestyle. Hans Thewissen has found the fossilised stomach contents of a Basilosaurus, which indicate that it ate fish, including sharks. The adjacent image is from Walking with Beasts.8. Janjucetus
Janjucetus was an early baleen whale that lived about 25 million years ago, and therefore a member of the order Mysticetes. It had not yet evolved baleen, as you can see from the image (also by Carl Buell, via Carl Zimmer's blog, The Loom). Despite not having any baleen, Janjucetus had acquired some features of modern day baleen whales, such as lower jawbones that were no longer fused at the front. It was about 3.5m long. It was a fierce creature, hunting much like today's leopard seals, a swift ambush predator that would strip flesh from bone.9. Aetiocetus
Aetiocetus was the first baleen whale (and therefore a member of Mysticetes) found with baleen. It also had teeth, but displayed troughs in the palate bone which in modern day baleen whales hold vascular tissue that supplies blood to the roots of the baleen plates. The adjacent image is from National Geographic, via Edward T. Babinski, who has written a great deal on whale evolution on his website.10. Squalodon
Squalodon lived between 15m and 30m years ago. It was a member of Odontocetes, which encompasses today's toothed whales. Other than its teeth, which resembled those of a shark's, it was very similar morphologically to today's dolphins, and also displays the first evidence of echolocation apparatus hat we know of. Squalodon had the beginnings of a melon-shaped forehead, which is how modern day whales amplify and direct their clicks. The adjacent image is from museumkennis.nl.There are many more species known in the family Archaocetes, that is, all cetaceans except Odontocetes and Mysticetes. We now know of about 30 species, most from greatly detailed fossils. Of ancient Odontocetes and Mysticetes, we know of at least another 20 species. Resolving the evolutionary history of the cetaceans is a great triumph for science. Unfortunately, an even greater challenge now presents itself - how to conserve the extant species of cetaceans?
11. Yangtze river dolphin and northern right whales
In 2007, a search by the Chinese government of the Yangtze River for several baiji (image on the left from the Guardian), or Yangtze River dolphins, to fill a long-hoped for nature sanctuary found precisely zero specimens. The baiji has not been seen in the wild since about 2005, and there are no specimens in captivity. The baiji is for all intents and purposes extinct, and the same fate may befall other freshwater dolphins, particularly the Ganges dolphin and the Amazon dolphin. 
The northern right whale may also go extinct, as there are fewer than 300 individuals left in the North Atlantic. Part of the reason why these whales have been reduced to so few is that the North Pacific population was decimated by large illegal catches by the Soviet Union's whaling fleet in the 1960s.With several countries beginning whaling again - Norway, Iceland and Japan so far - these whales may never recover. Vote against whaling - no one needs to eat whales, and particularly in Japan, to waste whale meat. Vote for conservation, otherwise interesting and magnificent animals such as river dolphins will disappear. There is no time to waste.
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