Whales evolved from small aquatic hoofed ancestors

Blogging on Peer-Reviewed ResearchTravel back in time to about 50 million years ago and you might catch a glimpse of a small, unassuming animal walking on slender legs tipped with hooves, by the rivers of southern Asia. It feeds on land but when it picks up signs of danger, it readily takes to the water and wades to safety.

Indohyus

The animal is called Indohyus (literally “India’s pig”) and though it may not look like it, it is the earliest known relative of today’s whales and dolphins. Known mostly through a few fossil teeth, a more complete skeleton was described for the first time last week by Hans Thewissen and colleagues from the Northeastern Ohio Universities. It shows what the missing link between whales and their deer-like ancestors might have looked like and how it probably behaved.Whales look so unlike other mammals that it’s hard to imagine the type of creature that they evolved from. Once they took to the water, their evolutionary journey is fairly clear. A series of incredible fossils have documented their transformation into the masterful swimmers of today’s oceans from early four-legged forms like Pakicetus and Ambulocetus (also discovered by Thewissen). But what did their ancestors look like when they still lived on land?

Hooves to flippers

Until now, we had little idea and their modern relatives have provided few clues. According to molecular evidence, the closest living relatives of whales are, quite surprisingly, the artiodactyls, a group of hoofed mammals that includes deer, cows, sheep, pigs, giraffes, camels and hippos.

They all have a characteristic even number of toes on each hoof and not a single one of them bears even a passing resemblance to whales and dolphins. Among the group, the hippos are evolutionarily closest and while they are at least at home in water, their family originated some 35 million years after the first whales and dolphins did.

Enter Indohyus, a small animal about 70cm long that lived 47 million years ago. It was a member of a family of mammals called the raoellids, prehistoric artiodactyls that lived at the same time as the earliest whales and hailed from the same place of origin – southern Asia. By analysing a fossilised skull and limbs collected from India, Thewissen found compelling evidence that the raoellids were a sister group to the ancestors of whales.

Even though Indohyus had the elegant legs of a small deer and walked around on hooves, it also had features found only in modern and fossil whales. Its jaws and teeth were similar to those of early whales, but the best evidence was the presence of a thickened knob of bone in its middle ear, called an involucrum. This structure helps modern whales to hear underwater, it’s only found in whales and their ancestors, and acts as a diagnostic feature for the group.

Based on these physical similarities, Thewissen suggests that the raoellids are a sister group to the whales. Both of these groups are evolutionary cousins to all modern artiodactyls. (As a note for journalists and creationists, Indohyus is not a direct ancestor of whales, as many news sites are claiming, and nor did whales ‘evolve from deer’!)

A swimming Indohyus

Life in the water

Indohyus‘s skeleton also suggests that it was partially adapted for life in the water. Its leg bones were unusually thick, a feature shared by other aquatic animals including hippos, sea otters and manatees. These heavier bones stop swimming mammals from floating by default and allow them to hang in the water and dive more easily.

Because Indohyus had slender legs and not paddle-shaped ones, Thewissen pictures it wading in shallow water, walking hippo-style along the river floor while its heavy bones provided ballast.

Thewissen found more clues about the animal’s lifestyle from its teeth, and particularly the levels of certain isotopes in their enamel. Levels of oxygen isotopes matched those of water-going mammals, providing further support for Indohyus‘s aquatic tendencies. Its large crushing molars are typical of plant-eaters and levels of carbon isotopes in them suggested that Indohyus either came onto land to graze (like hippos) or fed on plants and invertebrates in the water (like muskrats). In terms of behaviour, they were close to the modern mousedeer, a tiny, secretive deer that feeds on land but flees into streams when danger threatens.

Put together, this portrait of Indohyus‘s life also tells us about the changes that drove the evolution of whales, and it looks like it wasn’t a move to water. Whales and raoellids are evolutionary sisters and since early members of both groups were happy in the water, aquatic lifestyles must have pre-dated the origin of whales.

Instead, Thewissen suggests that the key step was a switch in diet. He speculates that whales developed from an Indohyus-like ancestor that fed on plants and possibly small invertebrates on land, but fled to water to escape predators. Over time, they slowly turned into meat-eaters and evolved to swim after nimble aquatic prey.

Video: Have a look at Thewissen talking about Indohyus and the origin of whales.

Images of Indohyus are painted by the extraordinary Carl Buell

Reference: Thewissen, J.G., Cooper, L.N., Clementz, M.T., Bajpai, S., Tiwari, B.N. (2007). Whales originated from aquatic artiodactyls in the Eocene epoch of India. Nature, 450(7173), 1190-1194. DOI: 10.1038/nature06343

About these ads

8 Responses

  1. fascinating… i’ve been thinking about the number of times ‘life’ went back to the sea since it first crawled out of it.

    Look at a seal- their rudimentary digits seem caught in the act of evolving into fins.

    Apparently they evolved from a ‘bear-like’ ancestor 23 million years ago.

    http://shlogblog.blogspot.com/

  2. Also…. what’s the current state of the “aquatic ape” hypothesis? has it been debunked?

  3. Wikipedia has a rather good entry on claims for and against the aquatic ape hypothesis. And I blogged earlier this year about new research on the origins of human bipedalism suggesting it has its roots in trees..

  4. Ha! “has it’s roots in trees”.. very funny.. intentional?

  5. AAT, the waterside theory of human evolution, is still often misrepresented. It says that after the human/chimp divergence c.5 Ma, the lineage leading to us was a lot more dependent on water than the chimp lineage. The general view is clear (human relatives/ancestors along African & Indian Ocean shores, eg, reaching Java & Flores, & venturing inland along rivers etc.), but the details are still discussed. For recent views, please google “aquarboreal” or have a look at http://groups.yahoo.com/group/AAT or http://users.ugent.be/~mvaneech/outthere.htm
    Yes, our upright locomotion has int origin in the trees, more specifically in swamp forests perhaps 20 Ma, where it was advantageous to wade (upright body) or float (airsacs) vertically & to climb arms overhead in the branches & to hang on branches above the swamp. See A.Filler’s 2007 paper in PLoS & his book “The upright ape” on the Moroto vertebra some 20 Ma that suggests that early apes had a vertical posture & locomotion (google “aquarboreal” aqua=water+arbor=tree).

  6. Thank you for being a quality source for science writing. It is appalling how the mainstream press reports on science topics. Your entry on Indohyus was the best I found.

  7. Thanks Keri and welcome to the site.

  8. I particularly liked the ‘note for journalists and creationists’!

Comments are closed.

Follow

Get every new post delivered to your Inbox.

Join 38 other followers

%d bloggers like this: