This article is reposted from the old WordPress incarnation of Not Exactly Rocket Science.
Travel 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.
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’!)
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
- I Spy With My Little Eye… [Last Updated On: November 7th, 2009] [Originally Added On: November 7th, 2009]
- A Crack Opens in the Ethiopian Landscape, Preparing the Way for a New Sea | 80beats [Last Updated On: November 8th, 2009] [Originally Added On: November 8th, 2009]
- The Politics of Addiction | The Intersection [Last Updated On: November 8th, 2009] [Originally Added On: November 8th, 2009]
- Finally! An iPhone App That Lets You Track Your Bathroom Habits | Discoblog [Last Updated On: November 8th, 2009] [Originally Added On: November 8th, 2009]
- Don’t Pack Your Bags Yet—New Planet-Finder Hobbled by Electronic Glitch | 80beats [Last Updated On: November 8th, 2009] [Originally Added On: November 8th, 2009]
- In Controversial Scent Lineups, a Dog’s Nose Picks Out the Perp | 80beats [Last Updated On: November 8th, 2009] [Originally Added On: November 8th, 2009]
- Are You a Cognitive Miser? | Cosmic Variance [Last Updated On: November 8th, 2009] [Originally Added On: November 8th, 2009]
- The Secret Lives and Loves of Great White Sharks | 80beats [Last Updated On: November 8th, 2009] [Originally Added On: November 8th, 2009]
- Britain’s New Protected Minority: Tree-Huggers | Discoblog [Last Updated On: November 8th, 2009] [Originally Added On: November 8th, 2009]
- Inspired by Maple Seeds, a Robotic Whirligig Takes To The Skies | Discoblog [Last Updated On: November 8th, 2009] [Originally Added On: November 8th, 2009]
- New Statesman on Accommodationism | The Intersection [Last Updated On: November 8th, 2009] [Originally Added On: November 8th, 2009]
- Laser-Powered Robot Climbs to Victory in the Space-Elevator Contest | 80beats [Last Updated On: November 8th, 2009] [Originally Added On: November 8th, 2009]
- Checking Back In With SEAPLEX | The Intersection [Last Updated On: November 8th, 2009] [Originally Added On: November 8th, 2009]
- Podcast: An Embarrassment of Genomes | The Loom [Last Updated On: November 8th, 2009] [Originally Added On: November 8th, 2009]
- A Baby Neutron Star, Swaddled in a Carbon Atmosphere | 80beats [Last Updated On: November 8th, 2009] [Originally Added On: November 8th, 2009]
- Butterfliiiies… iiinnnn… SPPPAAAAACCCCEEEEE! | Bad Astronomy [Last Updated On: November 8th, 2009] [Originally Added On: November 8th, 2009]
- My Slate Dialogue with Michael Specter Begins | The Intersection [Last Updated On: November 8th, 2009] [Originally Added On: November 8th, 2009]
- Musical, Fahrvergnügen-Inspired Staircase Makes Commuters Less Lazy | Discoblog [Last Updated On: November 8th, 2009] [Originally Added On: November 8th, 2009]
- Laser-Etched Fruit Is an Answer in Search of a Problem | Discoblog [Last Updated On: November 8th, 2009] [Originally Added On: November 8th, 2009]
- Ares and the carnivals | Bad Astronomy [Last Updated On: November 8th, 2009] [Originally Added On: November 8th, 2009]
- Toddler Gets a Telescoping, Prosthetic Arm Bone That Grows With Him | 80beats [Last Updated On: November 8th, 2009] [Originally Added On: November 8th, 2009]
- Neutered HIV Virus Delivers Treatment to Fatally Ill Boys | 80beats [Last Updated On: November 8th, 2009] [Originally Added On: November 8th, 2009]
- Specter’s First Reply: Denialism Kills People | The Intersection [Last Updated On: November 8th, 2009] [Originally Added On: November 8th, 2009]
- LRO sees a Moonslide | Bad Astronomy [Last Updated On: November 8th, 2009] [Originally Added On: November 8th, 2009]
- Al Gore’s New Book: A Focus on Solutions | The Intersection [Last Updated On: November 8th, 2009] [Originally Added On: November 8th, 2009]
- The Universe Has Us in Its Crosshairs | Bad Astronomy [Last Updated On: November 8th, 2009] [Originally Added On: November 8th, 2009]
- Makers of Universes | Cosmic Variance [Last Updated On: November 8th, 2009] [Originally Added On: November 8th, 2009]
- Can Your Pet Catch & Spread Swine Flu? Yes, If Your Pet’s a Ferret | 80beats [Last Updated On: November 8th, 2009] [Originally Added On: November 8th, 2009]
- Droid 2.0 Vs iPhone | The Intersection [Last Updated On: November 8th, 2009] [Originally Added On: November 8th, 2009]
- Tangled Bank News: An Excerpt and More | The Loom [Last Updated On: November 8th, 2009] [Originally Added On: November 8th, 2009]
- LHC Shut Down By Wayward Baguette, Dropped by Bird Saboteur | Discoblog [Last Updated On: November 8th, 2009] [Originally Added On: November 8th, 2009]
- Careidolia | Bad Astronomy [Last Updated On: November 8th, 2009] [Originally Added On: November 8th, 2009]
- Slate Reply to Specter Up–We Need a National Dialogue on Synthetic Biology | The Intersection [Last Updated On: November 8th, 2009] [Originally Added On: November 8th, 2009]
- Pray this doesn’t get passed | Bad Astronomy [Last Updated On: November 8th, 2009] [Originally Added On: November 8th, 2009]
- You Can’t Make This Stuff Up | Cosmic Variance [Last Updated On: November 8th, 2009] [Originally Added On: November 8th, 2009]
- Mother Tongue, Indeed: Newborn’s Cries Mimic Mama’s Accent | 80beats [Last Updated On: November 8th, 2009] [Originally Added On: November 8th, 2009]
- Ripped From the Journals: The Biggest Discoveries of the Week | 80beats [Last Updated On: November 8th, 2009] [Originally Added On: November 8th, 2009]
- Alternative Landscapes | The Loom [Last Updated On: November 8th, 2009] [Originally Added On: November 8th, 2009]
- Can an iPhone App Decipher Your Baby’s Cries? | Discoblog [Last Updated On: November 8th, 2009] [Originally Added On: November 8th, 2009]
- Reminder: Carl Sagan Day | Bad Astronomy [Last Updated On: November 8th, 2009] [Originally Added On: November 8th, 2009]
- Are There Pesticides in Your Soup? Dunk a Pollution Dipstick to Find Out. | 80beats [Last Updated On: November 8th, 2009] [Originally Added On: November 8th, 2009]
- Log in and Join the Conference [Last Updated On: November 8th, 2009] [Originally Added On: November 8th, 2009]
- Conference Ends [Last Updated On: November 8th, 2009] [Originally Added On: November 8th, 2009]
- Conference Archive Opens [Last Updated On: November 8th, 2009] [Originally Added On: November 8th, 2009]
- Galaxy Zoo [Last Updated On: November 8th, 2009] [Originally Added On: November 8th, 2009]
- .Astronomy 2009 Dates [Last Updated On: November 8th, 2009] [Originally Added On: November 8th, 2009]
- .Astronomy 2009: Programme and venue details [Last Updated On: November 8th, 2009] [Originally Added On: November 8th, 2009]
- .Astronomy Gets Some IYA Love [Last Updated On: November 8th, 2009] [Originally Added On: November 8th, 2009]
- 2009 Posters and Imagery [Last Updated On: November 8th, 2009] [Originally Added On: November 8th, 2009]
- 2009 Sponsors [Last Updated On: November 8th, 2009] [Originally Added On: November 8th, 2009]
- When in Holland… [Last Updated On: November 8th, 2009] [Originally Added On: November 8th, 2009]
- The WHAT Cloud? [Last Updated On: November 8th, 2009] [Originally Added On: November 8th, 2009]
- The Jewel Box [Last Updated On: November 8th, 2009] [Originally Added On: November 8th, 2009]
- Happy Halloween! [Last Updated On: November 8th, 2009] [Originally Added On: November 8th, 2009]
- Ares 1-X Launch [Last Updated On: November 8th, 2009] [Originally Added On: November 8th, 2009]
- Confessions of an Alien Hunter [Last Updated On: November 8th, 2009] [Originally Added On: November 8th, 2009]
- LRO Spies Apollo 17 Site [Last Updated On: November 8th, 2009] [Originally Added On: November 8th, 2009]
- Mercury in Color [Last Updated On: November 8th, 2009] [Originally Added On: November 8th, 2009]
- Hubble and M83 [Last Updated On: November 8th, 2009] [Originally Added On: November 8th, 2009]
- Cassini Flyby of Enceladus [Last Updated On: November 8th, 2009] [Originally Added On: November 8th, 2009]
- Preserving A Moth [Science Tattoo] | The Loom [Last Updated On: December 12th, 2009] [Originally Added On: December 12th, 2009]
- Another Russian rocket spiral lights up the sky | Bad Astronomy [Last Updated On: December 12th, 2009] [Originally Added On: December 12th, 2009]
- A (Very Gentle) Riddle to Complete Your Saturday [Last Updated On: December 12th, 2009] [Originally Added On: December 12th, 2009]
- Darwin Gets Swine Flu: The YouTube Edition | The Loom [Last Updated On: December 12th, 2009] [Originally Added On: December 12th, 2009]
- Happy Slothy Holidays | The Loom [Last Updated On: December 12th, 2009] [Originally Added On: December 12th, 2009]
- Jetting to Copenhagen | The Intersection [Last Updated On: December 12th, 2009] [Originally Added On: December 12th, 2009]
- Michael Gerson Attempts Thoughtfulness on “ClimateGate,” Then Gives it Up | The Intersection [Last Updated On: December 13th, 2009] [Originally Added On: December 13th, 2009]
- Incredible VISTA of the cosmos | Bad Astronomy [Last Updated On: December 13th, 2009] [Originally Added On: December 13th, 2009]
- Bundle up Sunday Night to Watch the Geminid Meteor Shower | 80beats [Last Updated On: December 13th, 2009] [Originally Added On: December 13th, 2009]
- “ClimateGate” a PR Disaster That Will Be “Taught in University Communications Courses” | The Intersection [Last Updated On: December 13th, 2009] [Originally Added On: December 13th, 2009]
- Is Google the Guardian Angel of Rainforests? | 80beats [Last Updated On: December 13th, 2009] [Originally Added On: December 13th, 2009]
- La ciencia es importante. Una vez mas. | Bad Astronomy [Last Updated On: December 13th, 2009] [Originally Added On: December 13th, 2009]
- Sensenbrenner Pulls an Inhofe, Asserts Global Warming is an “International Conspiracy” | The Intersection [Last Updated On: December 13th, 2009] [Originally Added On: December 13th, 2009]
- Let Kids Eat Dirt: Over-Cleanliness Linked to Heart Disease | 80beats [Last Updated On: December 13th, 2009] [Originally Added On: December 13th, 2009]
- What Are The Best Science Papers Of The Past Decade? | The Intersection [Last Updated On: December 13th, 2009] [Originally Added On: December 13th, 2009]
- Finally! Math Shows How to Cut Evenly Sized Pizza Slices | Discoblog [Last Updated On: December 13th, 2009] [Originally Added On: December 13th, 2009]
- Half-baked math | Bad Astronomy [Last Updated On: December 13th, 2009] [Originally Added On: December 13th, 2009]
- Can “Biological Passports” Save Sports From Doping? | 80beats [Last Updated On: December 13th, 2009] [Originally Added On: December 13th, 2009]
- Aiiiieeee! Slow down! | Bad Astronomy [Last Updated On: December 13th, 2009] [Originally Added On: December 13th, 2009]
- Weekly News Roundup: Bad Headlines, Martian moons, and Rotating Houses | Discoblog [Last Updated On: December 13th, 2009] [Originally Added On: December 13th, 2009]