"Long before it's in the papers"
December 20, 2007

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Whales evolved from raccoon-sized creature, researchers say

Dec. 20, 2007
World Science staff

Whales, dol­phins and por­pois­es—known col­lec­tively as cetacean­s—are be­lieved to have evolved from land mam­mals around 50 mil­lion years ago. But who pre­cisely their an­ces­tor was, is a mys­tery.

In this artist's con­cep­tion, In­do­hyus is seen div­ing in a stream, much as the mod­ern Af­ri­can Mouse­deer does when in dan­ger. (Cour­te­sy Carl Bu­ell)
New re­search sug­gests it was a raccoon-sized, hoofed mam­mal from In­dia known as a raoel­lid, which probably took to wa­ter in times of dan­ger, sci­en­tists say. The stu­dy, by J.G.M. “Hans” The­wis­sen of North­east­ern Ohio Uni­ver­s­i­ties Col­lege of Med­i­cine, ap­pears in this week’s is­sue of the re­search jour­nal Na­ture.

Sci­en­tists since Dar­win’s time have known that whales are mam­mals whose an­ces­tors walked on land. Thewis­sen and col­leagues iden­ti­fied a se­ries of in­ter­me­diate fos­sils doc­u­ment­ing whales’ dra­mat­ic ev­o­lu­tion­ary tran­si­tion from land to sea. 

The key piece of the puz­zle fell in­to place, they said, with the dis­cov­ery of fos­sils of In­do­hyus, a roughly 48-mil­lion-year-old raoel­lid from In­di­a’s Kash­mir re­gion. In­do­hyus emerged as whales’ clos­est known fos­sil rel­a­tive, the re­search­ers said.

The team stud­ied a lay­er of mud­stone with hun­dreds of bones of In­do­hyus, thought to have looked some­thing like a min­ia­ture deer. The struc­tures of the skull and ear re­gion of raoel­lids are very si­m­i­lar to those of early whales, the in­ves­ti­ga­tors found. More­o­ver, they wrote, their bone thick­ness and chem­i­cal ev­i­dence in­di­cate these crea­tures spent much time in wa­ter. 

Raoel­lids, how­ev­er, were mainly plant-eaters on land, so the spur for whale an­ces­tors to take to the wa­ter was probably an abun­dance of aquat­ic prey, ac­cord­ing to the re­search team. In­de­pend­ent mo­lec­u­lar ev­i­dence points to hip­pos as the clos­est rel­a­tives of to­day’s whales, but hip­pos don’t ap­pear in the fos­sil rec­ord un­til some 35 mil­lion years af­ter whales branched off from their land an­ces­tors.

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Whales, dolphins and porpoises—known collectively as cetaceans—are believed to have evolved from land mammals around 50 million years ago. But who precisely their ancestor was, is a mystery. New research suggests it was a raccoon-sized, hoofed animal from India known as a raoellid, which probably took to water in times of danger, scientists say. The study, by J.G.M. “Hans” Thewissen of Northeastern Ohio Universities College of Medicine, appears in this week’s issue of the research journal Nature. Scientists since Darwin’s time have known that whales are mammals whose ancestors walked on land. Thewissen and colleagues identified a series of intermediate fossils documenting whales’ dramatic evolutionary transition from land to sea. The key piece of the puzzle fell into place, they said, with the discovery of fossils of Indohyus, a roughly 48-million-year-old raoellid from India’s Kashmir region. Indohyus emerged as whales’ closest known fossil relative, the researchers said. The team studied a layer of mudstone with hundreds of bones of Indohyus, thought to have looked something like a miniature deer. The structures of the skull and ear region of raoellids are very similar to those of early whales, the investigators found. Moreover, they wrote, their bone thickness and chemical evidence indicate these creatures spent much time in water. Raoellids, however, were mainly plant-eaters on land, so the spur for whale ancestors to take to the water was probably an abundance of aquatic prey, according to the research team. Independent molecular evidence points to hippos as the closest relatives of today’s whales, but hippos don’t appear in the fossil record until some 35 million years after whales branched off from their terrestrial ancestors.