Scientists find missing link between whale and its closest relative, the hippo

A group of four-footed mammals that flourished worldwide for 40 million years and then died out in the ice ages is the missing link between the whale and its not-so-obvious nearest relative, the hippopotamus.

The conclusion by University of California, Berkeley, post-doctoral fellow Jean-Renaud Boisserie and his French colleagues finally puts to rest the long-standing notion that the hippo is actually related to the pig or to its close relative, the South American peccary. In doing so, the finding reconciles the fossil record with the 20-year-old claim that molecular evidence points to the whale as the closest relative of the hippo.

"The problem with hippos is, if you look at the general shape of the animal it could be related to horses, as the ancient Greeks thought, or pigs, as modern scientists thought, while molecular phylogeny shows a close relationship with whales," said Boisserie. "But cetaceans – whales, porpoises and dolphins – don't look anything like hippos. There is a 40-million-year gap between fossils of early cetaceans and early hippos."

In a paper appearing this week in the Online Early Edition of the Proceedings of the National Academy of Sciences, Boisserie and colleagues Michel Brunet and Fabrice Lihoreau fill in this gap by proposing that whales and hippos had a common water-loving ancestor 50 to 60 million years ago that evolved and split into two groups: the early cetaceans, which eventually spurned land altogether and became totally aquatic; and a large and diverse group of four-legged beasts called anthracotheres. The pig-like anthracotheres, which blossomed over a 40-million-year period into at least 37 distinct genera on all continents except Oceania and South America, died out less than 2 and a half million years ago, leaving only one descendent: the hippopotamus.

This proposal places whales squarely within the large group of cloven-hoofed mammals (even-toed ungulates) known collectively as the Artiodactyla – the group that includes cows, pigs, sheep, antelopes, camels, giraffes and most of the large land animals. Rather than separating whales from the rest of the mammals, the new study supports a 1997 proposal to place the legless whales and dolphins together with the cloven-hoofed mammals in a group named Cetartiodactyla.

"Our study shows that these groups are not as unrelated as thought by morphologists," Boisserie said, referring to scientists who classify organisms based on their physical characteristics or morphology. "Cetaceans are artiodactyls, but very derived artiodactyls."

The origin of hippos has been debated vociferously for nearly 200 years, ever since the animals were rediscovered by pioneering French paleontologist Georges Cuvier and others. Their conclusion that hippos are closely related to pigs and peccaries was based primarily on their interpretation of the ridges on the molars of these species, Boisserie said.

"In this particular case, you can't really rely on the dentition, however," Boisserie said. "Teeth are the best preserved and most numerous fossils, and analysis of teeth is very important in paleontology, but they are subject to lots of environmental processes and can quickly adapt to the outside world. So, most characteristics are not dependable indications of relationships between major groups of mammals. Teeth are not as reliable as people thought."

As scientists found more fossils of early hippos and anthracotheres, a competing hypothesis roiled the waters: that hippos are descendents of the anthracotheres.

All this was thrown into disarray in 1985 when UC Berkeley's Vincent Sarich, a pioneer of the field of molecular evolution and now a professor emeritus of anthropology, analyzed blood proteins and saw a close relationship between hippos and whales. A subsequent analysis of mitochondrial, nuclear and ribosomal DNA only solidified this relationship.

Though most biologists now agree that whales and hippos are first cousins, they continue to clash over how whales and hippos are related, and where they belong within the even-toed ungulates, the artiodactyls. A major roadblock to linking whales with hippos was the lack of any fossils that appeared intermediate between the two. In fact, it was a bit embarrassing for paleontologists because the claimed link between the two would mean that one of the major radiations of mammals – the one that led to cetaceans, which represent the most successful re-adaptation to life in water – had an origin deeply nested within the artiodactyls, and that morphologists had failed to recognize it.

This new analysis finally brings the fossil evidence into accord with the molecular data, showing that whales and hippos indeed are one another's closest relatives.

"This work provides another important step for the reconciliation between molecular- and morphology-based phylogenies, and indicates new tracks for research on emergence of cetaceans," Boisserie said.

Boisserie became a hippo specialist while digging with Brunet for early human ancestors in the African republic of Chad. Most hominid fossils earlier than about 2 million years ago are found in association with hippo fossils, implying that they lived in the same biotopes and that hippos later became a source of food for our distant ancestors. Hippos first developed in Africa 16 million years ago and exploded in number around 8 million years ago, Boisserie said.

Now a post-doctoral fellow in the Human Evolution Research Center run by integrative biology professor Tim White at UC Berkeley, Boisserie decided to attempt a resolution of the conflict between the molecular data and the fossil record. New whale fossils discovered in Pakistan in 2001, some of which have limb characteristics similar to artiodactyls, drew a more certain link between whales and artiodactyls. Boisserie and his colleagues conducted a phylogenetic analysis of new and previous hippo, whale and anthracothere fossils and were able to argue persuasively that anthracotheres are the missing link between hippos and cetaceans.

While the common ancestor of cetaceans and anthracotheres probably wasn't fully aquatic, it likely lived around water, he said. And while many anthracotheres appear to have been adapted to life in water, all of the youngest fossils of anthracotheres, hippos and cetaceans are aquatic or semi-aquatic.

"Our study is the most complete to date, including lots of different taxa and a lot of new characteristics," Boisserie said. "Our results are very robust and a good alternative to our findings is still to be formulated."

Brunet is associated with the Laboratoire de Géobiologie, Biochronologie et Paléontologie Humaine at the Université de Poitiers and with the Collège de France in Paris. Lihoreau is a post-doctoral fellow in the Département de Paléontologie of the Université de N'Djaména in Chad.

The work was supported in part by the Mission Paléoanthropologique Franco-Tchadienne, which is co-directed by Brunet and Patrick Vignaud of the Université de Poitiers, and in part by funds to Boisserie from the Fondation Fyssen, the French Ministère des Affaires Etrangères and the National Science Foundation's Revealing Hominid Origins Initiative, which is co-directed by Tim White and Clark Howell of UC Berkeley.

Narby: I've noticed that creationists and moon shot doubters use the same tactic. They pick at endless nits and imagined "holes" in the theory that we went to the moon, or that Evolution is true.

garybob: Refute my post #664 then.

You're kidding, right? Your post #664 is nothing but picking at nits and imagined "holes", and you want me to refute it?

Apparently you don't get it. You have no affirmative evidence. You are merely trying to cast doubt on science. Give me some positive evidence that points to an Intellegent Designer. God's signature in the cement. Something. Anything. (hint: a reply of "the fact that we exist is evidence" will not work. That's a philosophic argument suited for religion class, not science)

It's always possible to cast doubt on anything. Even reality itself.

Perhaps I don't even exist at all. Perhaps I'm only a reply-bot. Face it, you don't really KNOW that I'm actually a real person.

It's easy to create doubt, particularly among those ignorant of the subject.

Species A gives rise to species B1 and B2.

This is where I fall off the train. Could you please explain how species 'A' gives rise to species 'B1' and 'B2'?

Please note that I'm not talking about survival of the fittest, or natural selection. I accept natural selection as scientific fact.

As I said earlier, the question is not whether natural selection occurs. Of course it does, and it has an effect in maintaining the genetic fitness of a population. Creatures with severe birth defects do not survive to maturity and creatures which do not survive to reproduce do not leave descendants. These effects are unquestioned, but your claim asserts a great deal more than the fact that species avoid genetic deterioration due to natural attrition among the genetically unfit. You're basically claiming that this same force of attrition has a building effect so powerful that it can begin with Species 'A' and gradually craft its descendants over billions of years to produce such wonders as trees, flowers, ants, birds, and humans.

When Darwin wrote “The Origin of Species”, he could offer no good cases of natural selection because no one had looked for them. He drew instead an analogy with the artificial selection that animal and plant breeders use to improve domesticated varieties of animals and plants. By breeding only from the woolliest sheep, the most fertile chickens, and so on, breeders have been spectacularly successful in altering almost every imaginable characteristic of our domesticated animals and plants to the point where most of them differ from their wild ancestors far more than related species differ from them. This explanation is misleading because artificial selection is fundamentally different than natural selection. Human breeders produce variations among sheep or pigeons for purposes absent in nature, including sheer delight in seeing how much variation can be achieved. If the breeders were acting in the same manner as natural selection, which is interested only in having animals survive in the wild and be able to reproduce, then the extreme variation would not exist. When domesticated animals return to the wild, the most highly specialized breeds quickly perish and the survivors revert to the original wild type.

What artificial selection actually shows is that there are definite limits to the amount of variation that even the most highly skilled breeders can achieve. Breeding of domesticated animals has produced no new species, in the commonly accepted sense of new breeding communities that are infertile when crossed with the parent group. For example, all dogs form a single species because they are chemically capable of interbreeding, although inequality of size in some cases makes natural copulation impracticable. The fact is that selection gives tangible form to and gathers together all the varieties a genome is capable of producing, but it does not constitute an innovative evolutionary process. In other words, the reason that dogs don’t become as big as elephants, much less change into elephants, is not that we just haven’t been breeding them long enough. Dogs do not have the genetic capacity for that degree of change, and they stop getting bigger when the genetic limit is reached.

I’m simply saying that these questions that remain unanswered by science. I ask you again, what proof do we have that a species can make a dramatic change into a completely different species over any period of time, no matter how long? Have scientist provided proof that what they’ve been telling us is true, or most just believe it because they said so? That question seems so elementary and it often angers people, but I still beg the answer.

You guys possess an understanding of science that reaches far beyond what little I know. But one of the things that I admire about science is that it's built upon that wonderful foundation referred to as the "scientific method". The great advantage of the scientific method is that it is unprejudiced: one does not have to believe a given researcher, one can redo the experiment and determine whether his/her results are true or false. The results are not subject to opinion, a consensus, or verified by a vote. The conclusions will hold irrespective of the state of mind, or the religious persuasion, or the state of consciousness of the investigator and/or the subject of the investigation - and science doesn't make moral judgments.

How does one test the theory of evolution? There's no absolute way to objectivity test the assertions of creation or evolution. There aren't any eyewitnesses. Both ideas are left to propose a model and then compare it with nature for consistency. Can evolution be proved false? What part of the theory of evolution is open to invalidation, some small detail, or the entire principle?

How can you honestly subject evolution to the scientific method and consider it anything more than a theory?

I asked for the proof that evolution is a fact but you are compelled to jump in and insult me. Your opinion is not what I asked for.

Moon shot doubters and creationists are two sides of the same coin. They are both people who didn't work in the relevant field, but think they know enough to doubt the people who do know what they talking about.

I'm just amazed at the number of Christians arrogant to think that they know how God created the universe by simply reading a few hundred words in Genesis.

God created the universe, and He gave us the Bible. Yet you're willing to call His creation a "lie", but His word "truth". It is nonsensical to believe that there is a contradiction between the evidence of fossils and microbiology created by God and the Word of God. The contradiction is merely in your understanding, not in the evidence itself.

There can be no conflict between Genesis and science, except in the arguments of men.

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