Posted on 02/08/2005 3:50:43 AM PST by PatrickHenry
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.
A lion can breed with a tiger. What does that tell you?
Your "leap" is still a huge one. Dogs are still dogs--goats are still goats.
It'd be easier to work with insects--shorter life cycle, simpler creature.
Careful, your real motivations are showing...
"I think that pretty little artist's rendering (which has the horse in the wrong "swoop" of evolution, btw) ought to be recreated in stained glass and mounted on an altar to Hopeless Tenure Track and Our Lady of Perpetual Unemployability."
Good response!
A team looking for a new mascot? There are many mules in nature. I think, we once had this conversation. You said there was a mule that was not sterile, and I then inquired..."Who's his baby?"
I'm a dinosaur. My kids say so. ;)
re: dogs, wolf spp, coyotes etc.
Diminished fertility is a sign that speciation is not complete. There are ring species (seriously neat...adjacent neighbors can interbreed but when they reach the extreme ends of their range, overlapping populations can't breed.
>=can breed with X=can't breed with
a > b > c > d > e > f X a.
somewhere along the line they (let's say a and d) can breed but have fewer or weaker offspring.
That's similar to what goes on with wolves, dogs and coyotes. No one is going to seriously argue that a chihuahua male could impregnate a wolf female, or that a chihuahua female could either be impregnated by, or bear the offspring of, a wolf.
The biologists' definition of species is critters that don't interbreed in nature, but on the way to species is diminished fertility. That would explain the occasional hybrid in various canid populations.
The only way for more dinosaurs to come about would have been for other surviving reptiles, such as crocodiles, to evolve in that direction or for birds to regain their dino features. There was no environmental pressure pushing reptiles and birds back into the dinosaur direction.
Furthermore, the empty niches left by the extinct dinos were quickly filled by various other types of animal, such as mammals.
My "cylinder index" is currently at around 23 counting the 9.8hp Merc. that needs a tune-up to run. thanks for asking. shalom
How old are you, twelve?
It's a very easy thing to give an answer of yes or no. Is the wolf and the dog the same species?
By which definition of species?
(Hint: "Species" is a human classification. Nature makes no such distinctions.)
But yes, by most definitions of species, dogs and wolves are different species. Why do you ask (over and over and over again)?
If we agree that the wolf and the coyote are sufficiently different to be considered two distinct species, even though they are clearly closely related and quite similar animals both physically and tempermentally, and when wolves and coyotes interbreed they produce viable offspring who are not mules, then I would answer 'no,' the wolf and the dog are sufficiently different to be considered two distinct species.
Funny looking "gills":
Much of our increase in height is based on environmental issues, such as better nutrition, rather than genetic traits. We're not really getting genetically any taller, it's just that due to better nutrition and medical care, we are able to live up to our genetic potential these days when it comes to height.
As for how tall we can get as a species? In Earth's gravity, humans start to develop problems once they get up into the upper 6' range.
It is actually about a 12hp Merc. They put 9.8 on the label to get past state registration requirements.
This is where you lost all credibility.
But, there was no fly.
These are not scientists as much as they are priests. They don't present a reasonable theory, they insist I ascribe to it lest I be "ignorant".
I don't know how life began, and I don't credit these erstwhile Darwin-thumpers who claim to know, either. I have beliefs--but I don't call people "superstitious" or "ignorant" who don't happen to agree.
If this is science, it should be demonstrable and accountable--anything else is theorizing. Theorizing is OK--it's truth-seeking, but it's not truth itself.
"Naaa...just looking for evidence of something that is classified as both plant and animal."
At one time Slime Molds were thought to possibly be both. At least one group has their own kingdom now.
Fungi used to be classified as plants, now they are known to be closer to animals.
You didn't answer my question. Who is trying to prove that no gods exist?
Shoot. I was hoping for something more exciting like the Venus Fly-Trap.
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