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.
many thanks!
yes - the variable set is infinite
both dogs are german shepherds. the white beastie is not mine, she's an occasional day-guest.
Not really. That is certainly one of the considerations but isn't really the determining factor. Ring species throw this idea out the window - group A & B can interbreed, group B & C can interbreed, but group A and C can't interbreed. Which groups belong to the same species and which ones don't?
Evolution predicts that the concept of species will be murky and not concrete which is exactly what we observe.
The species concept is just a cataloging exercise that we as human use to classify life forms. Its very imperfect.
Did you ever address Chinese history and why their written documents on that time are somewhat lacking in describing how they were entirely wiped from the face of the earth?
Go to the photo and right click to get the URL address. Copy and paste inbetween the " ".
Not King Prout, but I live about 15 minutes from Lake Lanier in northern Atlanta suburb. Haven't seen any wolves but do see lots of possums! :-)
last time I checked, genetic surveys of mitochondrial DNA does not support the existence of a bottleneck -let alone a singularity- as recent as a mere 4,000 years ago.
care to explain?
ah, way up there on 400, yes?
the great skypixie waved his magic wand and hey-presto!
We call them targets.
you're playin' ketchup?
That's what the ancients said about the moon's path when they could not explain it aberrant behavior. Now we know better.
Last I checked, Mitochondrial DNA studies confirmed that thee was a single woman we came from, too
Care to explain that?
Remember, as an Evolutionist, you cant use GENESIS.
No. Based on past history we would end all progress in science and denigrate to a drug infested society with death the sentence to all "non-believers".
Yes. I check the replies to his question first but I didn't drive down to the replies to the replies.
Based on history (note that it's always in the past) the greatest advances in science and education have been in Christian countries.
But NOT when the fanatical Christians were in charge; that was the condition under discussion.
BTW, Christians couldn't even come up with a decent system for counting upon which all modern advances are based.
Sorry: you checked wrong.
It is a great pity if you are foolish enough to consider Discovery Channel the epitome of genetic science.
a clue for you: Discovery Channel has the execrable habit of making oversimplified overstatements going far beyond what the data will support, purely in the hunt for ratings.
the remains of a woman were found in north africa.
the remains were dated by measuring carbon radioisotope levels. If I recall correctly, the age of the remains was found to be around 20,000 years (that's a bit of an if: been a while since I reviewed the data on this specimen, and my memory is not perfect... however, I am quite certain the date was significantly older than a mere 6,000 years).
the bones contained some mitochondrial DNA fragments.
Some of those fragments are found in the mitochondrial DNA in all modern humans.
now, do you realise that it is entirely possible that this so-called "Eve" had NOT ONE DESCENDANT?
think about it.
HINT1: inheritance of mitochondria is entirely matrilineal
HINT2: mitochondrial DNA mutates very slowly
HINT3: "Eve's" great-great-great-great-great-great-grandmother had the same mitochondrial DNA that "Eve" herself did.
Think about it.
I mean, really, for once on this thread, THINK.
No, young-Earth Bible believing Christians was what was being referred to. They started most of our (and the world's) great universities you know.
BTW, Christians couldn't even come up with a decent system for counting upon which all modern advances are based.
Better spell out what you are referring to here. Arabic numerals?
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