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.
Yep, supposedly in humans. That seems to be reflected in the DNA, especially the mitochondrial DNA. However if there were only 20,000 individuals the number of possible alleles at any locus could be quite high.
It still refutes creationism.
OK, I'm backing out here, not being an expert. But by your definition every sexually reproducing individual is a hybrid.
I didn't say differing chromosome count is not a barrier. I said it is not an absolute barrier.
In a way, yes.
Most one celled critters don't have paired chromosomes the way the plants and animals that we are familiar with do. They are called "haploid" while the others are called "diploid"
The exchange or insertion of genetic information among haploid critters has far more impact than among diploids.
The redundancy of extra sets has a stablizing effect.
Just to make matters more confusing, especially among plants, there can be sets of four or more of each chromosome.
So why are the extremely long odds of evolution from chimp to human not as ridiculous as the supposed long odds of a bunch of modern animals being infected by the same virus?
That's a (very weak) argument refuting abiogenesis, not evolution. God could have created the first cell directly, it could have formed from a natural process, it could have come from outer space or it could have arisen in some way I can't imagine, and it would have no impact whatsoever on the evidence in favor of evolution. Evolution deals only with what happened after the first living organism was formed. It makes no statements about where that organisms came from originally.
For many that's the point of the debate -- you believe, or want to believe, that the existence of God is impossible so you want that established as conventional wisdom.
if we did uncover it then it would definitely not contravene the scientific underpinnings of evolution in the slightest
Well, it would actually since the modern thesis demands a common ancestor. Now, understand, I'm not saying that evolution isn't responsible for a great deal of biodiversity -- just that in no way does it explain everything and there are those who insist that it does to the point of silliness.
What evidence would it take for you to believe in God?
Please learn some science before you attempt to debate scientific questions. Precisely which theories, laws, etc. in physics, engineering or chemistry have been proven? Answer: None of them. Science can't prove anything. For example, most people think that gravity has been "proven." It has not. To prove the law of gravity, which is more correctly called the UNIVERSAL law of gravity, you would have to measure the attractive force between EVERY pair of objects in the entire universe. This is clearly impossible, so we can't say that this law is proven. What we can say is that there is a tremendous amount of evidence in favor of the law of gravity (in limited circumstances, relativity showed that the law of gravity is incomplete). It is not impossible that we could measure an attractive force between two objects that is NOT correctly described by the law of gravity. The same situation is applicable to EVERY law, theory or hypothesis in all of science.
But by your definition every sexually reproducing individual is a hybrid.
There would probably be a number of hybrid loci in every individual. However, it tends to lose meaning in nature.
Hybrids are generally used in selective breeding. For instance, pure strains of corn are bred to produce hybrid vigor and higher crop yields. If you plant the seeds produced from hybrids, your results will not be good.
Do you have a link to the haploid/diploid information on single celled organisms?
If my memory is correct, haploid is meiosis (reproductive cells in sexually reproducing organisms) and diploid are somatic cells.
I don't doubt that, in most cases. But at some point in evolution there must be discontinuities in chromosome count. If these are always absolute barriers to reproduction, there is a problem.
No biologist claims chimps turned into humans.
The virus question is very confusing.
I think you have been reading AIG or ICR sites for your information.
The issue is not infection by a virus; it is having viral genes inserted into a genome. Huge difference.
If you stake your faith on the absence of abiogenesis, you might end up like the flat earthers.
Just make up your mind that God did it and we may never fully understand how. Otherwise, you may find that the mustard seed rotted.
Fred.
Or is that another yet to be discovered missing link?
Here's what the article is about:
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.It is basically about connecting hippos to anthracotheres, heretofore considered a weak link. This page, for instance, cites difficulties making the connection.
In many, perhaps most, phylogenies, this taxon includes the Hippopotamidae. Early anthracotheroids were small. See the image of Anthracobunodon below. Later anthracotheroids were indisputably large and generally hippo-like in aquatic habit and adaptations [PS02]. Traditionally, the story was also told that the decline of the anthracotheres was closely correlated with the rise of the hippos, who, in turn were marginalized by bovids of various sorts [C88]. However, in critical details, the last anthracotheres were not very similar to the first hippos, and the timing no longer seems suggestive. Indeed, the entire clade disappeared too soon to have produced the first known hippo, Kenyapotamus [PS02] [????]. ATW040724.The study cited in this article answers that. It shows previously unrecognized characters linking the hippo and anthracotheres clades. Arguing from gaps is always a lousy idea.
Anyway, connecting hippos to anthracotheres leaves hippos diverging from something like this thing.
Meanwhile, the earliest "whale" looks like this.
Illustration by Carl Buell, and taken from http://www.neoucom.edu/Depts/Anat/Thewissen/whale_origins/whales/Pakicetid.html.
(So one's a little livelier than the other, but less transparent.) So your answer is, "Where's the thing bridging the gap?"
Even if we discover a possible path for abiogenesis, we are unlikely to know the actual historical path.
"If these are always absolute barriers to reproduction, there is a problem."
Let's say you had an insect whose chromosomes got "stuck". It produced 1000 eggs. All of a sudden you would have a new species with a thousand individuals.
For a Christian, constant quibbling over science is a dangerous game. Everytime science has explained something, we create more atheists, not do to the science but the silly arguments by Christian sects against science.
Mostly we keep getting the same false science posted from the same creationists' websites.
Look, I'm not a creationist pulling your chain. Most of the species that inflame the debate are mammals. The question is, how does the shift in chromosome count occur, and does it prevent cross mating. I have seen suggestions that humans and chimps could probably mate, despite differing chromosome count.
There are quite a few humans with anomolous chromosome counts, not all of which are retarded or sterile.
Not surprising.
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