Posted on 05/23/2006 4:08:38 AM PDT by PatrickHenry
When ancient whales finally parted company with the last remnants of their legs about 35 million years ago, a relatively sudden genetic event may have crowned an eons-long shrinking process.
An international group of scientists led by Hans Thewissen, Ph.D., a professor of anatomy at Northeastern Ohio Universities College of Medicine, has used developmental data from contemporary spotted dolphins and fossils of ancient whales to try to pinpoint the genetic changes that could have caused whales, dolphins and porpoises to lose their hind limbs.
More than 50 million years ago the ancestors of whales and dolphins were four-footed land animals, not unlike large dogs. They became the sleek swimmers we recognize today during the next 15 million years, losing their hind limbs in a dramatic example of evolutionary change.
"We can see from fossils that whales clearly lived on land - they actually share a common ancestor with hippos, camels and deer," said team member Martin Cohn, Ph.D., a developmental biologist and associate professor with the UF departments of zoology and anatomy and cell biology and a member of the UF Genetics Institute. "Their transition to an aquatic lifestyle occurred long before they eliminated their hind limbs. During the transition, their limbs became smaller, but they kept the same number and arrangement of hind limb bones as their terrestrial ancestors."
In findings to be published this week in the Proceedings of the National Academy of Sciences, scientists say the gradual shrinkage of the whales' hind limbs over 15 million years was the result of slowly accumulated genetic changes that influenced the size of the limbs and that these changes happened sometime late in development, during the fetal period.
However, the actual loss of the hind limb occurred much further along in the evolutionary process, when a drastic change occurred to inactivate a gene essential for limb development. This gene - called Sonic hedgehog - functions during the first quarter of gestation in the embryonic period of the animals' development, before the fetal period.
In all limbed vertebrates, Sonic hedgehog is required for normal limbs to develop beyond the knee and elbow joints. Because ancient whales' hind limbs remained perfectly formed all the way to the toes even as they became smaller suggests that Sonic hedgehog was still functioning to pattern the limb skeleton.
The new research shows that, near the end of 15 million years, with the hind limbs of ancient whales nonfunctional and all but gone, lack of Sonic hedgehog clearly comes into play. While the animals still may have developed embryonic hind limb buds, as happens in today's spotted dolphins, they didn't have the Sonic hedgehog required to grow a complete or even partial limb, although it is active elsewhere in the embryo.
The team also showed why Sonic hedgehog became inactive and all traces of hind limbs vanished at the end of this stage of whale evolution, said Cohn. A gene called Hand2, which normally functions as a switch to turn on Sonic hedgehog, was shown to be inactive in the hind limb buds of dolphins. Without it, limb development grinds to a halt.
"By integrating data from fossils with developmental data from embryonic dolphins, we were able to trace these genetic changes to the point in time when they happened," Thewissen said.
"Studies on swimming in mammals show that a sleek body is necessary for efficient swimming, because projecting organs such as rudimentary hind limbs cause a lot of drag, and slow a swimmer down," said Thewissen, who spends about a month every year in Pakistan and India collecting fossils that document the land-to-water transition of whales.
Researchers say the findings tend to support traditional evolutionary theory, a la Charles Darwin, that says minor changes over vast expanses of time add up to big changes. And while Sonic hedgehog's role in the evolution of hind limbs in ancient whales is becoming apparent, it is still not fully defined.
"It's clear when ancient whales lost all vestiges of the limb it was probably triggered by loss of Sonic hedgehog," said Clifford Tabin, Ph.D., a professor of genetics at Harvard Medical School who was not involved in the research. "But it's hard to say for certain because you're looking at events long after they occurred. As they suggest, there could have been a continual decrease in Sonic as the limbs reduced until the modern version of the animal arrived."
The study itself, combining fossil and developmental data, is notable, Tabin said.
"Whales went through this remarkable transformation to become more like the ancestral fish," Tabin said. "Convergence of evolutionary studies and developmental genetics give us another piece in this growing tapestry of how genetic changes lead to morphological change. It is a remarkable process that was achieved simply and led to profound consequences in how whales were able to survive. Only now in the last five years are we developing this understanding of how the world of evolution is controlled genetically."
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In addition to UF and Northeastern Ohio Universities College of Medicine, scientists from the Natural History Museum of Los Angeles County and the Indian Institute of Technology were involved in the research. Financial support was provided by the National Science Foundation, the National Institutes of Health and the Indian Department of Science and Technology, New Delhi.
Good points!
<< Evolution implies you gotta die and evolve, die and evolve, die and evolve, die and evolve... >>
Not quite. You either reproduce or die before you can. And individuals do not evolve -- populations do.
If you don't understand that statement, you have no basis upon which to argue against evolution. The first step in refuting something is to understand what it is you are refuting. Making stupid statements that have no relation to the facts does not refute anything -- it just makes you look ignorant.
Fortunately -- ignorance, unlike stupidity, can be cured.
Soup myth.
Star Trek V was better.
<< And the Evos posting their beliefs aren't emotional? >>
Oh, they use emotions, just as all do. They are passionate about some things -- just as others are. But they are not basing their beliefs -- in this area -- on their feelings. As shown in the massive evidence they reference -- they are basing their beliefs about evolution on extremely well-supported conclusions derived from the real world -- not from feelings.
Your comments about basing your religious beliefs on emotions are just more evidence to support what we are saying -- that you are refusing to look at evidence because your feelings will not allow you to consider it. You erroneously believe that if you were to accept the evidence, you would have to overthrow your belief in God.
That is not necessary. A large percentage of those who accept the scientific evidence for common descent are religious people. What you cannot do is accept that evidence and continue to hold to a literalistic interpretation of the beginning of Genesis.
This is why I asked you -- and you have ignored my question: If I were to demonstrate for you that the evidence supporting common descent is overwhelming, and that your characterization of the theory of evolution is erroneous -- what would you do?
Why not just answer that simple question?
"Fortunately -- ignorance, unlike stupidity, can be cured."
I don't know, Mr Smart@ss, you tell me? How many hours in paleontology do you have? I have a few. Your argument sure is compelling.
Then where'd the apes come from? I know quite about the THESIS. You're right, I know nothing about the theory, because it doesn't qualify as one.
<< A scientific fact requires observation. Like I said in another post, you can't test or observe non-life creating life, transitional forms, or primitive organisms, to name a few. >>
Is it a scientific fact that Pluto has been circling the Sun for at least several thousand years? The fact is that Pluto has not been observed to have circled the Sun even once! So -- is it not a fact?
Life from non-life is not part of the theory of common descent -- so it has nothing to do with this discussion. Whether life came about as an act of God or not is not something that evolution speaks to.
Transitional forms have been observed; speciation has been observed, both in the lab and in nature. If you think speciation has not occurred, then answer this simple question: why can't domestic wheat inter-breed with emmer grass? If you don't understand the question, you are not ready to refute anything about transitional forms.
And primitive organisms have been observed. What an odd statement.
Where'd the non-ape primates come from? CrackJack box?
Yeah, it's a THESIS. If you weren't ignorant, you'd know the difference! LOL
"Fortunately -- ignorance, unlike stupidity, can be cured."
<< I don't know, Mr Smart@ss, you tell me? How many hours in paleontology do you have? I have a few. Your argument sure is compelling. >>
Not trying to be a smart-ass. It's just that your statements give clear evidence that you have no idea what evolution is about. The other alternative is that you do know, but you are pretending to be ignorant for some reason. I just assumed you were being honest.
How many hours have I had in paleontology? You mean in college classes? None. Most of my science hours were in biology and physics, before I made the mistake of switching to humanities. I have put in many thousands of hours since then, studying books and papers written by paleontologists, evolutionary biologists, geologists, and astronomers.
I'm not too good in chemistry, though -- so you can easily stump me there. If I were to start arguing about chemistry -- and say some off-the-wall things about chemistry, the way you have done about evolution -- and if you were knowledgeable in chemistry -- you would have every right to tell ME that MY ignorance is curable.
That is -- if I'm not just stupid! LOL! Like I said -- that ain't curable.
Assuming that you have had many more hours in paleontology than I -- that makes me wonder, then, about my assumption that you were honestly misunderstanding things. It's hard for me to believe that someone can have a lot of hours in paleontology -- even if he doesn't believe what he was taught there -- and say the things you are saying.
Something's not right here.
To be honest with you, I've not read any study that is compelling enough for me to believe this is any different than believing in UFO abductions, pixies and brownies hiding in tress. I really don't have any trouble if it's factual, but I haven't seen anything that makes me "buy into it".
I'm kinda clowning around with ya--sorry. But if you boil it down, it amounts to "die and evolve", especially if you ascribe to the idea of natural selection. Because at one point the thought was adapting to an environment that promoted the species through it's offspring. Mutations, if you will.
Maybe I was spending too much time playing with bananas.
<< But if you boil it down, it amounts to "die and evolve", especially if you ascribe to the idea of natural selection. >>
Even creationists accept natural selection. They even accept "evolution" within "kinds" -- even though they never can seem to tell us what "kinds" are, or what the barrier is that keeps the "evolution" from going further.
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