"40 Quatloos on the corporeal being"
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.
Literary version: "Why, sometimes I've believed as many as six impossible things before breakfast."That's a keeper!Charles Lutwidge Dodgson, Alice in Wonderland
Creationist version:
"Why, sometimes I've declared as many as six things impossible before breakfast."
An excellent observation. GarySpFc has managed to engage in at least five and possibly six logical fallacies in a single sentence. That is quite an accomplishment.
I always thought it was:
"Everything is possible when you don't understand how anything happens."
Wilma the Whale & the SS Guppy. (I googled).
Yep.
My brother is a biology major, and he's in the same camp. He gets to shake his head sadly on a regular basis. :)
<< GarySpFc has managed to engage in at least five and possibly six logical fallacies in a single sentence. That is quite an accomplishment. >>
And I am sure he's proud of it, too. After all -- "God's thoughts are higher than our thoughts." We are just using "man made reasoning." I heard that same crap endlessly even when I WAS a believer -- every time I noticed a ridiculous logical fallacy and had the temerity to say so.
but you can run a car on water, right? :P
(Sorry! I couldn't help it!)
Yup. There have been "throwbacks".
Are there metrics to determine a certain level of change over time? For example, the shortening of limbs or the fusing of toes into a flipper? I am curious about the rate of change that took place for certain transitions.
I agree. The holy rollers use "athiest" as an insult (since it is the only way they can convonce themselves they are "superior").
I always feel sorry for the inferiority complex these folk carry around on their shoulder.
Just look for the guy ordering the duck with mango salsa.
"40 Quatloos on the corporeal being"
I am going to helpfully preempt any "In the Year 2525" references...
Let's be fair, however. They also quite often (with many recent examples in these threads) us "religious" and "religion" as an insult, in conjunction with claiming that evolution is "a religion".
Interestingly, in doing so, and in many allied and similar arguments, they are adapting almost wholesale the philosophical presuppositions of the most superficial and aggressive sort of "scientific atheism": Principally the notion that the Bible should be evaluated by the standards of secular science, and must be literally true by those standards, else it (and any devotional system derived from it) logically should be discounted and abandoned entirely. Sure, they reach a different conclusion, but at the same time concede and validate the core logic of the most caustic anti-religious bigotry!
It's quite a stunning performance, both for smug and bilious cynicism, and astonishing self-destructiveness.
Good point: "Atheist" is an epithet, as is "religious."
And the contradictory criteria are, indeed, the icing on the cake.
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