I was happy to see that my post on Tuesday about the evolution of whales attracted a lot of readers. One commenter asked about seals and manatees. As other commenters kindly explained, those mammals descend from other ancestors (relatives of bears and elephants, respectively) that independently moved into the water. This transition has occurred many times since vertebrates moved on land. In some cases, the animals have adapted completely to the water (such as marine reptiles). In other cases, the transition has not been so complete. Other relatives of elephants evolved into desmostylians, sometimes called "sea-bears." They looked like polar bears but fed on aquatic plants. (pdfs here and here) The picture here is of Paleoparadoxia, from Siberia.
And then there were the aquatic sloths. Yup--sloths. Here's a post I wrote on their transition to the sea.
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Bears? Cool! So Darwin was almost right!
PS - I just finished Soul Made Flesh, and it made me go to Westminster Abbey when I was in London so I could see Willis' tomb. It's a wonderful book - thanks for writing it.
Here's a couple of peculiar questions...
1) How much did transition to and from water have to do with the development... loss of... and re-development of legs in snakes? (Including leg-less amphibians). What other environmental factor beside water that can cause the loss of legs?
2) Most snake species are living on land, and have been for millions of years.. but most have not re-developed legs. Of course, many species spend much of their life between land and water, which likely inhibits re-development... though, take for instance the Sidewinder, why no legs [yet]? Yes, I understand a little about how Darwinism works, the creature already has developed physical adaptations which give it advantages in its eco-system, that may be superior to say, legs...
[My question is an exception to the Python, which already has lovely little hind spurs].
It seems the fossil record tells us, terrestrial snakes are overdue for some legs sometime in the future??
". . . The fossil Haasiophis was about 3 feet long and also had legs, but its jaw structure suggested to Tchernov et al. that it was more closely related to the larger, living snakes of today and that both Pachyrhachis and Haasiophis were not primitive at all, but advanced snakes that had re-evolved legs. (Many living pythons retain rudimentary hind limbs, so re-evolving limbs is a possibility.) The limbs of Pachyrhachis and Haasiophis are too small in relation to body size to have had any locomotor function; they may have been used as an aid in mating, as are the hind limb buds of pythons today."
Why should an animal with legs evolve into one without them? Legs seem to be quite a useful bit of equipment; every other terrestrial vertebrae has them. Only the fishes are legless; the whales, to adapt to the fishes watery environment, have lost their hind limbs completely and their forelimbs have evolved into paddles. Perhaps the sea snakes are a more advanced form of snake, for only in aquatic animals does leglessness confer an advantage -- or at least, it does not handicap the animal. . .
-Excerpts, Richard Ellis, AquaGenesis
"Worldwide there are about 2,700 different species of snakes, very few of which are marine animals. "And so we are back to not knowing what kind of an origin snakes had," Rieppel said. Dr. Harry Greene, professor of ecology and evolutionary biology at Cornell University, said the most intriguing aspect about the West Bank fossils is they may show that certain "atavistic" traits can re-evolve if the right genes are triggered. The West Bank fossils may be snakes whose limbs re-evolved, making them "real snakes, just extinct real snakes" with legs, Greene said. Greene postulates that if animals like the West Bank fossils could re-evolve limbs, then other animals that have certain genes they never lost but whose "triggers" are dormant could re-evolve those traits. Maybe humans will end up with tails again."
- Argument About Snake Evolution Rekindled by Fossil
By Patrick Rizzo
Very simply put, evolution doesn't give organisms what they need, it only selects for the good mutations. Even though legged whales have been found, it may just be too hard for snakes to re-evolve legs.
Why should an animal with legs evolve into one without them?
Because legs can get in the way in certain lifestyles, such as burrowing ones. It's thought that the ancestors of snakes were small lizard-like things that went underground, losing their external ear openings, eyelids, and legs... which are bad if you live in dirt. glass lizards are another example of an organism that has lost it's legs because it has adapted to a burrowing lifestyle (although it still has eyelids and ear openings)
TAW wrote: ancestors of snakes ... went underground, losing their external ear openings, eyelids, and legs.. adapted to a burrowing lifestyle
That sounds very much like the answer I was searching for.
This too perhaps, sounds like a factor that can inhibit hind limb development. I mean, consider how awkward it would be if snakes had only their hind limbs to push along the rest of its body, without front limbs to balance, guide and walk? Unless the species is bipedal, even then, survival would be extremely difficult.
LENNY FLANK WRITES: "As an aside, we now know, from genetic analysis, why snakes don't have vestigial FRONT limbs. During the evolutionary appearance of snakes, there was a change in one of the HOX genes that shifted the body plan forward a bit. As a result, snakes now have no neck vertebrae --- they are all thoracic and abdominal. Since, genetically, front limbs appear right where the cervical vertebrae begin, snakes can't have front limbs. The vestigial rear limbs appear where the abdominal vertebrae meet the tail. As photos show, the tail of a snake is extremely short. So, even though a snake LOOKS like it is all neck or all tail, in reality, it is all body."
I'm thrilled to see that the last several threads have evolved into interesting study groupls--entertaining learning experiences--rather than degenerating into the usual whack-a-troll shindigs.
Those can be amusing, too, but not always in a good way.
Nor do I think it's shameless pandering to simply thank and praise Carl, on occasion, for his knowledgeable and stimulating posts. I doubt he makes any significant amount of money from these posts and, even to the extent that many of the posts may be mental "spin-offs" from his other, more lucrative, research and writing projects, I also doubt that he gets enough ego-gratification in return to justify the investment in time and energy.
This is just a guy who is so genuinely excited and interested in all this cool stuff that he can't help but want to share it, discuss it, debate it...like a rocker or rapper who has too many musical ideas to constrain himself to one band. We're fortunate to find ourselves in our favorite pub on the night a rock star happens to drop in with his side-band to try out a few new tunes in an informal setting.
Just because Carl isn't doing this strictly for the money or the ego-boost, I see no harm whatsoever in saying "thanks!" on a relatively loud and frequent basis...
Or, as Lenny Flank might say, :=>
doubt he makes any significant amount of money from these posts
safe guess he makes nothing from posts... blogs can be great for bringing awareness for books.
a rock star
the only similarity I see between Zimmer and a rock star, is religious righters say El Diablo is their inspiration.
Actually, I think the religious right hates Zimmer's kind worse than rock stars. Afterall, at least half of what comes out of rock music is based on passages from revelation 13.
I think one of the main reasons we don't see snakes re-evolving legs is that evolution generally works through modification of existing structures. Snakes simply no longer have anything leg-like enough for evolution to work with, so instead of re-evolving legs, they tend to evolve other structures and locomotive behaviors from features their bodies DO have. That's why we see swimming snakes, tree-climbing snakes, and even "flying" snakes, each with their own unique modifications for those types of locomotion. They've all evolved their novel movement methods and structures from the basic snake body. They haven't developed legs because evolution could always find easier ways to adapt to any locomotive needs.
This can also explain why most land creatures have legs in the first place. They (we) have all evolved from aquatic creatures that had leg-analogies, such as fins. Evolution used what was available and turned them into true legs, and has kept them ever since. If the first land-transitional creatures had been appendage-less, then perhaps we'd all have snake-like bodies now, and legged creatures would be the odd ones out.
(This is my first post here. Please excuse any mistakes or misconceptions. I'm no expert in this.)
I loved the bit in At the Water's Edge about the sea going sloths. What fascinating creatures they would have been to see. This blog may not bring Mr. Zimmer any money but all the good karma should be worth something. I'm reading his books as fast as I can, can't believe I've missed them all these years. Thanks Carl.
Paul Maybury Jr.: I'm reading his books as fast as I can, can't believe I've missed them all these years
from the look of his archives and photo, he's not "that" old.
Speaking of kudos. Was browsing some of his articles this morning, and found this very interesting piece on Limpets.
Carl is right. Limpets are an oddly "designed" species. I began studying to learn more about limpets after beginning beachcombing, (yeah, *chuckles* I live near the ocean and love it) started blogging on the shells I found. I could only wonder "how" do these things ever stay attached to the rock in the waves? Further, how can they possibly avoid predators, like the ever-looming threat of moon snails? (Keyhole limpet "design" is really like a welcome mat for Polinices duplicatus "come on over for dinner, the hole is already drilled for you!")
it is simply a terrible design! Unlike most mollusks [for the most part] who can close up inside, or withdraw into their shell for protection, not so easy for the limpet. [there's other bad designs, where some species like barnea truncata the Fallen Angelwing, if I'm not mistaken is another species, almost as bad as limpet design, can not completely close their shell]... which would seem to leave them vulnerable to predators. Notice the curve near the hinge of the shell...
the keyhole limpet is an obscure little shell I've been collecting and I've been asking myself the same questions, Carl addressed. I've got some photos of keyhole limpets if anyone is interested in what these things look like.