Mammal Evolution

[cadbrowser]

I am needing some checking of my work in answer to a question posed to me and also any corrections and further information that could aid in the layman's understanding a bit better.

I was asked this:

Whales and seals and such are mammals. The evolution theory holds they were land animals which went back to the sea and developed the skin, flippers and such. The questions I have include, if we came from the sea aren’t the gill genes still present in us and the mammals should have started growing gills? And two, why are we sure that mammals originated on land, rather than in the sea?

This was my reply:

My understanding on this is that there is fossil evidence (including substantial transitional fossils) that traces "cetaceans' (whales, dolphins, and porpoises) back to it's land ancestor the Indohyus (48 million years ago).

The first known footprints on land date to 530 million years ago (predates plants). Between this date and the one previously mentioned, as far as I know there is no way to tie the two together from a purely ancestral aspect. Don't forget there was the Permian-Triassic extinction event (250 million years ago) that eliminated close to 95% of all marine species. In fact the evidence shows it took life on land at least 30 million years to recover. Which indicates that a lot of potential fossil evidence linking the two back to its original Eukaryotic cell ancestor is most likely lost (1.85 billion years ago).

Amniotes desceneded from reptiliomorph amphibians that lived on land during the late Carboniferouos period (300 million years ago to 325 million years ago). Within a few million years two lineages branched off, the Synapsids (includes mammals) and the Sauropsids (turtles, lizards, snakes, crocodilians, dinosaurs, & birds). They had both lungs and gills.

So technically ALL life originated in the oceans, some developed the ability to live on land. Some of those decided to return to the oceans.

Why do wales still have lungs and not gills is due to their sheer size and the efficiency of lungs over gills in that respects.

"With an unlimited, easy supply of oxygen, whales and dolphins can maintain a high metabolism and high levels of activity in any part of the ocean. They can grow faster than fish, to much larger sizes, safe from predators."

There is a great article on New Scientist website that explains why whales don't have gills. The above quote is taken from that article.

Any corrections to my reply are welcomed. Thank you in advance.

[CaptainChewbacca]

Basically the reason they didn't go back to gills is they didn't NEED to. Adaptation comes out of necessity, and marine mammals get by quite successfully holding their breath and surfacing when they need to. Genes aren't an on/off switch, and there hasn't been the time or the necessity for mammals to go back to gills.

[Vehrec]

I should point out to this fellow that the 'genes for gills' idea is an oversimplification, and that the structures in the embryo that become gills in fish are in mammals re-purposed to do other tasks. Without our re-purposed gill arches, mammals would lack ear bones, the Hyloid bone and thyroid gland, most of the cartilage of the throat and the soft palate, and even parts of the upper and lower jaws. Now try to imagine how long a theoretical mammal without these traits would last, even if they had gills. Early in development, every mammal has clear, well developed arches along the throat that look like gills, but as they develop the arches reduce in size and mingle somewhat to form the final tissues of the head, face and throat. Gills also present a problem for a warm-blooded animal-you need to take blood coming from the heart, pass it through fine filaments in cold water, and then restore lost heat once it's oxygenated. There's a very good reason why tuna and great whites are only able to moderately boost their core temperature above that of the surrounding water.

As to his second point, science is sure of very little-but it can tell us some of the oldest whales are found in Pakistan in rocks about 50 million years old.Now, since the oldest mammal fossils are more like 200 million years old, either whales came from land dwelling animals, or we are missing something around 150 million years of whale fossils from the early, mid and late Mesozoic. Whales also lack something he has-hind limbs. If whales do have them, they are incredibly vestigial, and fossil whales show a clear decline in the size of such limbs. A mechanism for a large mammal that cannot support it's own weight to leave the water seems dubious-a small animal entering the water passes the test of parsimony much easier.

And if you take a look at Ambulocetus or Indohyus, you don't see something that looks like a whale anyways, they're clearly evolving from land animals. Pictures are worth a thousand words here.

[cadbrowser]

Thank you guys for commenting. I know I probably could've done a more in depth study and reply but I really didn't want to waste too much of my time on this YEC "looking for answers". He still thinks a rat will turn into a dog, cat, or horse via evolution. So I had to tell him to go back and read some basic literature on what evolution is; not what he has read from a YEC website what it is.

Round and round I tell you...ugh.

[Lord of the Abyss]

It's my understanding that seadwelling mammals can't use gills, because they'd die from hypothermia. The enormous surface area needed to extract oxygen from the water would also serve as a very good heat dissipator; not a problem for cold blooded fish, but a huge problem for mammals. Water's much better at absorbing heat than air, so not being chilled to death is a major concern for mammals; just look at all the blubber whales and seals and the like use for insulation. You can't really insulate a gas exchange organ though, it has to contact whatever you are extracting the gases from.

[Alyrium Denryle]

The gill arches were re-purposed into the inner ear bones and other structures. The other structures needed for gills are no longer functional. Think about it. A four-chambered mammal heart routes blood to the lungs, which then comes back around through the heart again and out to the body. Fish gills work with a two chambered heart that acts as a one-way pump. This is way more than re-activating gills--the developmental pathways for which are no longer present after 200 million years. Large parts of the mammalian physiology would have to be changed and rebuilt from scratch. Evolution tends to re-work existing structures, and maintaining air-breathing is easier after all the water-breathing physiology has been re-purposed into other structures.

[Dass.Kapital]

Wrote interesting and informative stuff....

Spoiler

So...just to ask a question. Could a similar mechanism that Penguins use to keep their feet both warm (And not frost bitten) but NOT vent body heat off into the cold Antarctic ice develop/be used so that said heat loss would be slowed down in said said gill structure?

Just asking a different but related question to topic. I also think it's an interesting co-thread with the toic regarding "What if Mermaid(Folk?)" were real.

[PeZook]

I had some success with this simple explanation: Evolution doesn't plan ahead. It literally doesn't know whether we could have gills or not because it's a process, not a conscious being. So selection pressures act on what's there, and what's there is a warm-blooded air-breathing creature trying to get food, survive and breed in a new environment. So it will select for (slight, incremental) changes in what systems currently exist, not proactively plan ten steps ahead to reintroduce water breathing.