Gradually bridging insurmountable gaps

I’ve been reading the Living Fossils Expose Evolution thread and, whilst new posts are accumulating at a rate with which I could never hope to keep up, all sorts of things cropped up that I felt the urge to reply to. Most of them were distantly off-topic though, so I thought beginning a new thread on the one which bothered me most was the best approach.My ain gripe was Calypsis4’s incredulity at some seemingly impossible (to him) evolutionary transitions — such as that from water-breathing to air-breathing animals. I thought we could then go on to discuss some of the other transitions causing him problems, such as how ungulates could evolve into aquatic mammals, but if the mods think it would keep things tidier to stick to the restricted topic of water-breathing fish evolving into air-breathing tetrapods then fair enough.[Yes, please, let's keep the focus on the evolution of air-breathing tetrapods. --Admin]From water-breathers to air-breathersAs stated by Calypsis, the problem is as follows: One important point to mention before we being is that there are no morphing organisms involved. There are, in fact, organisms which morph from water-breathers to air-breathers over the course of their lives, but we're discussing evolutionary change here. 'How did a population evolved from breathing water to breathing air?' is the question (and the problem is ‘air’ as opposed to ‘oxygen’ — all the creatures involved breathe oxygen, the question is which medium they extract it from).The key assumption which makes this evolutionary transition a problem is that it’s only possible for the same organism to do one of the two — breathing water or breathing air. There’s no realistic way any species could change from obligate water breathers to obligate air-breathers in one generation. Fortunately, there’s no reason for them to do so.The dominant lifeforms* in my home aquarium are three-spot gouramis (I think). They look pretty much like this chap from Wikimedia: They can often be observed to swim to the top of the tank and stick their heads out of the water whilst opening and closing their mouths. It looks like they’re taking big gulps of air. I was prompted to do all sorts of reading into what I saw going on in the fish tank, and was pleasantly surprised to discover that they are, in fact, breathing air. They possess what’s called a labyrinth organ, formed through the invasion of blood vessels into the bones of their gill arches and so named because it’s labyrinthine structure leaves a large surface area through which oxygen can be absorbed from the air — much like our lung.*Or, in fairness, the biggest and most aggressive. The dominant lifeforms are clearly the snails.These fish are capable of breathing both underwater and in the open air using different organs, and they’re not the only ones. European eels sometimes have to wriggle overland for a while when they’re migrating back to the sea, and they don’t hold their breath while doing so — they can absorb oxygen from the air around them through their skin. Mudskippers use the same tactic, though these are even better adapted for breathing out of water. They increase the available surface area by also absorbing oxygen through the lining of their mouths and throats. Note that the two below on the bare rock are not holding their breath — they’re happily breathing. Our ancestors didn’t go through any breath-holding while they were in the process of transitioning from an aquatic to terrestrial existence, because they already had lungs. We can see this by looking at our nearest relatives amongst fish — the aptly named lungfish. As you can probably guess, these fish have fully formed lungs for breathing air, and some species can survive out of water for up to two years.The first vertebrates to make a life for themselves out of water never had to hold their breath. They had lungs, and probably also absorbed oxygen through their skin and the lining of their mouths — just as many amphibians still do today. We know that all these methods of respiration evolved amongst fish still living in the water because we can see them in fish that live in water today. As they became more terrestrial and less reliant on water, their air-breathing equipment would become more important and their water-breathing less important. Some lineages might begin to lose their gills at a certain point in life, again as many (most?) amphibians do today, and some lineages would eventually cease developing them altogether, like ours.The key point is that there’s no real problem posed by the transition from water- to air-breathing. All that’s required is an overlap period of being able to do both. As many species can do this today, there’s no reason our ancestors couldn’t accomplish the same feat.Edited by Admin, : Add moderator comment.

Thread copied to the Evolution of Air-breathing Tetrapods thread in the Biological Evolution forum, this copy of the thread has been closed.