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Fins To Feet

One of the most iconic images of evolution include that of an industrious fish crawling out of the water to be the first animal to walk on land.

Details about this transition from water to land are very exciting but typically have been very hard to come by.

We know that 365 million years ago or so, some fish fins evolved enough into limbs to allow them to travel on land eventually becoming the mighty tetrapods which were the first vertebrate land dwellers. These early tetrapods eventually became reptiles, amphibians, and of course mammals.

I’m sure those first fin limbs weren’t even close to optimal for land use. We’ve all tried to walk with swim fins on haven’t we? Not very efficient is it? How did fish fins become full-fledged tetrapod limbs then?

The first step in this transition may have been uncovered recently by scientists as described in the Journal Nature.

What they did was inhibit the expression of a set of genes in a zebra fish embryo. The results were fins that were dramatically shortened.

According to Marie-Andree Akimenko, a biologist and one of the study’s authors.

“If this kind of gene expression happened in an ancestral fish that may have led to limb development,”

You know those bony supports inside of some fish fins? The precursors to those in the embryo are called actinotrichia which are these slender thread-like fibers. It’s those fibers in the embryo which are prevented from forming. The idea then is that without those embryonic fibers, it would be easier for limbs and digits to eventually evolve.

I found two facts that best support this theory.
One is that all land animals that have been studied don’t have these genes nor, I assume, do they have mutated versions of them.

Second, The best support may have been discovered when they compared a zebrafish embryo and a mouse embryo.

Dr Akimenk said:

“When we compared fin development and limb development, the early steps are very similar…But at one point there is a divergence, and that correlates with the beginning of the expression of these genes.”

The next step in this research is to insert these genes into mice and see what happens. That could be interesting.

Still, this is just the easy outer-edge of the puzzle. There’s still a lot missing inside this puzzle like how did the fin change into 8 digits of the oldest tesrapods, why do 5 digits predominate now?

I have two problems with this.
First,

You would think that stunted fins would be selected against since it seems like quite a disadvantage; like Pixar’s Nemo. Presumably, it could happen in some fins and not all or maybe all the fins were affected but not to a debilitating degree. There’s lots of ways to explain that away.

Second,

It’s generally accepted that lobe-finned fish evolved into tetrapods and not the ray-finned fish.

Lobe-finned fish are an ancient and almost extinct lineage of fish. They had two primary characteristics that strongly support them as the first land animals. Their fins were strong appendages that could easily support their weight and they had swim bladders which are believed to have eventually evolved into lungs.

Ray-finned fish with their diaphanous fins are generally what you think of when you think of a fish and it’s what this research focused on.

I must just be missing something but if anyone knows what that is, please share.

5 comments to Fins To Feet

  • nighthiker

    I think this would be too basic a mistake for the researches (and Nature’s peer review) to commit , so I’m likely wrong, but… Didn’t lobe-finned fish appear before their ray-finned cousins? If so, wouldn’t it be more logical to assume those genes appeared in ray finned fish, rather than disappeared in the tetrapod ancestors? The fish that gained those genes then diverged into all the ray finned fish we see today, while the ones that didn’t originated the tetrapods before mostly dying out. So when they take those genes out the fish simply revert to something closer to how they were before the gene appeared, i.e. closer to lobe-finned fish. It makes more sense and still fits the embryology findings mentioned, it looks to me.

  • nighthiker

    Errata:

    After looking it up online (my copy of “The Greatest Show” isn’t at hand at the moment), it actually looks like ray-finned fish came first (at least have older, if incomplete, fossil records) than lobe-finned ones – they were just much less common than lobe-finned until the late Devonian. If so, then I guess I was wrong indeed.

  • Evan,

    For one point, the “swim bladder” apparently evolved as a lung, then transformed into a flotation organ. For another, ray-finned fishes also have them.

    For another, it’s possible that Zhang and Akimenko are explaining how a ray-finned ancestor evolved into the lobe-finned fishes. I don’t have easy access to Nature and can’t check–does Steve have their online version?

  • Bob, I have no idea why the preceding message is headed “Evan”. Sorry.

  • azinyk

    I think that there are something like 6 species of lungfish and 2 species of lobe-finned fish, compared to about 25,000 species of ray-finned fishes. So there might be more ray-finned genomes sequenced to compare to.

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