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Understanding Evolution - Origin of Limbs

I recently came across and answered a question on Quora that reveals a misunderstanding I've seen before over evolution. The question was If theory of evolution is true, why aren't there more semi-evolved species with hands coming out of their skulls or other half-baked monstrosities?. It seems to be that some people think that evolution works by creating partially complete versions of what will later be a fully fledged feature of an organism. Obviously, that's not the case. So I figured it might help to explain how a trait like our limbs actually did evolve.

Evolution doesn't have any foresight or planning. Every organism that has ever lived is both the end result of all the evolutionary history leading up to it, and a transitional form to whatever it's future descendants might look like (assuming its lineage doesn't go extinct).

In contrast to what that Quora question implied, evolution doesn't work like this:

Not How Evolution Works
(not how evolution works)

Evolution didn't provide us with a partial arm in anticipation of a fully developed arm down the road. We didn't sprout a bicep, then a forearm, and finally a hand to complete our limbs. Genetics just doesn't work that way. If an organism doesn't have any limbs to begin with, it's not going to create a monstrosity with a partial human limb. Evolution only works incrementally, and every step has to be functional, or else the organism won't survive to reproduce.

So, if our ancient ancestors didn't have limbs, but we do now, how did it happen? What were the types of changes that were small enough to be possible through genetic mutation, but useful enough to benefit those organisms?

First of all, our ancient ancestors looked something like this:

MetaSpriggina Reconstruction & Fossil
(source: New fossil find pinpoints the origin of jaws in vertebrates)

Just to be clear, it's very unlikely that that's actually our direct ancestor. It's probably a cousin of our direct ancestor. So, it's more like our great-great-great aunt/uncle. But, just like you're more similar to your human aunts and uncles than to unrelated strangers, our ancestors were probably pretty similar to that creature. And of course, you can go back and find even simpler ancestors that are more ancient, but I didn't want to go through our entire evolutionary history for this entry.

So, that critter is a very early chordate. It's got some of the same features we do - eyes, a brain, a 'spinal' cord, etc. But it's also missing some of the features we have now - a jaw, bones, and most notably for this discussion, limbs. So, what could those first rudiments of limbs have looked like? Probably something like this:

Hypothetical evolution of paired fins and their skeletal supports
(source: Origin and Comparative Anatomy of the Pectoral Limb: Anthony F. DePalma MD, FACS)

Now, I'm going to be completely honest. That picture is informed speculation based on studying modern animals, genetics, and embryology, since no fossils have been found preserving that type of soft tissue, but it should be clear how skin folds leading to fins would have been a pretty small mutation that would have benefitted our swimming ancestors. From that creature in the top of the image, it's also a small step to then split up those now existing fins into several independent fins - giving those organsims more freedom in how to use those fins.

Once our ancestors had simple fins, then it was a series of small incremental steps to get to more complex fins, like shown in this next picture:

Diagrams illustrating hypothetical evolution of the extremities of diapnoan (I), ganoid (H) and elasmobranch (G) from a fin fold supported by a series of similar radial cartilages.
(source: Origin and Comparative Anatomy of the Pectoral Limb: Anthony F. DePalma MD, FACS)

I like this one because it also shows the branching nature of evolution. It wasn't just primitive fins in a straight line to our direct ancestors. As those populations split up, each newly independent population took its own evolutionary path, each finding different strategies for modifying fins. In one particular lineage (L), it created a fin with fairly robust bones.

Now, that lineage kept on splitting, too. Some populations remained in aquatic habitats, so that today you can still find coelocanths which have fins with robust bones like that. But some of the populations became increasingly specialized for amphibious habitats, and eventually some populations even became specialized for fully terrestrial habitats. Take a look at this next picture:

Tetrapod Limb Development
(source: Berkeley - The origin of tetrapods)

Like the caveat I gave above, most likely none of those organisms are direct ancestors of any of the others, but the oldest ones are similar to the actual direct ancestors (they're all known animals from fossil remains). And you can see how each newer organism and newer limb is only a slight change from the previous one, as well as how each organism has a fully developed limb that serves it quite well.

And remember, as populations split off and each go their own evolutionary way, they can all develop their own independent adaptations and ways of using different features. So, from those early tetrapods, different lineages have modified those limbs in remarkably different directions, but all showing the underlying similarity:

Similarity of Vertebrate Limbs
(source: Pinterest)

It's not just limbs. That's how virtually all features have developed over the course of evolution. You've probably seen octopuses and squids, and know that they have pretty complex eyes, with an iris, cornea, lens, retina, etc. But since we know evolution is true, it must follow that even something as complex as that eye must have evolved through small, incremental changes, with each new change being beneficial. And in fact, we actually do have a pretty good idea how it happened:

Stages of eye complexity in mollusks
(source: Evergreen Comparative Physiology of Vision - Cephalopods)

Note that those intermediate steps are found in existing animals. They're not just hypothetically a type of eye that could exist. They're types of eyes that we know, for sure, exist and are used by different species of animals.

So, just to recap, evolution doesn't have foresight or a plan. For that matter, it's not a conscious entity at all, even if anthropomorphizing sometimes helps to explain it. Evolution only works through small incremental changes, and each of the changes has to be beneficial if the organisms are going to survive and pass those changes on to future generations. Every organism alive, past and present, is in a sense the end result of all the evolutionary history leading up to it. But in another sense, as long as they don't go extinct, evolution never stops, so every organism is also a transitional form to whatever its descendants might be.

---

Here are two links with more info on vertebrate limb evolution. I already linked to the first one up above, but I wanted to make sure to call it out as being especially informative.

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