Atomic Nerds

So, when I reduced myself to outright begging my readers to hand me content ideas, David actually obliged me with the following question:

Really, though, I guess my question is more along the lines of ‘why do we keep remnants of design that are no longer useful?’, or maybe how long does it take for something like that to go away? Is it really that all the common knowledge useless body parts aren’t actually useless, or is there something deeper that I’m missing about evolution?

The answer to the very last question is essentially, “yes”, and the first and second ones have two related answers.

The first answer- “why do we keep remnants of design that are no longer useful”- is essentially “because there’s usually no reason not to”. Selective pressure is an efficient driver of change, but it only works when there’s actually pressure. If an element of design isn’t particularly useful anymore, but also isn’t actively detrimental, it will stick around. This is especially true in terms of large features like limbs and organs- those are formed during embryonic development, and development is necessarily a highly conserved state because the vast majority of mutations that affect development are either lethal right there or produce a seriously crippled result. One of the complications with molecular “clocks” and dating is that while different regions of the genome may mutate at constant rates, the speed at which different regions accumulate variations is NOT constant; there are some things you can mess with almost endlessly without serious consequences to the organism, usually in old bits of broken code that no longer have a function anyway, and other things that develop variation extremely slowly because almost all possible variants produce a non-viable organism. When we say some feature is “conserved”, we mean it’s one of those vulnerable areas where mutation is almost always a disaster. This is why human fetuses still go through stages with gills, tails, fur, and all sorts of other things adult mammals have long since disposed of that do the fetus not a bit of good aside from a bit of wasted energy- it takes a tremendous amount of time for anything to change at all there, especially with no selective pressures pushing every single step in the “right” direction hard.

Selective pressure does have SOME influence on limbs and organs that are no longer useful- structures that are very expensive or that impede the organism in its new role create pressure to reduce them. This is why some pythons have vestigial pelvises for hind limbs that no longer exist, but no hind legs to drag around. Some legless lizards, being far more recently slitherers, do this step one better by having complete limbs hidden underneath the skin- a little expensive and will no doubt reduce even further over time as the snakes’ limbs did, but still there as they are not actively interfering with the lizard. All in all though, things that do not create any significant cost to an organism will stick around for an extremely long time, and will probably leave some form of trace on the organism more or less for its entire run as a lineage, if only as a patch of scrambled code.

Selection can also actively interfere in the reduction of an organ in some unexpected ways- for example, the human appendix, which not only represents the shadowy remains of the caecum more herbivorous primates have but also represents an active ongoing health hazard in the form of a nasty potential for fatal infections. The more the appendix fades away, the narrower the “neck” of the organ connecting it to the rest of the intestine gets- BUT, the narrower the neck gets, the more likely it is to get blocked up with crap, become horribly infected, and kill its owner. The last few stages in “disappearing” the appendix are actively selected AGAINST- therefore we’ll probably still be carrying the thing around for quite a long time to come, at least until surgery is such a feature of human existence across the entire planet that many generations have that selective pressure removed. Still, the sheer inertia involved still means that will take quite a long time- as it is with another feature of our relatively recent ancestors that were far more serious plant consumers, the wisdom teeth. These represent a feature with active selection against- the tendency to become impacted and seriously screw up your jaw- and they also represent a feature that is fading out of the human population. I was only born with three of my wisdom teeth (which all grew in various horrifying directions), with the fourth entirely missing, and some people have fewer or none at all. Ironically, unlike the appendix, surgery as a common feature of human life will retard the disappearance of the wisdom tooth- dental problems used to be something very likely to be eventually fatal if not merely crippling, as opposed to a reason to moan about novocaine and drills.

Someone in the audience is no doubt eager to point out about now that the human appendix DOES do something, which is house lymphoid tissue that produces various cells important to the immune system, like other organs with the kind of lymphoid tissue associated with the gut in general. This is true, but it’s important to emphasize at this point that vestigial does not mean completely useless. It CAN, as in the case of the python’s pelvis, but it doesn’t NEED to. What makes the human appendix vestigial isn’t its uselessness, it’s that it is directly homologous- derived from the same structures and apparently having once had the same function- as a larger and much more active part of the digestive systems of many mammals, including many primates- the ceacum. The ceacum is a big pouch found in animals that eat a lot of plants that’s essentially a bacterial storage bag; it houses colonies of friendly bacteria that can break down the celluloses found in plants, which no animal can do with its own enzymatic toolkit. Ceacums in larger animals not only do this, they ALSO produce those same immune system cells- and do it far better and in far higher volume than humans use their appendixes for. This kind of work is easy to recruit an appendix for- it’s derived from tissues associated with the lymphatic system*, and it was probably what that bit of digestive system was doing before it became recruited by herbivorous mammals to be a caecum. As most have noticed, the capabilities provided by the human appendix in the realm of immune maintenance are so minor that there is no apparent difference between individuals that have had theirs removed and those that haven’t.

One of the things that makes evolution as (relatively) fast and robust a system as it is- and it’s a nonintuitive thing, to judge by the number of people that object to evolution because it’s supposedly TOO fast and robust in practice for the mechanisms behind it to possibly be responsible- is that it isn’t particularly linear as we think of it. Oh, sure, it is in ONE sense- each generation is derived from the prior generation, and it gets whatever genetic hand it was dealt by whatever system of reproduction created it. How it isn’t linear is in how systems are developed; it doesn’t evolve one part and then evolve another part and then ta-daa, a complex system to do a complex thing is produced. This is the fallacy that “irreducible complexity” rests on, but as I’ve gone on at some length before, that’s not how things actually *work*.

How most people think of genomes is as being carefully drawn blueprints, with maybe a few things sketched in here or there, but overall careful plans for the eventual organism. How most people think of organisms is as being much like things that blueprints are for- like, say, cars: a carefully machined and implemented design. They can hardly be blamed, as this is certainly what they look most like, and that’s certainly the narrative that’s easiest for a species that designs and builds things to understand, and it’s a useful narrative for when you’re trying to fix the damn things when something breaks. However, it’s generally a lot messier than that; genomes contain tons of information that will never be used, because it’s only useful in certain contexts, like in ice ages or droughts or an extinct disease or something we did when we were amphibians or something that’s only actually information if it’s in combination with certain other genes- and that’s not even getting into what happens when a virus gets in and half-rewrites the whole thing. Which happens often. The resultant organism is as much a product of how much information that makes actual sense can be wrung out of the genome plus various environmental factors (that often signal parts of the genome to activate or deactivate**) plus the more conserved aspects of the genome that really *do* act more or less like a blueprint.

Organisms, in other words, are less like cars than they are like ambulatory junkyards, at least on the inside. Parts aren’t machined to order whenever something new is needed, they’re picked up and adapted from something else- whether that be a bit of old code, or a larger part that’s sitting around not doing much in its current role that could be used for another. The classic teaching example of this is the panda’s thumb. Pandas are bears that have adapted an entirely vegetarian diet, unlike all their more sensible*** bear relatives. In order to manipulate the bamboo that became their diet, a really useful thing for them would be an protuberance at an angle to the rest of their digits that could help them hold things in place against their “palm”, much like primates have enjoyed for basically their entire history; however, being bears, their fifth digit had long since become completely dedicated to being a claw and was, in that role, unavailable for recruitment as a proper thumb. What pandas have for a “thumb”- something that can press against the pad and be a rudimentary grasper- is a really enlarged and protruding radial sesamoid bone, one of the fiddly little bones in your hand that helps hold the whole complicated thing together to create some kind of flexibility in a bony structure. It’s not a fraction so flexible as a digit, but it’s good enough to do the job- and good enough is all that counts in evolution until somebody comes up with something a little better. Stephen Jay Gould termed this overall phenomenon- organisms using a part being used for one function for another- “exaptation”.

Exaptation is the other thing that can happen to structures that are no longer needed for their original function: they get folded into another system, and over enough time, may well become an indispensable part of that system. This is how we can come across genes that code for synapses in humans in an ancient creature that has no real nervous system at all and therefore no need for synapses- they’re not “genes for synapses” at all, or at least they weren’t always. The sponge uses them for other kinds of cellular junctions****- they’re junction genes, which nearly everything subsequent to the sponge level of multicellular organism has specialized into synapse genes.

Improvise, adapt, overcome isn’t just a military motto- at the core of things, it’s the motto of life itself. It just doesn’t run half as high speed and low drag.

*Yes, I know this bit is vague. In order to properly un-vague it, I’d have to get pretty extensively into developmental embryology. Neither of us wants that. Trust me.

**This is called epigenetics. Not a concept you encounter often outside of college, as evidenced by periodic articles in otherwise scientifically sober-sided popular science that tout newly discovered examples as some kind of challenge to “Darwinism”.

***Sometimes specialized adaptations that are a good move in the short run are really *bad* for a species in the long run. Pandas are one such. Explanation of why deserves a separate post.

****Cell communication functions, which go well beyond nerves, which are just cells really specialized for communication. Remember my threat about developmental embryology? I can do it with this too, and once again, neither of us will like it.

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