Breathing problems recap the evolutionary tree —in reverse

I am following a course on “physics of organs”, and found something interesting:

(…) when a key component of the respiratory system fails in a higher organism, external respiration [breathing] becomes more like that of an organism lower on the evolutionary tree.

For example,

  1. a failure of a mammal’s air pump makes this individual behave more like a unicellular aquatic organism without cilia [water pump organs].
  2. A reduction in the surface area of the alveoli [minuscule air sacs] in a mammal creates the same problems faced by an amphibian with simple sack-like lungs.
  3. A major shunt in the circulatory system [mixing of high and low oxygenized blood] makes a mammal behave more like a fish.
  4. In severe anemia [lack/dysfunction of red blood cells for oxygen transport], a mammal faces the same problems as a lower life form with a less efficient respiratory pigment [oxygen transportation].
    From Medical Physiology by Boron & Boulpaep

Now, while dysfunctional breathing can easily lead to regression “down” the evolutionary tree, the opposite is not the case: we don’t see movement “up” the evolutionary tree due to dysfunction. For me, this is an interesting illustration of the explanatory power of biological evolution.

I am curious about two things:

  • How do young-earth creationists deal with such observations? Do they picture God as “incrementally” designing all species in His mind before creating them?

  • What are other clear examples of such “stepwise functionality” across organisms that recaps the evolutionary tree? Maybe the biologists in the room can help out here.

Not a biologist, but the “executive function” and the prefrontal cortex (PFC) of the brain are similar to what you describe with lungs and respiration. The PFC is located at the very front of the frontal lobes, in simplest terms. It was the last area of the brain to appear in evolution, and it is also the last area of the brain to mature in the life of the individual. (It does not complete development until the age of 25 or so.) The “executive function” of the PFC is just that – the CEO of the mind. In functional terms, it makes decisions, forms plans, focuses attention, etc. Philosophers would associate it with “reason” and “will,” though that is an oversimplification.

Interestingly, when placed under stress, the PFC is the first thing to go “offline.” For example, adrenaline shuts off the rational mind (PFC), which is too slow, and we simply react “by instinct,” which is what happens when the evolutionarily older and, thus, more “automatic” systems of the brain take over. This is why soldiers are trained to respond to commands without question, and why athletes often “crack” under pressure.

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First, I think that the wording by Boron and Boulpaep is a bit of a mess. Phrases like “behave like a fish” are unclear and potentially ridiculous, and all of their points seem questionable to me as generalizations. We can grant the basic claim to discuss your question, but I’m not sure it’s robust enough to justify their list of claims.

But I think the basic claim/hypothesis is that backup systems or default mechanisms in physiology are more likely to be phylogenetically older than the more adapted systems that characterize a lineage (alveoli, etc.). I haven’t seen this hypothesis discussed in depth in the literature, so I think it probably hasn’t gotten traction among evolutionary biologists. (I could be wrong, and I haven’t exhaustively searched.) For the hypothesis to be robust, especially as an explanatory achievement of common descent, I think we would want to see a strong trend phylogenetically. And I think we would want solid phylogenetic answers to the myriad exceptions, namely those cases when there is no backup system physiologically, or when the backup system is itself a recent adaptation. Such cases surely exist, and in fact they would have to, because adaptation does NOT require “advancement” or increased complexity, indeed not even improved function. In the phylogenetically-limited but well known examples of trait loss (e.g. blind cavefish) and neoteny (in lots of lineages), we see changes that can be crudely described as “regression,” and so it is less of a “challenge” for evolution to turn things around and go “back up.” All of these metaphors, of course, are contaminated by a “ladder of progress” vision of evolution that is utterly incorrect.

I agree that the mammalian diving reflex is the kind of thing that makes little sense outside of common ancestry, but I’m not sure that these physiologists have made a very strong case for an overall trend.


I think the brain is a beautiful example indeed, perhaps the most beautiful one. A very rough approximation of the brain’s architecture is that the origin of “deeper”, subcortical structures goes further back in evolutionary history, while more “superficial” cortical structures host the more recent innovations of our brain.

I am saying “rough” because it does not apply perfectly (innovations can take place anywhere), but there is a lot of information that supports this pattern. What you mentioned about the prefrontal cortex is a good example. Another example is that our subcortex is where our most “ancient” senses like smell and taste are embedded and where we find structures (like the amygdala) crucial to basic elements of affect like enjoyment and fear. In contrast, the neocortex hosts evolutionarily “recent” senses like sight and hearing and is associated with advanced functions like conscious control (like you said).

Perhaps for others who read along a picture can help to illustrate the neocortex and the subcortex:

I agree, though it is merely a single paragraph in a textbook, and they are referring back to points they clarified in previous paragraphs. For example, they mentioned earlier that fish do not have a separation between oxygenized and de-oxygenized blood flows, like mammals do. So when they say “behave like a fish”, they are referring specifically to that piece of information. Context is crucial, I guess.

Yes, and I do not think that claim needs much justification within an evolutionary context. Circulatory mechanisms only became needed when multicellular organisms became so large that simple diffusion was insufficient to transport fuel and waste for the cells. In general, many optimizations (like circulation) only became necessary as a result of other developments (like multicellular life). So given the progression we observe over the course of evolutionary history, we could expect to see a similar progression in the way our bodies function.

I assume you mean to say that evolution does not guarantee “progress” in terms of complexity or something like that. I agree. But as a description of evolutionary history, it is accurate to say that evolution has resulted in “progress” in terms of complexity, by any measure, from prebiotic chemistry to the first cells to the first mammals. Also, there is, for example, this work in computational ecology indicating that an increase in complexity over time could be guided by evolutionary processes.

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