Biological Revolutions of the Mind: An animal perspective


(Picture from here.)

I’ve been thinking about the long view recently. Specifically, the age of life on this planet. Wikipedia’s timeline of evolution is interesting but it compresses as it approaches us, in effect giving the same weight of the rise of complex cells to the rise of insects. This tends to trivialize the magnitude of complex cells.

I think we can only consider a few points in the timeline as truly revolutionary.

  1. Abiogenesis revolution: the rise of living systems. Most of these are likely extinct.
  2. Prokaryotic revolution: bacteria and the like. About 3.8 billion years ago.
  3. Eukaryotic revolution: complex cells such as found in amoebas and human beings. Say, 2 billion years ago
  4. Multicellular revolution
  5. Neurozoan revolution: Anything with a nervous system. This allows integrated system response to local stimulation. Somewhere between 1 billion and 600 million years ago.
  6. Cognozoan revolution: Centralized response driven life forms. Life forms with brains or central nervous systems (CNS.) Say 600 million years ago.

Neurozoan appears to be the invention of John Opie (2010.) Lots of animals have nervous systems and no brain– hydras, for example.

Cognozoan is my own. Do you like it?

I do not use the term “revolution” lightly. While nothing really compares in scale to the abiogenesis, each of these so-called revolutions changed the landscape of earth. Once we had prokaryotes we had the ability have true inheritance via DNA and variability in metabolism. We got photosynthesis and an oxygen atmosphere. Once we had eucaryotes we could have fungi– which pretty much created something called “soil.” Multicellular systems comprise all complex life forms from trees to sea anemones. I suspect the roles of predator and prey, as we know them, originated here.

Neurozoans: all free living animals. Period. Full stop. End of story.

Once we introduce cognozoans, we have abstract behavior. Mating dances. Specialized predator/prey strategies. Fish. Crabs. Beetles. Starfish. Squids. Anything with a CNS. Most animals past the size of mesozoans have a brain. One could venture a guess that a central processing system is required beyond a certain level of behavioral complexity.

While this is an animal centric perspective I may be addressing some plant concerns as well.

I’m going to be going over these revolutions in the next few weeks and talking about them one at a time. Each of them is worth a blog entry of their own.

This is going to be fun.

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Biological Revolutions of the Mind: An animal perspective — 6 Comments

  1. Looking forward to it … specially your take on the ‘oxygen catastrophe’ … whether it was a catastrophe depends on your POV I guess!

  2. 1. Protists are eukaryotes. Bacteria are not protists. Did you perhaps mean prokaryotic revolution?
    2. Metazoans are, by definition, a subset of animals. Trees are neither metazoans nor descendants thereof.
    3. Systemic responses to local stimuli do not require a nervous system. See, for example, a basic plant physiology text.

    From my perspective, the 6-revolution approach seems far too narrow and zoocentric.

  3. I had one of my protagonists once write a textbook titled Eukaryotes and Prokaryotes — he was a microbiologist. Having heroes smarter than yourself means that they write books you could not.

  4. Here I thought no one ever read this. Corrections are made above with a couple of caveat.

    I did not say systemic responses. I said integrated systemic responses– which, according to my reading, plants do not do. They have general and specific responses but these responses are not integrated within the organism. My reading of plant physiology is that the apparent integrated responses are in fact specific responses that result in the appearance of integrated responses.

    An integrated response might be a spider responding to a fly caught in a web. The spider takes disparate inputs and responds to what those disparate inputs might mean. An apparent integrated response might be a plant growing towards a light to a window. It looks like it’s seeking the light. In point of fact it’s actually differential growth based on auxin production and light– i.e., a single biochemical response that is operationally different in different parts of the plant.

    I am in no way disparaging the chemistry of plants or the marvelous organization of trees. I am saying it’s a different adaptation than what animals did. Animals developed an integrated response system– CNS and brains. Plants had to solve the same problem but did it in an organizationally different manner.

    It might be interesting to do this from a plant centric point of view. The revolutions for the plant world would be different. Photosynthesis in an obvious one, of course. Invention of structural cellulose. Creation of xylem and phloem. Leaves. Flowers. I may do it yet.

    I considered including the land invasion by both plants and animals. I might talk about it by itself one of these days. It’s big but I didn’t think it was big enough compared to creation and organization of life. After all, if we held hands all around the world, 5/8 of us would drown.

  5. Argh! Cat just erased a very long comment.

    I think we may disagree about what constitutes integrated systemic response, though I don’t argue that a central nervous system wasn’t a fantastic way for animals to achieve it. To me, a plant that can flower in response to appropriate photoperiod treatment of one leaf, turn on systemic defenses in response to local infection, or close leaf stomata when roots begin to dry, seems very integrated indeed. In addition, the level of integration required to balance and coordinate plant responses to multiple effectors is pretty darned impressive. It is true, however, that a stationary, high surface-area organism, which frequently has different parts exposed to different environmental conditions, would be expected to have less centralized control than a compact, mobile organism. For example, independent control of phototropism gives a plant much more potential for exploiting available light resources than having all stems locked into growing in some average “best” direction. A phototropic animal, in constrast, would have problems if it tried to move in several directions at once.

    I am do accept all of your revolutions as significant revolutions, just not *the only important* revolutions in the grand scheme of evolution. Even without going into times after the divergence of eukaryotic kingdoms, I couldn’t possibly leave photosynthesis off the list. A world limited to chemotroph calories would be very different indeed, and the ozone layer made possible by oxygenic photosynthesis was a prerequisite for quite a few of the later “revolutions.” (Just how many I’m not sure, though there are those who argue that we wouldn’t even have made it to eukaryotes.) I might also allow slots for some of the less well publicized bacterial evolutionary leaps whose impacts are discussed in Cavalier-Smith’s ‘Cell evolution and Earth history: stasis and revolution’ [Phil. Trans. Royal Soc. B 361 (2006) 961] rather than thinking of them as mere increments between prokaryotic and eukaryotic revolutions. That’s a very long time to compress!