“For nature moves continuously from lifeless things through things that are alive but not animals.” ~Aristotle
Aristotle was fascinated by sponges. He could never quite figure out what they were nor why they seemed to mysteriously lay at the boundary between Plant and Animal. To him plants after all were only capable of reproduction and providing sustenance for other organisms, meanwhile animals had the capacity to sense and to feel. But sponges seemed to have characteristics of both kingdoms: like plants they’re sessile, i.e., they don’t move but instead are rooted in place. And yet, like animals, some sponges are sensitive to touch, a decidedly unplantlike characteristic according to the great philosopher (Stott, 2012). (Work in recent years on the senses of plants would throw a decided wrench into Aristotle’s works, blurring the line between plant and animal even further. See What a Plant Knows for some interesting reading on plant sensation.)
Why are sponges so interesting? Well, we know nowadays that they are indeed metazoans or “animals” despite their primitive characteristics. And they’re multicellular even though those cells aren’t strictly organized into different tissues like the organ systems of animals we usually think of. For instance, they neither have a nervous system nor musculature. But Aristotle, the Father of Empiricism, was exceptionally perceptive because although sponges are animals, they’re one of the earliest known forms of animal arising over 500 million years ago in the Cambrian period and in some respect they do lie at the boundary between Plant and Animal.
But here’s an interesting question: what does a sponge and your brain have in common? Beyond any of the obvious jokes, sponges exhibit what may be a very primitive pre-nervous system. While they don’t have what we would consider nerve cells that communicate with one another via synapses, nor do they have cells that resemble the typical structure of a neuron, they do however have some very familiar chemical (paracrine) signals which cells send to one another that are reminiscent of the paracrine signals in our own brains.
A prime example of this, sponges appear to have NMDA receptors, the well-known glutamate receptor partially responsible for neuronal excitation as well as long-term potentiaion (i.e., learning) [1]. So while they definitely don’t have brains like us or any cell structurally resembling a neuron, sponges nevertheless have a cellular system which utilizes some very familiar neurotransmitters.
Even though most scientists define the animalian nervous system by its characteristic synapses which allow unidirectional flow of information via electrochemical charges, a smaller group of scientists who specifically study the early evolution of the nervous system in primitive organisms have proposed that neurosecretion of stimulatory chemicals is an older system from which the nervous system was exapted or “borrowed” [2, for review].
The sea squirt, a closer though still primitive relative of ours, likewise utilizes NMDA paracrine signaling [3]. In this instance, however, the sea squirt actually has a nervous system. –The weird thing about these little guys though is that they only have a “brain” during their larval stage in which they’re mobile and in search of a place to hunker down and live the remainder of their lives. Presumably the only purpose for this brain is to orchestrate the tail movements which allow the tadpole-like larva to go out in search of their new home. Once they find that special rock, they latch on with their “chin” area, face down, their little tails sticking out, and begin their metamorphosis. Below is an image of the tunicate larva borrowed from here.
And below are some beautiful examples of adult sea squirts [4, 5, 6].
Okay, so you’ve probably heard the phrase, “Ontogeny recapitulates phylogeny,” meaning that stages of an organism’s development correspond to stages in evolutionary development. (This phrase is actually more useful when referring to aspects of embryonic development, such as the gill slits that mammals still exhibit in embryo which are a likely remnant of our oceanic ancestors.)
So I’m borrowing this phrase and mutating it a bit for our purposes here to read: synaptogenesis recapitulates mitosis. What do I mean by this? Well, mitosis or cell division is an ancient process. It’s a basic requisite capacity for any organism to produce progeny and continue its lineage. Synaptogenesis, or the production of a synapse through which a neuron will ultimately communicate with another neuron, on the other hand is a less ancient process and one which arose in Animalia presumably somewhere between the evolution of sponges and sea squirts [7].
Now the interesting thing is that many of the chemical pathways which are utilized for cell division are also used for synapse development. Admittedly, the idea that all forms of cell growth use similar mechanisms may seem like a no-brainer to some people. But is it really? If all forms of cell growth share redundancy with one another and the earliest form of that growth is rooted in cell division, then one can conjecture that chemical pathways used for mitosis were borrowed for neurite and synapse formation. It may seem like a no-brainer but when you think about it and all the tiny steps that had to evolve to make such a process possible… it’s actually quite astounding. Rather than forming new chemical pathways, the process of synapse development exapted some of the pathways used for mitosis. Hey, why reinvent the wheel after all?
“As can be seen at both the gross morphometric and molecular levels, evolution is known for the exaptation of one aspect of ontogenesis, utilizing it for another. Just as the articular and quadrate bones of the reptilian jaws have been coopted to form the ossicles of the middle ear in mammals, so is the electrochemical nervous system reflected in the paracrine pre-nervous system of phylogenically ancient metazoa. And so while mitosis of epithelial-like progenitor cells and neuritogenesis and synaptogenesis of their neuronal offspring are unique processes, the redundancy in pathway activation suggests that these latter processes may have been exapted from the mitotic cycle” (p. 114, Williams & Casanova, 2011).
Which takes us back to our beloved sponge. You remember that the sponge exhibits traits of what we could consider “neurosecretion” even though it doesn’t have cells that resemble neurons or anything that could be regarded as a synapse? Just as the mitotic cycle is reflected in the development of the synapse, perhaps these early forms of neurosecretion are the predecessors of the nervous system. While it’s lovely to sit back in an armchair and conjecture theory, let’s rely on some basic observations to inform our opinions:
First, we know that sponges don’t have a nervous system, they don’t have anything that could be regarded as musculature, and they’re definitely multicellular with some differentiation in various cell types but not anything we would define strictly as “tissues”. Now the basic purpose of a nervous system is to process sensory information and communicate a needed response to the musculature. Using the sea squirt as a good example for contrast, this tunicate has a tail that allows it to swim about in its larval stage, it also has a very primitive eye, and it has a nerve cord to provide communication between those two systems: incoming sensory, outgoing motor. But once it makes its permanent home and becomes sessile, its tail disappears, its eye disappears, and most interestingly, its nervous system disappears. All of it is apoptosed so that all its energies can be devoted to digestion. An adult sea squirt is basically a big tube with stomach and intestines.
But even though the sponge doesn’t have a nervous system in order to coordinate sensation with movement nor any muscles to carry out such movements, many sponges are capable of basic forms of contraction. Nikel (2010) gives his reasons as to why such contractions are suggestive of an integrated, communicative pre-nervous system:
1) These contractions usually occur rhythmically.
2) Many sponges react to external mechanical stimulation by immediately contracting.
3) Certain chemical transmitters can be used to induce contraction in sponges in vitro.
Presumably these contractions are carried out through coordinated paracrine communications between cells, reminiscent of our own nervous system. Therefore, even though sponges don’t have a nervous system or muscles sensu stricto, they have many systems already in place which are suggestive of a proto-nervous system, sensu lato. Behaviorally and chemically, sponges fulfill the basic requirements of a nervous system without having a nervous system: to communicate and coordinate incoming sensory information with movement.
Okay, so back to the title of this post, because I’m sure you’re all dying to hear the punch line. Stop me if you’re heard this one: a sea squirt and a sponge walk into a bar. They sit down, both order a beer, and the sea squirt decides to strike up a conversation.
“So, tell me who’s your favorite character in The Wizard of Oz.”
“Oh, the Scarecrow, definitely,” the sponge says. “If I only had a brain!”
Waaa waaa waaaaaaaa. 
