We don’t have much in common with the sea sponge, a sedentary animal so primeval that not only does it not have a brain, it doesn’t even have tissues. It has no organs. Sea sponges have no nervous system, no digestive tract and no circulation. They have no muscles. The sea sponge is so primitive it makes a coral look advanced, and it’s been around at least 600 million years.
The sponge isn’t even a metazoan, which is any multicellular animal whose cells are organized in tissues and organs. Metazoans are thought to have evolved from single-celled animals that discovered that the group life – still single animals, but bunching together – had advantages.
But the sponge, seemingly a sac of skin and not much else, does have some specialist cells. In fact, the animal has 18 types of cells, according to a new paper in Science, which is a lot more than had been thought. Still not muscles or nerves, but wait for it.
Note that author Jacob Musser of the European Molecular Biology Laboratory and colleagues worked on a freshwater sponge. Most sponges are sea creatures, but not all. The team’s study of the sponge’s cell types, coupled with genetics, gave them the insight: an animal like this may have been the starting point for the evolution of the nervous system, which is basically a collection of cells that communicate with one another.
Thus, the wonder that is our brain, which may have shrunk about 3,000 years ago but is still pretty impressive, may have begun way back in a tiny primordial animal that ate – can we really say it “felt hungry” – and then needed to void. Makes sense when you think about it.
Water in, garbage out
A sea sponge is a multicellular balloon of double-layered skin: inner layer and outer layer. Seawater enters through pores in that skin, carrying along nutrients and oxygen. Inside, the sponge’s void has hairlike filaments that speed things along. And from time to time, the sponge contracts its version of a body to flush debris from the inner void. Lacking a digestive system, it has no anus.
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They eat by absorbing nutrients from the water. But what mechanism exactly is telling the sponge it’s time to flush?
It turns out that even though the sponge has a very simple body organization, its genetic repertoire is quite complex. They have some genes expressed in the nerve cells or muscles of metazoans – including genes involved in creating synapses: the inter-neuron junctions through which nerve cells communicate.
Anyway, these genes expressed in our bodies today seem to have originated in said sponge, or a common ancestor to us and said sponge. What role do these genes play in the micro-beast? They are involved in feeding and flushing. Among the specialist cells the authors identified are flat cells both outside and inside the skin, called pinacocytes. Sort of proto-muscles, they play a role in the animal contracting and expanding. Intriguingly, pinacocytes aren’t sponge-specific and may therefore have evolved even earlier.
They also found phagocytes, which are the amorphous (shape-shifting) cells that actually engulf and digest the nutrients gleaned from the water. That is how they eat. Your white blood cells are also phagocytes, eating the germs they locate in your bloodstream.
They found nerve-like cells (called neuroids) closely associated with the digestive choanocytes, the hairlike filaments that filter the water for toothsome microorganisms. The neuroids express genes related to communication between our nerve cells, i.e,. synaptic communication.
We are starting to see where all this is going. The sponge may not have tissues or neurons, but genes active in specialist cells involved in its feeding and cleansing are today related to the functionality of our muscles and brains.
The 18 types of cells in the freshwater sponge, many of which hadn’t been known before, were detected by Musser and the team using “whole-body single-cell RNA sequencing,” correlated by x-ray and electron microscopy. Thus, they deduced that the sponge has cellular communication around the digestive chambers, regulating the feeding and control of its interior environment.
How is it that genes in a primordial animal that lived 600 million years ago still exist in advanced animals? Conservation. If a gene fulfills a role that is important enough, and if change in its form (mutation) causes it to become nonfunctional, and if its non-functionality is fatal – enough to cause the animal not to survive – then the gene will persist unchanged over the generations, down the centuries and eons, because mutants in that gene will die.
One example of an extremely conserved gene is ubiquitin, which is practically identical in all organisms because if it changes, survival is not possible. Ubiquitin, a protein involved in cellular protein regulation, is so old it makes the Precambrian sponge look like a puling child. It isn’t that it didn’t evolve, it’s more that it can’t evolve.
“Around [2.5 billion] years ago, a unicellular organism with radically novel features, ancestor of all eukaryotes, dwelt the earth,” wrote Alice Zuin et al in 2014. Eukaryotes are all beings with cellular nucleus (the others being bacteria, archaea and viruses).
This cell was the last common ancestor to us and the T. rex and the palm tree, etc. And it had the same functionally capable ubiquitin molecule that all eukaryotic species contain today. The same. Ubiquitin systems have enormously expanded in the last 2.5 billion years, involving hundreds of enzymes and insanely complicated processes. But the simplest genetic arrangement encoding a fully-equipped ubiquitin signaling system is five genes in an archaea (which used to be called archaebacterial).
Back to the sea sponge and your brain. Evidently, some characteristics go back to the dawn of life itself, not that we know when that happened. But genes coding for at least some synaptic proteins go back at least to that sponge.
It bears adding that maybe neurons had more than one avenue of evolution. The comb jelly has such a radically different nervous system than anything else that some suspect neurons evolved (at least) twice, once in the sponge and once in the comb jelly.
So is this the end of the debate? Did that spark of connectivity begin in the belly of a sponge? Did our philosophy originate in a hungry little sac of skin? What do comb jellies have to do with anything? Stay tuned.