I previously offered a puzzle to think about:
You are given two flasks, A and B, each filled with a 10 mls of a solution that contains all the nutrients needed for Tetrahymena to survive and grow.
On Friday, you are given a tube that is filled with thriving Tetrahymena and transfer 10,000 of these cells to flask A. Drawing from the same tube, you then add 1,000 cells to flask B. You go home and celebrate Easter, returning to the lab on Monday. You find the cells in flask A to be thriving, having spawned millions of new cells. But when you look at flask B, the cells have not grown and divided. On the contrary, they all died!
Why did the cells in flask B all die in a sea of food?
The only difference between the two flasks if the density of cells. Flask A starts with 10,000 cells in 10 mls, thus it has a density of 1000 cells/ml. Flask B has a density of 100 cells per ml. Why is this significant?
If we think in terms of front-loading, perhaps we should look to complex, metazoan features to explain this protozoan phenomenon. And sure enough, this density-dependent survival is also well known among scientists who culture mammalian cells. In other words, if you take a few human cells and put them into a Petri dish with lots of nutrients, those cells will also die instead of developing into a massive population of cells. So why is this?
Consider this figure, something you might find in any textbook on human physiology:
So just as the cells of your body communicate with each other, and control each other with secreted factors and hormones, the single-celled Tetrahymena do likewise. When the cell density of Tetrahymena falls below 750 cells/ml (empirically determined), there is an insufficient concentration of some secreted factor that says, “Live!” In other words, what we commonly call growth factors. So not only do these single-celled organisms require nutrients to survive, they also need some growth factors which are provided by a population of fellow Tetrahymena cells. It’s not just the environment that matters.
If you think about this, that simple fact can really help us re-orient our thinking about life. First, there doesn’t seem to be any good reason for thinking this phenomenon is an artifact of existing in the lab. When it comes to the need for growth factors among cultured mammalian cells, we know the same requirement exists in mammalian bodies. So out in the wild, the same cell density rule may very well apply for Tetrahymena. Which would mean that they don’t truly exist as solitary scavengers, but instead they travel in packs. I’m not sure anyone has every studied Tetrahymena in the wild (can it even be done?), but it is neat to think of these cells as moving through a puddle as a pack, looking for some prey.
What’s more, this insight should challenge the popular reductionist notion of “unicellular life.” If some Tetrahymena cell strayed too far from the pack, to experience the environment truly as a single cell, it would die. This means that while Tetrahymena are not physically connected to each other, they are chemically connected. That is, their existence is effectively multicellular.
Take it to the next step. What if this dependence on communication from other cells is not particular to Tetrahymena, but is actually a universal feature of all eukaryotic cells? In fact, what if it is a universal feature of all cells? If this is the case people, multicellularity is as old as life itself. Meaning that there was no need to frontload the appearance of the multicellular state. It was always there. Instead, front-loading would involve ways to nudge and exploit this inherent tendency for multicellularity toward some objective. An objective that might naturally flow from this biotic inherency. Perhaps, something like…..metazoan-type complexity?
But let’s not so quickly leave our Tetrahymena pack behind. There’s more to this story.