Measuring in at between 1 and 2 millimeters, the stentor is a thousand times longer than most bacteria and a billiontimes the volume. On top of that, for a single cell, it’s extremely complex. Hair-like structures called cilia beat around its mouth—more of an opening, really, than like human puckers—sucking in food like algae, even spitting out bits the cell doesn’t care for. Again, a single cell without a brain of its own.
More remarkable still for something without a central nervous system, it can flee predators, but at the same time ignore repetitive yet harmless stimuli. “If you’re living near the train tracks, you don’t get scared when the train goes by,” says Marshall. “And so a stentor, if you bump it again and again at the same intensity it will just learn to ignore it. It’s learning without a brain.” Should a stentor detect a threat, it’ll fire out a cloud of blue pigment, perhaps as a distraction, like an octopus might ink.
Everything about this organism makes it clear we think too simplistically about how cells develop and function. We have assumed that cells are merely simple machines which can perform some basic task when fed a particular input.
The stentor’s weirdness even penetrates right down to its genome. Last month, Marshall and colleagues sequenced it for the first time, hoping it could give insight into the stentor’s regenerative powers, and revealed particular oddities with what are known as introns. These are spacers within strands of DNA. In human DNA, introns can stretch over a thousand letters. In stentor? It’s 15, the shortest of any known organism.
Why exactly? Add that to the long list of stentor secrets. “One possibility is the cell is just trying to make its genome as compact as it can,” says Marshall. “Now, why that would be in a gigantic cell, I don’t really know.” On top of all that, a stentor stores hundreds of thousands of copies of its genome in its oversized single-cell body. In humans, each cell stores a measly two copies. For the stentor, having so many copies spread throughout its giant body might mean instructions are available wherever an injury occurs.
When we first sequenced the human genome, it was widely assumed that it would be significantly different and more complex than that of other organisms. Much to our surprise, the human genome doesn't seem like anything special on paper, it's not extraordinarily long, and in fact is largely identical to that of a chimpanzee. The closer we look, the more it looks like very subtle changes to the structure of the DNA strand can have profound effects on the developing embryo.
I love learning about organisms such as the stentor as it highlights how dynamic life can be. It's easy to get lost in the assumption that all thought occurs in the brain and that it will be relatively simple to load the connections between the neurons into a computer and suddenly you'll be able to 'live' for ever. I think this organism does a good job of reminding us that the other cells in our bodies are not simple machines which can be cut out of the system and be forgotten. Similar to how DNA expression is incredible subtle and is dramatically altered by the minor details, I think we will discover something similar as we try and "digitize" our consciousness.