Other Minds: The Octopus and the Evolution of Intelligent Life. Peter Godfrey-Smith
they were really a departure from a “single-celled” mode of life, but they were not organized in anything like the way that our animal bodies are organized.
When picturing this world, we might also presume that because there are no animals, there’s no behavior, and no sensing of the world outside. Again, not so. Single-celled organisms can sense and react. Much of what they do counts as behavior only in a very broad sense, but they can control how they move and what chemicals they make, in response to what they detect going on around them. In order for any organism to do this, one part of it must be receptive, able to see or smell or hear, and another part must be active, able to make something useful happen. The organism must also establish a connection of some sort, an arc, between these two parts.
One of the best-studied systems of this kind is seen in the familiar E. coli bacteria, which live in vast numbers inside and around us. E. coli has a sense of taste, or smell; it can detect welcome and unwelcome chemicals around it, and it can react by moving toward concentrations of some chemicals and away from others. The exterior of each E. coli cell has an array of sensors – collections of molecules bridging the cell’s outer membrane. That’s the “input” part of the system. The “output” part is composed of flagella, the long filaments with which the cell swims. An E. coli bacterium has two main motions: it can run or tumble. When it runs, it moves in a straight line, and when it tumbles, as you might expect, it randomly changes direction. A cell continually switches between these two activities, but if it detects an increasing concentration of food, its tumbling is reduced.
A bacterium is so small that its sensors alone can give it no indication of the direction that a good or bad chemical is coming from. To overcome this problem, the bacterium uses time to help it deal with space. The cell is not interested in how much of a chemical is present at any given moment, but rather in whether that concentration is increasing or decreasing. After all, if the cell swam in a straight line simply because the concentration of a desirable chemical was high, it might travel away from chemical nirvana, not toward it, depending on the direction it’s pointing. The bacterium solves this problem in an ingenious manner: as it senses its world, one mechanism registers what conditions are like right now, and another records how things were a few moments ago. The bacterium will swim in a straight line as long as the chemicals it senses seem better now than those it sensed a moment ago. If not, it’s preferable to change course.
Bacteria are one among several kinds of single-celled life, and they are simpler in many ways than the cells that eventually came together to make animals. Those cells, eukaryotes, are larger and have an elaborate internal structure. Arising perhaps 1.5 billion years ago, they are the descendants of a process in which one small bacterium-like cell swallowed another. Single-celled eukaryotes, in many cases, have more complicated capacities to taste and swim, and they also edge close to a particularly important sense: vision.
Light, for living things, has a dual role. For many it is an intrinsically important resource, a source of energy. It can also be a source of information, an indicator of other things. This second use, so familiar to us, is not easily achieved by a tiny organism. Much of the use of light by single-celled organisms is for solar power; like plants, they sunbathe. Various bacteria can sense light and respond to its presence. Organisms so small have a difficult time determining the direction light is coming from, let alone focusing an image, but a range of single-celled eukaryotes, and perhaps a few remarkable bacteria, do have the beginnings of seeing. The eukaryotes have “eyespots,” patches that are sensitive to light, connected to something that shades or focuses the incoming light, making it more informative. Some eukaryotes seek light, some avoid it, and some switch between the two; they follow light when they want to take in energy, and avoid it when their energy supplies are full. Others seek out light when it is not too strong and avoid it when the intensity becomes dangerous. In all these cases, there is a control system connecting the eyespot with a mechanism that enables the cell to swim.
Much of the sensing done by these tiny organisms is aimed at finding food and avoiding toxins. Even in the earliest work on E. coli, though, it seemed that something else was going on. They were also attracted to chemicals they could not eat. Biologists who work on these organisms are more and more inclined to see the senses of bacteria as attuned to the presence and activities of other cells around them, not just to washes of edible and inedible chemicals. The receptors on the surfaces of bacterial cells are sensitive to many things, and these include chemicals that bacteria themselves tend to excrete for various reasons – sometimes just as overflow of metabolic processes. This may not sound like much, but it opens an important door. Once the same chemicals are being sensed and produced, there is the possibility of coordination between cells. We have reached the birth of social behavior.
An example is quorum sensing. If a chemical is both produced and sensed by a particular kind of bacterium, it can be used by those bacteria to assess how many individuals of the same kind are around. By doing this, they can work out whether enough bacteria are nearby for it to be worthwhile to produce a chemical that does its job only if many cells make it at once.
An early case of quorum sensing to be uncovered involves – appropriately for this book – the sea and a cephalopod. Bacteria living inside a Hawaiian squid produce light by a chemical reaction, but only if enough other bacteria are around to join in. The bacteria control their illumination by detecting the local concentration of an “inducer” molecule, which is made by the bacteria and gives each individual a sense of how many potential light producers are around. As well as lighting up, the bacteria follow the rule that the more of this chemical you sense, the more you make.
When enough light is being produced, the squid who house the bacteria gain the benefit of camouflage. This is because they hunt at night, when moonlight would normally cast their body’s shadow down to predators below. Their internal lights cancel the shadow. Meanwhile, the bacteria seem to benefit from the hospitable living quarters provided by the squid.
This aquatic setting is the right one to have in mind when thinking about these early stages in life’s history – though in the evolutionary story we are at a point long before there were any squid. The chemistry of life is an aquatic chemistry. We can get by on land only by carrying a huge amount of salt water around with us. And many of the evolutionary moves made at these early stages – those giving birth to sensing, behavior, and coordination – would have depended on the sea’s free movement of chemicals.
So far, all the cells we’ve met are sensitive to external conditions. Some also have a special sensitivity to other organisms, including organisms of the same kind. Within that category, some cells show a sensitivity to chemicals that other organisms make to be perceived, as opposed to chemicals made as mere byproducts. That last category – chemicals that are made because they’ll be perceived and responded to by others – brings us to the threshold of signaling and communication.
We’re arriving at two thresholds, though, not one. In a world of single-celled aquatic life, we’ve seen how individuals can sense their surroundings and signal to others. But we’re about to look at the transition from single-celled life to many-celled life. Once that transition is under way, the signaling and sensing that connected one organism to another become the basis of new interactions which take place within the new forms of life now emerging. Sensing and signaling between organisms gives rise to sensing and signaling within an organism. A cell’s means for sensing the external environment become a means to sense what other cells within the same organism are up to, and what they might be saying. A cell’s “environment” is largely made up of other cells, and the viability of the new, larger organism will depend on coordination between these parts.
~ Living Together
Animals are multicellular; we contain many cells that act in concert. The evolution of animals began when some cells submerged their individuality, becoming parts of large joint ventures. The transition to a multicellular form of life occurred many times, leading once to animals, once to plants, on other occasions to fungi, various seaweeds, and less conspicuous organisms. Most likely, the origin of animals did not stem from a meeting between lone cells who drifted together. Rather, animals arose from a cell whose daughters did not separate