Thinking Beyond Science

About 600 Million years ago no one knows what the animals in question looked like in any detail, but they perhaps had the form of small, flattened worms. They may have been just millimeters long, perhaps a little larger. They might have swum, might have crawled on the sea floor, or both. They might have had simple eyes, or at least light-sensitive patches, on each side. If so, little else may have defined “head” and “tail.” They did have nervous systems.

These might have comprised nets of nerves spread throughout the body, or they might have included some clustering into a tiny brain. What these animals ate, how they lived and reproduced —all are unknown. But they had one feature of great interest from an evolutionary point of view, a feature visible only in retrospect. These creatures were the last common ancestors of yourself and an octopus, of mammals and cephalopods. They’re the “last” common ancestors in the sense of most recent, the last in a line.

The history of animals has the shape of a tree. A single “root” gives rise to a series of branchings as we follow the process forward in time. One species splits into two, and each of those species splits again (if it does not die out first). If a species splits, and both sides survive and split repeatedly, the result may be the evolution of two or more clusters of species, each cluster distinct enough from the others to be picked out with a familiar name- the mammals, the birds. The big differences between animals alive now-between beetles and elephants, for example— originated in tiny insignificant splits of this sort, many millions of years ago. A branching took place and left two new groups of organisms, one on each side, that were initially similar to each other, but evolved independently from that point on.

For most of the Earth’s history, then, there was life, but no ani-mals. What we had, over vast stretches of time, was a world of single-celled organisms in the sea. Much of life today goes on in exactly that form.

When picturing this long era before animals, one might start by visualizing single-celled organisms as solitary beings: countless tiny islands, doing nothing more than floating about, taking in food (somehow), and dividing into two. But single-celled life is, and probably was, far more entangled than that; many of these organisms live in association with others, sometimes in mere truce and coexistence, sometimes in genuine collaboration. Some of the early collaborations were probably so tight that 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.

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 organ-ism. 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 an 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.

Source : Other Minds by Peter Godfrey-Smith

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