Other Minds: The Octopus and the Evolution of Intelligent Life. Peter Godfrey-Smith

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СКАЧАТЬ 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 properly during cell division. Usually, when a single-celled organism divides into two, the daughters go their separate ways, but not always. Imagine a ball of cells that forms when one cell divides and the results stay together – and the process repeats several times. The cells in the clump probably ate bacteria as they hovered together in the sea.

      The next stages in the history are unclear; a couple of rival scenarios are on the table, based on different kinds of evidence. In one scenario, perhaps the majority view, some of these balls of cells forsook their suspended life and settled on the sea floor. There they began feeding by filtering water through channels in their bodies; the result was the evolution of the sponge.

      A sponge? It seems that one could hardly pick a more implausible ancestor: sponges, after all, do not move. They look like an immediate dead end. However, only the adult sponge is stationary. The babies, or larvae, are another matter. They are often swimmers, who search for a place to settle and become an adult sponge. Sponge larvae have no brains, but they have sensors on their bodies sniffing their world. Perhaps some of these larvae opted to keep swimming, rather than settle down. They remained mobile, became sexually mature while suspended in the water, and began a new kind of life. They became the mothers of all the other animals, leaving their relatives fixed to the sea floor.

      The scenario I just described is motivated by the view that sponges are the living animals most distantly related to us. Distant does not mean old; present-day sponges are the products of as much evolution as we are. But for various reasons, if sponges did branch off very early, they are thought to offer clues to what the earliest animals were like. Recent work, however, suggests that sponges might not, after all, be the animals most distantly related to us; instead, this title may belong to the comb jellies.

      A comb jelly, or ctenophore, looks like a very delicate jellyfish. It’s an almost transparent globe, with colorful bands of hair-like strands running down its body. Comb jellies have often been seen as cousins of jellyfish, but the observable similarities might be misleading; they might have split off from the line leading to other animals even before sponges did. If this is true, it does not mean that our ancestor looked like a present-day comb jelly. But the comb jelly scenario does motivate a different picture of the early evolutionary stages. Again we start with a clump of cells, but then imagine that this clump folds into a filmy globe-like form, and swims in a simple rhythm as it lives suspended in the water column. The evolution of animals proceeds from there – from a hovering ghost-like mother, rather than a wriggling sponge larva who refused to settle down.

      When multicellular organisms arise, the cells that were once organisms in their own right begin to work as parts of larger units. If the new organism is to be any more than a clump of cells glued together, it requires coordination. Earlier I described the forms of sensing and acting seen in single-celled life. In multicellular organisms, these sensory and behavioral systems become more complicated. Further, the very existence of these new entities – animal bodies – depends on those capacities for sensing and action. Sensing and signaling between organisms gives rise to sensing and signaling within them. The “behavioral” capacities of cells that once lived as whole organisms become the basis for coordination within the new multicellular organism.

      Animals give that coordination several roles. One role is seen also in other multicellular organisms, such as plants: signaling between cells is used to build the organism, to bring it into being. Another role exists on a faster time scale, and is especially characteristic of animal life. In all but a few animals, the chemical interactions between some cells become the basis for a nervous system, small or large. And in some of these animals, a mass of such cells concentrated together, sparking in a chemo-electrical storm of repurposed signaling, become a brain.

      ~ Neurons and Nervous Systems

      A nervous system is made of many parts, but the most significant are the unusually shaped cells called neurons. Their long strands and elaborate branchings form a maze through our heads and bodies.

      The activity of neurons depends on two things. One is their electrical excitability, seen especially in the action potential, an electrical spasm that moves along a cell in a chain reaction. The other is chemical sensing and signaling. A neuron will release a tiny spray of chemicals into the gap or “cleft” between it and another neuron. These chemicals, when they are detected at the other side, can help trigger (or in some cases suppress) an action potential in that adjoining cell. This chemical influence is the residue of ancient signaling between organisms, pressed inward. The action potential, too, existed in cells before animals evolved, and exists today outside them. The first one ever measured, in fact, was in a plant, the Venus flytrap, at the instigation of Charles Darwin in the nineteenth century. Even some single-celled organisms have action potentials.

      What nervous systems make possible is not cell-to-cell signaling itself – that is common – but particular kinds of signaling. Nervous systems are fast, first of all. Except in a few cases like the Venus flytrap, plants act on a slower time scale. Second, the neuron’s long, tenuous projections enable one cell to reach some distance through the brain or body and affect just a few distant cells; influence is targeted. Evolution has transformed cell-to-cell signaling from an activity in which cells simply broadcast their signals to whoever is close enough and listening into something different: an organized network. In a nervous system like our own, the result is a continual electrical clamor, a symphony of tiny cellular fits, mediated by sprays of chemicals across the gaps where one cell reaches out to another.

      This internal tumult is also expensive. Neurons cost a great deal of energy to run and maintain. Creating their electrical spasms is like the continual charging and discharging of a battery, hundreds of times each second. In an animal like us, a large proportion of the energy taken in as food, nearly a quarter in our case, is spent just keeping the brain running. Any nervous system is a very costly machine. Soon I’ll turn to the history of this machine, when it might have evolved and how. First, I’ll spend some time on a general question about why.

Why is it worth having such a brain, or any nervous system? What are they for? As I see it, two pictures guide people’s thinking СКАЧАТЬ