LITMIR.BIZ

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body plan is friendly, it turns out, to many kinds of complex behavior. The diversification and entanglement of lives that took place in the Cambrian was mostly the work of bilaterians.

      Before pressing on into the world of bilaterian evolution, let’s pause and ask: which animal produces the most sophisticated behavior, which is the smartest, without a bilaterian body plan? Questions like this are notoriously hard to answer in an unbiased way, but in this case, the answer is clear. The most behaviorally sophisticated animals outside the bilaterians are the – terrifying – box jellyfish, the Cubozoa.

      With their soft bodies and sparse fossil record, it is hard to work out when different kinds of jellyfish evolved, but cubozoans are thought to be late arrivals, originating in the Cambrian or after. A general feature of cnidarians, as I noted above, is their stinging cells. Some cubozoans have truly brutal venom in their stingers, strong enough to have killed large numbers of humans. In northeastern Australia, the presence of box jellyfish clears the beaches completely each summer; for a good part of the year it’s too dangerous to swim off the shore at all, except in netted enclosures. To compound the problem, these jellyfish are invisible in the water. They also have the most complex behaviors of any non-bilaterian. Around the top of their body are two dozen sophisticated eyes – eyes with lenses and retinas, like ours. The Cubozoa can swim at about three knots, and some can navigate by watching external landmarks on the shore. Box jellyfish, the lethal behavioral pinnacle of non-bilaterian evolution, are also products of the new world that began in the Cambrian.

      ~ Senses

      Nervous systems evolved before the bilaterian body plan, but this body created vast new possibilities for their use. During the Cambrian the relations between one animal and another became a more important factor in the lives of each. Behavior became directed on other animals – watching, seizing, and evading. From early in the Cambrian we see fossils that display the machinery of these interactions: eyes, claws, antennae. These animals also have obvious marks of mobility: legs and fins. Legs and fins don’t necessarily show that one animal was interacting with others. Claws, in contrast, have little ambiguity.

      In the Ediacaran, other animals might be there around you, without being especially relevant. In the Cambrian, each animal becomes an important part of the environment of others. This entanglement of one life in another, and its evolutionary consequences, is due to behavior and the mechanisms controlling it. From this point on, the mind evolved in response to other minds.

      When I say that, you might reply that the term “mind” is out of place. In this chapter, I won’t argue with that. Fine. What is the case, though, is that the senses, the nervous systems, and the behaviors of each animal began to evolve in response to the senses, nervous systems, and behaviors of others. The actions of one animal created opportunities for and demands on others. If a yard-long, fast-swimming anomalocarid is swooping down toward you, like a giant predatory cockroach with two grasping appendages on its head poised and ready, it’s a very good thing to know, somehow, that this is happening, and to take evasive action.

      The senses may well have been crucial to the Cambrian: organisms opened up to the world, especially to each other. The first sophisticated eyes seem to have appeared, eyes that can form an image. The Cambrian witnessed the appearance of both the compound eyes seen today in insects and camera eyes like our own. Imagine the behavioral and evolutionary consequences of being able to see the objects around you for the first time, especially objects at some distance and in motion. The biologist Andrew Parker has argued that the invention of eyes was the decisive event in the Cambrian. Others have developed broader views, but with a similar flavor. As the paleontologist Roy Plotnick and his colleagues put it, the result of this sensory opening was a “Cambrian information revolution.” With an influx of sensory information comes a need for complex internal processing. When more is known, decisions become more complicated. (Is the anomalocarid more likely to intercept me if I flee to that hole, or that other one?) An image-forming eye makes possible actions that would be unthinkable without it.

      Jim Gehling, my Ediacaran guide, and the British paleontologist Graham Budd have offered scenarios for how the feedback process generating these changes got under way. Near the close of the Ediacaran, Gehling suspects that scavenging arose, followed by predation. Animals went from feeding on microbial mats to feeding on the dead, and then began hunting the living. As Budd sees it, animal behavior itself changed the way resources were distributed in the Ediacaran. Imagine a world with edible microbial mats stretching before you like an endless swampy lawn. Slow-moving grazers wander over the mats, consuming this rather uniform resource. Other animals fed without moving. These animals then become a new kind of resource; they are big concentrations of nutritious carbon compounds. Nutrition is now less spread out than it was. It exists in patches. These animals might first have only been consumed by others after they had died. But this soon changed. Scavenging became predation.

      If the fossil record is taken at face value, it seems that one group set the pace: the arthropods. This group today includes insects, crabs, and spiders. Early in the Cambrian we see the rise of trilobites, which are prototypical arthropods with shells, jointed legs, and compound eyes. In the photograph of the Dickinsonia fossil on page 29, you’ll find two much smaller fossils just below it, above the letters “A” and “B.” These animals are just millimeters long, and Gehling thinks they might be precursors of trilobites – still soft-bodied, but with hints of a trilobite design. In this picture, Dickinsonia is present in its classic Ediacaran mode, with no apparent limbs, head, or protection, while purposeful little bugs lurk beneath. The image reminds me of a drawing in a book about the dinosaurs and their decline that I owned as a child. A huge dinosaur towered over a few small and mischievous-looking mammals, shrew-like creatures, at its feet. I think they had their eye on a clutch of dinosaur eggs. The trilobite precursors look intent on a similar goal, with the lilypad-bathmat Dickinsonia oblivious above.

      Michael Trestman, another philosopher, has offered an interesting way of looking at all these animals. Consider, he says, the category of animals who have complex active bodies. These are animals who can move quickly, and who can seize and manipulate objects. Their bodies have appendages that can move in many directions, and they have senses, such as eyes, which can track distant objects. Trestman says that only three of the major animal groups produced some species with complex active bodies (CABs). Those groups are arthropods, chordates (animals like us with a nerve cord down their back), and one group of mollusks, the cephalopods. This trio might seem to make up a large category, because these are the sorts of animals that tend to come to our minds, but it is a small group in many ways. There are about thirty-four animal phyla – basic animal body plans. Only three phyla contain some animals with CABs, and within one of those three, the mollusks, the only animals that count are cephalopods.

      With these ancient stages of the historical story in place, I’ll return to the divide between two views of nervous systems and their evolution – the sensory-motor and action-shaping views. Earlier I introduced the distinction, linked it to two roles that signals can have in social life (sexton and Revere versus the rowboat), and noted that the two roles are different but also compatible. What might be the historical significance of this divide? Can the distinction be fit in some natural way onto the march of millennia from the Ediacaran, to the Cambrian, to more recent times? It does seem possible that there was a shift in the roles nervous systems were performing. Although tracking events in the outside world might always be worth doing to some extent, the Cambrian sees a great increase in the importance of this side of life. There’s more that’s worth watching, and more that needs to be done in response to what’s seen. Not paying attention, for the first time, means getting eaten by the swooping anomalocarid. Perhaps, then, the very first nervous systems primarily served to coordinate actions – first animating the body of an ancient cnidarian, then shaping the actions of Ediacarans. But if there was such an era, by the Cambrian it was over.

      This is one possibility among many, though, and our imaginations, shaped by lives lived in modern bodies, underestimate the range of options. Possibilities abound. Here is one developed by the biologist Detlev Arendt and his colleagues. As they see it, nervous systems originated twice. But they don’t mean that they evolved in two kinds of animals; rather, they originated

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