as the problem of teamwork. The players on a football team must combine their actions into a whole, and at least in some kinds of football, this would be a substantial task even if the other team always never varied its moves. An orchestra must solve the same problem. The problem that teams and orchestras face is confronted by some individual organisms, too. This issue is largely peculiar to animals; it’s a problem for multicellular organisms, not single-celled ones, and only a problem for those multicellular organisms whose lifestyle involves complex actions. It’s not much of a problem for bacteria, and not a big problem for seaweed.
Above I treated interactions between neurons as a kind of signaling. Though the analogy is not complete, it is helpful again here as a way of understanding these two visions of the role of early nervous systems. Recall the story of the ride of Paul Revere at the start of the American Revolution in 1775, as told (with considerable poetic license) by Henry Wadsworth Longfellow. The sexton of the Old North Church in Boston was able to observe the movements of the British Army, and he used a lantern code to send a message to Paul Revere (“one if by land; two if by sea”). The sexton was like a sensor, Revere like a muscle, and the sexton’s lantern acted like a nervous connection.
The story of Revere is often used to get people to think about communication in an exact way. And so it does. But it also nudges us toward thinking about a particular kind of communication, which solves a particular kind of problem. Consider a different, though still familiar, situation. Suppose you are in a boat with several rowers, each with one oar. The rowers together can propel the boat forward, but even if they are vigorous, their individual actions will not get the boat to go anywhere unless they coordinate what they’re doing. It doesn’t matter exactly when they pull their oar, as long as they pull at the same time. One way to deal with this situation is to have someone call the “stroke.”
Communication in everyday life serves both roles: there is a sexton-and-Revere or sensory-motor role, based on a division between those who see and those who act, and there is a purely coordinative role, as seen in the rowers. Both of these roles can be played at the same time and there’s no conflict between them. Getting a boat to move requires the coordination of micro-actions, but someone also needs to watch where the boat is going. The person calling the stroke, the coxswain or “cox,” usually acts as the crew’s eyes and as a coordinator of micro-actions. The same combination can be seen in a nervous system.
Though there’s no essential clash between these roles, the distinction itself is important. Through much of the twentieth century, a sensory-motor view of the evolution of nervous systems was simply assumed, and it took some time for the second view, the one based on internal coordination, to become clear. Chris Pantin, an English biologist, developed the second view in the 1950s and it has been revived recently by Fred Keijzer, a philosopher. They rightly point out that it’s easy to fall into the habit of thinking of each “action” as a single unit, in which case the only problem left to solve is coordinating these acts with the senses, working out when to do X rather than Y. As organisms get bigger and can do more, that picture becomes more and more inaccurate. It ignores the problem of how an organism is able to do X or Y in the first place. Pressing an alternative to the sensory-motor theory was a good thing. I’ll call this the action-shaping view of the role played by early nervous systems.
Returning to the history, what did the first animals with nervous systems look like? How should we picture their lives? We don’t yet know. Much of the research in this area has been focused on the cnidarians (pronounced “nye-dair-ians”), a group of animals that includes jellyfish, anemones, and corals. They are very distantly related to us, but not as distantly as sponges, and they do have nervous systems. Though the early branchings in the tree of animals remain murky, it is common to think that the animal with the first nervous system might have been jellyfish-like – something soft, with no shell or skeleton, probably hovering in the water. Picture a filmy lightbulb in which the rhythms of nervous activity first began.
This might have occurred something like 700 million years ago. That date is based entirely on genetic evidence; there are no fossils of animals this old. From looking at rocks of this age, you’d think that all was still and silent. But DNA evidence strongly suggests that many of the crucial branching points in the history of animals must have occurred around that time, and that means that animals were doing something back then. The uncertainty about these crucial stages is frustrating for someone who wants to understand the evolution of brains and minds. As we get a little closer to the present, the picture starts to become clearer.
~ The Garden
In 1946, an Australian geologist, Reginald Sprigg, was exploring some abandoned mines in the outback of South Australia. Sprigg had been sent to find out whether some of the mines might be worth working again. He was several hundred miles from the nearest sea, in a remote area called the Ediacara Hills. Sprigg was eating his lunch, the story has it, when he turned over a rock and noticed what looked like some delicate fossils of jellyfish. As a geologist, he knew the rocks were so old that the finding was important. But he was not an established researcher of fossils, and when he wrote up his paper, few people took it seriously. The journal Nature rejected it, and Sprigg then worked his way down from journal to journal, until his article on what he called “Early Cambrian (?) Jellyfishes” appeared in the Transactions of the Royal Society of South Australia in 1947, alongside such papers as “On the Weights of Some Australian Mammals.” The paper had a quiet career at first, and it took another decade or so before anyone realized what Sprigg had found.
At the time, scientists familiar with the fossil record were well aware of the importance of the Cambrian period, which began about 542 million years ago. In the “Cambrian explosion,” a great range of the animal body plans we know today first appeared. Sprigg’s discoveries turned out to be the first fossil record of animals living before that time. Sprigg did not realize this in 1947 – he dated his jellyfish as early Cambrian. But as similar fossils were found in other places around the world and people took more note of Sprigg’s outback jellyfish, it became clear that they dated from well before the Cambrian, and were probably not jellyfish, in most cases, at all. The period in prehistory now known as the Ediacaran (named after the hills Sprigg was exploring, and pronounced “Eedee-ac-aran”) runs from 635 to about 542 million years ago. With the Ediacaran fossils we get our first direct evidence of what the lives of very early animals might have been like – how big they were, how numerous, how they lived.
The nearest large city to Sprigg’s site is Adelaide, where a large collection of Ediacaran fossils is kept in the South Australian Museum. I was shown around the exhibits by Jim Gehling, who knew Sprigg and has worked on the fossils since 1972. I was surprised at how dense with life the ancient environment was; the Ediacaran was not about a few lone individuals. Many rock slabs Gehling has collected contain dozens of fossils of different sizes. Among the more prominent is Dickinsonia, which has fine stripe-like segments and looks a bit like a lily pad, or a bath mat. (A picture of a Dickinsonia in the South Australian Museum’s collection appears just below this paragraph.) But if you focus on the large fossils, you miss most of the life that is present. Several times, Gehling walked up to what looked like a scrappy and nondescript bit of one of the rocks and pressed a piece of Silly Putty into it; when he took it back, the putty revealed a fine and detailed imprint of a tiny animal.
Ediacaran animals weren’t tiny – many were several inches in length, some up to three feet. They seem to have mostly lived on the sea floor, on and amid mats made of living material – clods of bacteria and other microbes. Their world was a kind of undersea swamp. Many were probably motionless as adults, anchored in place. Some might have been early sponges and corals. Others had body forms that have since been entirely abandoned by evolution – three-sided and four-sided designs, some with quilted arrangements of plant-like fronds. Many Ediacarans seem to have lived quiet lives of very limited mobility on the bottom of the sea.
DNA evidence, though, suggests strongly that there were nervous systems present at this time – probably in some of the animals on the wall in Adelaide. Which ones? Among them are some animals who appear to have moved under their own steam. The clearest case