Critters with hard shells, like Drepanaspis or any number of mollusks during those ancient times, appear to have been under intense selection pressure from predators, evolving tough armor and sea bottom–loving habits in response.[126] But even shells and armor could be crushed by the giant jawed fishes of the day, like Dunkleosteus, who had the prerequisite size, skeletal structure, and muscle strength to do the job, as shown by Mark Westneat, curator of zoology at the Field Museum of Natural History and director of the Biodiversity Synthesis Center.[127] So if you were smart, in an evolutionary sense, you, as a jawless fish and potential meal, couldn’t rely on armor alone. Best not even to test your armor in the first place. Run!
Cowards get a bad rap. But dying sucks, remember? Cowards survive, for the moment, by running away. Running or swimming or flying away turns out to be the nearly universal response of animals to danger. Only those glued to a rock, hiding in a burrow, blessed with camouflage, or hormone-crazed in mating season don’t flee to escape from danger. As we’ve seen, fish rely on the lateral line to detect dangers, and we built one using an infrared (IR) proximity detector, a small device that emits an IR pulse and uses the time it takes for the pulse to bounce back off an object to calculate the distance to the object. The mechanism is quite different, but the function is the same.[128]
Nicole, our chief engineer on the Tadro4 project, put an IR sensor on each side of PreyRo along with the two photoresistors serving as eyespots (Figure 6.7). The onboard microcontroller continuously samples both kinds of paired sense organs. If you remember the subsumption architecture we described in Chapter 5, then you can probably see the solution. At the default, or lowest level, PreyRo forages and feeds, using the difference in light intensity between the two photoresistors to calculate the direction to the light source. PreyRo is constantly making adjustments in its heading to make that difference zero, where a difference of zero means that it’s headed straight up the light-intensity gradient. And so PreyRo forages to feed on the light. Until, that is, the escape behavior overrides the forage-and-feed behavioral module.
The escape behavior is triggered when either the left or right IR proximity sensor detects an object within a preset threshold distance. If the left sensor triggers, then PreyRo interrupts the forage-and-flee behavior and initiates a fixed turning maneuver to move quickly to the right. The opposite is true if the right sensor triggers. Because this “predator detection threshold” can be altered in the programming of the microcontroller, we could evolve it.
This sensory character gives us a crude way to test Rob’s prediction that the evolution of the vertebrate nervous system—characterized in part by its paired sensory systems—has to evolve first in order to permit vertebrae to evolve. In other words, we predict that the evolution of vertebrae is contingent upon the prior evolution of, in this specific case, the sensitivity of the predator-detection system. This makes functional sense: why would the enhanced propulsion that vertebrae bring ever evolve without some means of detecting when it’s time to use it? The only way you know is to have a sensory system capable of detecting the predators. And although the eyes you use for foraging and feeding give you some predator-detection capabilities, you can’t see at night or in the dark of the deep. A lateral line, however, works anytime and anywhere. But just because a pattern makes functional sense to us doesn’t have any bearing on how and why that pattern actually occurred.
If the proposed functional codependence were true, I’d be sorely tempted to call the pattern “contingent-sequential evolution.” Sorry for the mouth-full phrase, but one can’t be too careful when speaking of evolutionary phenomena. The contingency refers specifically to an identified causal interaction of the characters. Without a causal mechanism, we just have a correlation. Correlations may occur by accident, for no other reason than two unrelated things happen to share a pattern. However, we never want to ignore correlations because functionally codependent systems are always, in some way, correlated.
When the pattern of the evolution of two or more characters are correlated, then that pattern is called—you guessed it—“correlated evolution.” When the correlation has a causal basis, it is called—you’ll never guess it—“concerted evolution.” Maybe you can see why, for a specific sequenced pattern of concerted evolution, I’ve proposed the phrase “contingent-sequential evolution.” Whatever the pattern of concerted evolution, to claim that you have one requires that you show the specific functional codependence between or among the characters. Categories of potential functional mechanism include genetic, developmental, and physiological.
Whereas concerted evolution means that two or more characters evolved because of their interactions, when no interactions are present we call that a pattern of “mosaic evolution.” Mosaic evolution was introduced in Chapter 2 so that we could make the important point that species are not “primitive” or “derived” but are, instead, mosaics of both ancestral and derived characters. Mosaic evolution is a fact of life, but it’s not the only fact of life. Concerted evolution is also a fact. If we look at enough characters, we will see both kinds of character evolution in any species: mosaic and concerted.[129]
In addition to our predicted lateral-line 
vertebrae pattern of sequential-contingent evolution, we also expect concerted evolution within the propulsion system itself. A character of all fishes, extinct and living, that varies like crazy is the shape of the caudal fin. The earliest known vertebrate, Haikouichthys,[130] has a caudal fin that tapers to a point, like an eel. Yet our Tadro4 target, Drepanaspis, has a twolobed caudal fin that splays apart, forming a sharp vertical trailing edge, or “Kutta condition,” to use the hydrodynamic lingo.[131] These two kinds of tails, tapered and splayed, are just some of the kinds that we see. In terms of propulsion, what matters is the length of that trailing edge, measured as the “span” of the caudal fin. The trailing edge is where the body sheds so-called bound vorticity into the water. If we were to revisit Lighthill’s Elongated Body Theory (EBT), which we used earlier to propel our digi-Tad3s, then we’d see that propulsive power generated by the fish is proportional to the square of the tail’s span. That square term is huge: a slightly larger tail span should help produce much more power.
Because both the tail’s span and the vertebral column are involved in generating propulsion, we predict that the two characters will show concerted evolution. Here’s why, specifically. In order for the square-of-the-span magic to work, what I told you above assumes that everything else about the motion of the fish’s body stays the same, including how far it moves its caudal fin side to side, what we measure as the lateral amplitude of the caudal fin. But that caudal fin amplitude will decrease when you put a bigger caudal fin on the body—it has to. It’s like when you, ignoring the better judgment of your parents, used to stick your hand out the window when speeding down the highway. Palm down, hand parallel to the road? No problem. Rotate your hand ninety degrees. Boom! Your hand flies backward and you ram your arm into the window frame. Ouch. The difference is drag, which is low in the first position and high in the second position.
126
C. E. Brett and S. E. Walker, “Predators and Predation in Paleozoic Marine Environments,” in
127
Westneat, an expert on the biomechanics of fish feeding, produced this jaw-dropping analysis: P. S. L. Anderson and M. W. Westneat, “Feeding Mechanics and Bite Force Modeling of the Skull of
128
An important point to keep in mind here, if you want to try this system in your own Evolvabot, is that because Tadros are surface swimmers, the IR sensors mounted above the water line are working in air.
129
To beat a dead horse, as the saying goes, keep in mind that correlated evolution can turn out to be either concerted or mosaic. In spite of this flogging, you shouldn’t be deterred from investigating this character evolution approach. Here’s a great place to start: Michael I. Coates and Martin J. Cohn, “Developmental and Evolutionary Perspectives on Major Transformation in Body Organization—Vertebrate Axial and Appendicular Patterning: The Early Development of Paired Appendages,”
130
D.-G. Shu, S. Conway Morris, J. Han, Z.-F. Zhang, K. Yasui, P. Janvier, L. Chen, X.-L. Zhang, J.-N. Liu, Y. Li, and H.-Q. Lui, “Head and Backbone of the Early Cambrian Vertebrate
131
The term “Kutta condition” describes the physical situation at the trailing edge that is used in the Runge-Kutta theorem of inviscid flow. Runge-Kutta is used to estimate the pattern of flow around a body, including the so-called separation and stagnation points. The pattern of flow around a fish’s body is constantly changing as the fish undulates its body, and the caudal fin is the place where water that has interacted with the body is shed rearward, creating a wake. The wake, in turn, can be thought of as evidence of the momentum that the fish has transferred from its body in order to move forward, thanks to Newton’s third law.