View - Kowalewski, M. - Virginia Tech

PALEONTOLOGICAL SOCIETY PAPERS, V. 8, 2002sides of the same behavior, and detritus feedersare bound to engulf countless living microbes. Mostorganisms are not confined to a single mode of life,so the same organism may be predator, scavenger,parasite, etc.—and, of course, prey. Phenomena innature tend to have fuzzy edges, and terminologyshould not lead us to forget that.Neither predator nor prey has to be an animal,so the definition allows for a discussion of theevolutionary mechanisms that might have beenpresent long before crown-group animals werearound. The definition encompasses so much oforganism–organism interactions, however, thatconstraints are necessary to keep the chapter withinbounds. The constraints will mainly be practical—I will deal with processes that either have left directevidence among fossils or at least have the potentialto have influenced the fossil record recounting theearly history of life on Earth, up till about 500million years ago. Also, emphasis will shift fromprokaryote and protist predators during the earlypart of the interval covered, to multicellularsuspension/filter feeders and grazers on plankticand benthic microbes, and finally to macrophagouspredators—animals eating animals.Because successful predation by definitionleads to the death of the prey, its selectiveimportance is considerable. Interactions betweenorganisms are generally regarded as a major factorin evolution (though see Gould, 1985, 2002), andsuch interactions that lead to the failure ofindividuals to reproduce should have the strongesteffect of all. The evolutionary effects are strongerin prey than in predator taxa (Dawkins and Krebs,1979; Vermeij, 1987; Abrams, 2000), because ofthe unequal nature of the interactions—at eachencounter the prey risks it life, the predator onlyits meal (the “life–dinner principle” of Dawkinsand Krebs, 1979). Experimental work in modernecosystems confirms that the introduction ofpredators may cause a rapid evolution of variousdefensive characters in the prey species(Thompson, 1998; Agrawal et al., 1999). When thepredators respond by evolving more efficientmeans of predation, the feedback loop sets up thefamiliar “arms-race” (Dawkins and Krebs, 1979)or escalation (Vermeij, 1987, 1994, 2002) scenario.Nonetheless, our ability to identify theevolutionary effects of predation in the fossil recordis limited, because evolution is the sum of alleffects, and controlled experiments are generallynot possible. We have very incomplete informationabout the nature of the encounters between predatorand prey, and in most cases we know the identityof only one of the participants. We know equallylittle about population structure, competition levels,environmental stress, etc. The fossil record mayyield trends through time, but interpreting them issimilarly difficult. For example, trends towardlarger size in both predators and prey may beinterpreted as causally related but may equally wellbe responses to the same external factors.For the vast stretches of pre-Phanerozoic timecovered in this chapter, matters would seem evenworse. Fossils are scarce, they are mostly microbial,and their mode of life cannot in general be deducedfrom their morphology. For most of the time period,direct evidence of predation is lacking. The fossilrecord is generally silent with regard to animals olderthan about 600 million years, and only indirectevidence suggests that animals or animal-likeorganisms and predatorial modes of life existedearlier. Clearly, formulating and testing ecological/evolutionary hypotheses in this setting is difficultor impossible. Nonetheless, theoreticalconsiderations of the role(s) of predation in earlyevolution, set against paleontological data, help usto interpret the sparse early fossil record and toevaluate hypotheses regarding the role of predationas a driving force in the evolution from an almostexclusively microbial biosphere to one characterizedby multicellular organisms and the complex foodwebs of modern ecosystems. Although thecausalities and triggers of this process are far fromunderstood, the evolution of predatory modes of lifeis likely to have played a central role, certainly indetermining the course of evolution in a number oflineages and perhaps also as a major shaping forcein the radiation of multicellular and unicellularorganisms during the Cambrian explosion, one ofthe most significant and certainly the most manifestrestructuring of the biosphere during Earth history.290