View - Kowalewski, M. - Virginia Tech

PALEONTOLOGICAL SOCIETY PAPERS, V. 8, 2002emergence of macrophagous predators and secondorderconsumers. Also, the amount and quality ofinformation available from the respective stages hasforced a shift in the level of analysis from speculationbased on general biological principles, to hypothesesbased on indirect evidence in the fossil record, andto hypotheses more firmly based on fossil evidence.The theme has been escalation in the sense ofVermeij (1987), but brought to bear on the earlyhistory of life up into the Cambrian (whereVermeij’s story begins). The etymology of“escalation”—from Spanish escala, ladder, andLatin scala, with the same meaning—suggestsstepwise rather than gradual shifts, and Vermeij(1987) stresses the pattern of punctuated equilibria(Eldredge and Gould, 1972) when analyzing theanatomy of escalation. Although not dealing withpunctuated equilibria at the species level, I havefocused on some important steps (or escalations)in which predation is likely to have played a keyrole (Fig. 1). They all represent the attainment of ahigher level of organization (in the sense ofcombining previously existing systems to a newwhole), and so correspond to some of the majortransitions in evolution as discussed by MaynardSmith and Szathmáry (1995).Step 1.—From prokaryotic to eukaryotic.Predation was in all probability the determiningfactor in this event, and the resulting organismscombine characters of predators and prey in a waythat opens new evolutionary possibilities. The timefor this step is poorly constrained to around 2.7Ga (when geochemical evidence suggests that thehost and at least one of the guest symbionts wereavailable).Step 2.—From unicellular to multicellular. Thisstep was taken many times independently, but as ameans of producing bigger organisms it may reflectpredatorial pressures from cell-engulfing eukaryotes.At least in some lineages this happened soon after 2Ga (when the atmospheric oxygen had gone up andthe first non-stromatolite macrofossils appear).Step 3.—The appearance of mobile selectivepredators on bacteria and protists. This is the mostuncertain event of them all, for it may go back asfar as 2 Ga or it may be not much older than theCambrian explosion. The combined indicationsfrom the decline of stromatolites and thediversification of acritarchs suggest that it mayhave begun around 1 Ga.Step 4.—From simple, mostly microbial,ecosystems to ones with complex food webs andsecond- and higher-order consumers. Theappearance of macrophagous predators is thetelltale sign, and it took place no later than a fewmillion years before the beginning of the Cambrian,or around 550 Ma.FUTURE RESEARCHDIRECTIONSThe really interesting new research results arealways the unexpected ones. Any recipe for futureresearch that I may attempt to write will be one forstale cookies—the unpredictable cannot bepredicted. It should be clear from this review of theearly history of predation, however, that there areenormous gaps in our knowledge of the ecologicalinterplay between organisms up to about thePrecambrian–Cambrian transition (after that the gapsare only huge). The partial filling of some of thesegaps is something that one might humbly wish for.For example: Where did the main organismalinteractions take place that led to the Cambrianexplosion of animals? The planktic habitat has ofold been considered difficult to analyze from fossils,both because planktic organisms tend to be fragileand because in order to be preserved at all theyneed to be shifted out of their habitat. Thediscoveries that delicate animal tissues, such asminute arthropod limbs (Müller and Walossek,1985; Butterfield, 1997) and embryonalblastomeres (Zhang and Pratt, 1994; Bengtson andYue 1997; Xiao et al., 1998; Yue and Bengtson,1999; Xiao and Knoll, 2000), may be exquisitelypreserved by carbonization or phosphatization in 3-dimensional detail in rocks of this age spell greatpromise for the investigation of early animals,whether they be planktic or benthic. The extensivephosphorite deposits from the time period still guardmany secrets, and a suitable target in these rocksmay be fecal pellets, today an important medium of306