THREE PROVOCATIVE PATTERNS IN HIERARCHICAL EVOLUTION

One of the most salient and certain trends in the history of life is the increase in hierarchical structure of organisms. Episodes of increase include the following: the origin of the eukaryotic cell from symbiotic associations of prokaryotes, of multicellular individuals from clones of eukaryotic cells, and of individuated colonies from associations of multicellular organisms. In each case, the increase was hierarchical in that lower-level entities combined to form a higher-level individual. The trend is well known but has not been well documented, and in particular, the timing of first occurrences of levels has not been established with any rigor. One problem is that no objective scale has been developed, i.e., no operational and consistent set of morphological criteria has been devised for recognizing the various hierarchical levels in fossils. Also, as hierarchy is treated conventionally, only the four major levels in the list above can be recognized in the fossil record, and thus the resolution of any pattern that can be documented is quite limited.

Here I propose a scale that is operational and applicable in a consistent way across levels. It also offers moderately high resolution in that each major level is subdivided into three discrete minor levels. I use the scale together with the body-fossil record to identify first occurrences of organisms at each major and minor level over the history of life. Two extreme alternative views of the data are offered. Both reveal a long-term trend in the maximum, i.e., a fairly regular increase over time in the degree of hierarchical structure of the hierarchically deepest organism in existence. The data should be treated skeptically, but if we accept them at face value, three provocative patterns emerge: 1) In one view, the levels seem not to arise precisely in order, raising the counter-intuitive possibility that in evolution, higher levels might occasionally arise before lower ones. 2) Waiting times between first occurrences of successive minor levels decrease over time (in one view), suggesting that hierarchical evolution is accelerating. 3) In both views, much of the hierarchy morphospace seems to be unoccupied. All three patterns are unexplained (although explanations can be devised), and all warrant further investigation.