MCSHEA, D.W.: Three Provocative Patterns in Hierarchical Evolution

The hierarchical structure of organisms increased a number of times in the history of life, notably in three episodes: the origin of the eukaryotic cell from symbiotic associations of prokaryotes; the emergence of the first multicellular individuals from clones of eukaryotic cells; and the origin of the first 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. These episodes suggest a trend in the maximum, an increase in the number of levels present in the hierarchically deepest organism on Earth. Such a trend is widely acknowledged but not well documented. One problem is that no objective scale has been developed, no set of criteria has been devised, that would enable us to assess and compare hierarchical structuring in fossil organisms in a consistent way. Also, as hierarchy is treated conventionally, only four major levels are recognized � prokaryotic cell, eukaryotic cell, multicellular individual, and colony � and thus the resolution of any pattern that can be documented is quite limited. Here I propose a scale that is operational and that also offers moderately high resolution, with each major level subdivided into three minor levels. And I use the scale together with the body-fossil record to generate first-occurrence data and document a trend. The data should be treated skeptically, but if we accept them at face value, three other patterns emerge: 1) Hierarchical evolution is accelerating. 2) Much of the hierarchy morphospace is unoccupied. 3) The minor levels do not arise precisely in order, i.e., higher levels occasionally arise before lower ones. All three patterns are unexplained (although possible explanations can be devised) and warrant further investigation.