HIERARCHICAL COMPLEXITY AND THE LIMITS OF ORGANISM SIZE
The mean and maximum sizes of life have increased dramatically over the course of the Geozoic (approximately 3.8 Ga to present). The maximum size across all life has increased by approximately 20 orders of magnitude and the mean size by perhaps ten. At the broadest temporal scale, there were two punctuations in size increase associated with increases in complexity as eukaryotic cells evolved from prokaryotic cells approximately 1.9 Ga and as multicellular life diversified from unicellular ancestors approximately 0.6 Ga. However, the quantitative relationship between organism size and structural complexity remains poorly documented. We assessed the relationship between complexity and size using a dataset that includes organism size (biovolume) and hierarchical level of complexity (degrees of aggregation and individuation; McShea 2001) for more than 34,000 genera spanning all three domains of the tree of life as well as the entire Geozoic. The distributions of sizes within each of the hierarchical levels show two key patterns. First, the mean size of genera within each hierarchical level of complexity is, on average, one to three orders of magnitude larger than at the next lower level. Second, there is no overlap at the extremes of the size distributions between hierarchical levels. Third, the range of possible sizes increases from one level of hierarchical organization to the next. Together, size data spanning the tree of life provide the first quantitative support for previously suggested evolutionary expansion away from a left wall of minimal size and complexity as well as a rescaling of the right wall of maximal size concomitant with structural innovation. In addition, size data provide clear evidence that although higher levels of structural complexity enable larger maximum sizes and a wider range of sizes, they also rescale the left wall of minimum size. Given the fundamental functional importance of size, the tradeoffs between size and complexity illustrate why structural innovations have never led to the competitive exclusion of less structurally complex classes of organisms.