Analysis of the fossil record shows that life has undergone two profound jumps in size — from bacteria to eukaryotic cells, and from single-celled to multi-celled organisms.
In each case, possible body size increased by a factor of one to two million. After the second jump, say comparative zoologists, bodies hit the limits of Earthly possibility.
"If you look at a blue whale today, it’s about ten times larger than any other animal," said Stanford University comparative zoologist Jonathan Payne, co-author of a study in the Proceedings of the National Academy of Sciences. "The mass or volume of the largest animals we see in the Cambrian and Ordovician radiations" — 2 billion and 450 million years ago, respectively — "were a million times bigger than anything that had come before."
Both of those jumps coincided with massive increases in atmospheric oxygen, said Payne. Another such increase in geochemical nutrients is unlikely.
"If there were a species whose individuals were a million times the mass of a blue whale, their nutrient demands would be so large that you couldn’t have many species like that on the planet. Potentially you could have just one," he said. "It’d be like having a few hundred million blue whales: there’s just not much food left to go around."
Citation: "Two-phase increase in the maximum size of life over 3.5 billion years reflects biological innovation and environmental opportunity." By Jonathan L. Payne, Alison G. Boyer, James H. Brown, Seth Finnegan, Michal Kowalewski, Richard A. Krause, Jr., S. Kathleen Lyons, Craig R. McClain, Daniel W. McShea, Philip M. Novack-Gottshall, Felisa A. Smith, Jennifer A. Stempien, and Steve C. Wang. Proceedings of the National Academy of Sciences early edition. DOI: 10.1073/pnas.0806314106, Dec. 22, 2008.
Images: 1. Flickr/mikebaird 2. Evolutionary body size chart / PNAS