Size Limits of Very Small Microorganisms

FOREWORD


The world was galvanized in August 1996 by the announcement of possible evidence for relic biogenic activity in the martian meteorite ALH84001. Prominent among the five features cited in support of this startling hypothesis was the observation of "carbonate globules and features resembling terrestrial microorganisms, terrestrial biogenic carbonate structures, or microfossils."1 The structures, revealed in electron micrographs, range in length from 10 to 200 nm. One reason for skepticism about the claim that these have biologic origin is that the martian structures are generally much smaller than the terrestrial objects to which they were compared.

Regardless of one's conclusions about the meteoric evidence for life on Mars, the public fanfare was very effective in focusing attention on scientific questions central to understanding if and how we can recognize extraterrestrial life. The topic of the workshop whose findings are reported here, the size limits of very small organisms, is an important one for the interpretation of the carbonate structures on ALH84001, as preparation for future investigations of other samples carried to Earth naturally or in spacecraft and as a litmus test of how well we understand biological organization in general. An indicator of the interest and excitement catalyzed by the Mars announcement is that nearly all the panelists invited to participate accepted with alacrity.

Nearly two dozen researchers applied their diverse expertise to the problem of extrapolating from what we know about Earth's abundant microbial population and the laws of physics and chemistry to draw conclusions about size limits for putative extraterrestrial life forms. Extrapolation is necessary because the only thing we can be reasonably confident about is that extraterrestrial organisms will differ in significant ways from those we find around us. Our life forms have been described by a Nobel Prize-winning biologist as Rube Goldberg contraptions assembled over eons by the stochastic processes of evolution—one can hardly expect that these would be exactly reproduced in other environments.

Part of the legacy of the ALH84001 meteorite is a significant increase in the vigor of NASA's programs in astrobiology, the exploration of the context and possible evidence for life elsewhere in the solar system, and the search for other planetary systems that might harbor life. These are pursuits that stimulate scientists and the public alike. They also demand the highest standards of scientific rigor—as stated by the late Carl Sagan, extraordinary claims demand extraordinary evidence. This volume lives up to those standards.

Claude R. Canizares
Chair, Space Studies Board


1D.S. McKay, et al. (1996). Science 273:924-930.





Last update 12/28/00 at 3:26 pm
Site managed by Anne Simmons, Space Studies Board




Copyright Statement.
The National Academies Current Projects Publications Directories Search Site Map Feedback DEPS SSB Previous Report Menu Next