350 Annals New York Academy of Sciences 



The microorganisms referred to in this paper are pecuhar in that they grow 

 in environments that are lethal to most other forms of Ufe. One can instruc- 

 tively reverse the point of view that has been taken here and ask why it is 

 that most organisms live under "common" conditions. The answer is, of 

 course, because life as a whole is selectively adapted to growth in common 

 environments. If the waters of the earth were predominantly acid, growth 

 at neutral pH values would be regarded as an oddity. Thus, the fact that 

 most living species conform physiologically and ecologically to average Earth 

 conditions should not be taken to indicate any inherent environmentally based 

 physicochemical conservatism of living matter. Adaptation has taken place. 



Environments of the Earth that are sterile or nearly so mostly fall into one 

 of two categories: nonaqueous environments, and noncirculatory aqueous en- 

 vironments. The first category is so obviously restrictive in a biological sense 

 that it requires no further comment. The second refers to rock-enclosed waters 

 that do not readily enter into the hydrological cycle. Oil brines, for example, 

 that are perfectly sealed in place, seem to be sterile (Shturm, personal communi- 

 cation), and deeply buried wet sediments usually have low to negligible bac- 

 terial populations. In small enclosed systems extinction becomes increasingly 

 probable with time because of the small numbers of organisms involved, the 

 accumulation of metabolic waste products, and the general decrease in free 

 energy of the system as a function of time. Continuous circulation negates 

 these factors and in addition permits occasional injections of diverse micro- 

 organisms into new environments, to which they may become adapted over 

 many generations. Given the presence of circulating water, it seems rather 

 unlikely that any aqueous environment could remain indefinitely sterile over 

 geologically long periods of time. The powers of microbial reproduction and 

 variation are so immense in an evolutionary sense as to make this a virtual 

 impossibility. This assumes, of course, that some energy source is available 

 for metabolism in the environment concerned; but this is not a restrictive 

 limitation either biologically or geochemically. 



Returning to the cjuestion of extraterrestrial life, the problem involved seems 

 not so much to be whether organisms could live elsewhere under conditions 

 that we would regard as unusual on Earth, as it is to account for the origin of 

 life itself. In relation to the possibility of life on Mars, for example, the ques- 

 tions should be of two types: (1) whether conditions there were ever favorable 

 for the origin or introduction of life; and (2) whether subsequent conditions 

 have been favorable for the persistence of such life as might have been formed. 

 The second question is far less critical at the present time than is the first. 

 To appreciate the potentiahties of adaptation one need only contemplate how 

 an Ordovician observer might have viewed the likelihood of birds flying in the 

 air, the possibility of an animal maintaining a temperature of 37° ± 1° C. for 

 virtually all of its lifespan over a period of 100 years, or the existence of plants 

 that trap and feed on animals. What can the leper know of the scorpion's 

 sting? And what does the blind man know of the firefly's light? 



References 



Allen, M. B. 1959. Studies with Cvanidium caldarium, an anomalously pigmented chloro- 

 phyte. Arch. f. Mikrobiol. 32: 270-277. 



