Planet of the Apes: Curiosity about the definition of life
August 13, 2012 By Faye Flam
After a triumphant landing, the Curiosity rover is ready to search Mars for signs of past life or suitability for life. Several readers have raised concerns that NASA scientists might fail to recognize life if it isn’t based on carbon or is otherwise radically different from our kind of life.
1. I’m not sure how to define life, or if definitions of biological constructs are important to understanding the basic principles of biology or levels of biological organization. I find it mildly annoying, however, when articles like this include speculation on how other life forms might have evolved on other planets. Realistically, life on this planet involves human cells with the following characteristics: 3,000,000,000 base pairs of DNA; 3 metres of DNA; 20,000-25,000 genes; 10,000-20,000 protein species; 4,000,000 ribosomes; 60,000,000 tRNA molecules; 300,000 mRNA molecules; 10,000,000,000 protein molecules; 1,000 miRNA species.
Also, there are 50,000,000,000,000 cells in the body and 80,000,000,000 neurons in the brain Given the odds against non carbon-based life on other planets, perhaps some of us need definitions to tell us what life is. Others will look at what’s known about our adaptive evolution via ecological, social, neurogenic, and socio-cognitive niche construction and perhaps laugh a little at the speculations of theorists who are not likely to have the last laugh.
2. Re: “Making up arbitrary definitions (like life/non-life) doesn’t help us in finding interesting stuff at all.” The ability of life to recognize self / non-self differences is what’s interesting. How does a virus recognize a potential host cell that does not recognize the virus. How does one cell of the organism called brewer’s yeast recognize the ‘sexual orientation’ of another cell it can mate with? The level of recognition we attribute to other sensory input, is olfactory/pheromonal in every other species on this planet. That suggests we should be sniffing around on other planets rather than looking to ‘see’ if there are other carbon based life forms — or any other life forms that recognize self / non-self differences as is required for nutrient acquisition and reproduction.
3. Re: Cynicism. I don’t want to be cynical about anything, I want to be accurate. The processes are not random, and cannot accurately be attributed to random mutations. In all species from microbes to man the chemical senses are responsible for self / non-self recognition, which is a requirement for nutrient acquisition; the metabolism of nutrients to pheromones; and the pheromonal control of reproduction. Some of us have known that for more than 15 years. “Parenthetically it is interesting to note even the yeast Saccharomyces cerevisiae has a gene-based equivalent of sexual orientation (i.e., a-factor and alpha-factor physiologies). These differences arise from different epigenetic modifications of an otherwise identical MAT locus.” — From fertilization to adult sexual behavior. Diamond M, Binstock T, Kohl JV. Horm Behav. 1996 Dec;30(4):333-53.