Astrobiologist Sara Imari Walker commented recently that current definitions of life are a “mess”: “A popular definition that “life is a self-sustaining chemical system capable of Darwinian evolution” excludes any organisms that can’t reproduce because they are not capable of evolution—therefore mules and many senior citizens are excluded if you read the definition too strictly.”
Well, now that she mentions it, there are over 100 definitions of life. Many don’t particularly make sense, as she points out. Also, there are many ways evolution can happen other than Darwinian evolution (horizontal gene transfer, for example). Why be exclusive?
More at Mind Matters News.
See also: Particle physicist offers 75 reasons we don’t see aliens. But Oxford’s Future of Humanity Institute gives high odds that we are the only intelligent beings in the galaxy (Lots more linked.)