From an article by Max Tegmark, PhD at kurzweilai.net:
In his new book Life 3.0: Being Human in the Age of Artificial Intelligence, MIT physicist and AI researcher Max Tegmark explores the future of technology, life, and intelligence.
The question of how to define life is notoriously controversial. Competing definitions abound, some of which include highly specific requirements such as being composed of cells, which might disqualify both future intelligent machines and extraterrestrial civilizations. Since we don’t want to limit our thinking about the future of life to the species we’ve encountered so far, let’s instead define life very broadly, simply as a process that can retain its complexity and replicate.
What’s replicated isn’t matter (made of atoms) but information (made of bits) specifying how the atoms are arranged. When a bacterium makes a copy of its DNA, no new atoms are created, but a new set of atoms are arranged in the same pattern as the original, thereby copying the information.
In other words, we can think of life as a self-replicating information-processing system whose information (software) determines both its behavior and the blueprints for its hardware.
Like our Universe itself, life gradually grew more complex and interesting, and as I’ll now explain, I find it helpful to classify life forms into three levels of sophistication: Life 1.0, 2.0 and 3.0.