Before long, our planet was teeming with a diverse panoply of life forms. It’s still an open question how, when and where life first appeared in our Universe, but there is strong evidence that here on Earth life first appeared about 4 billion years ago. 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. 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. 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. What’s replicated isn’t matter (made of atoms) but information (made of bits) specifying how the atoms are arranged.
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.
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.
The question of how to define life is notoriously controversial. The following is an excerpt from Life 3.0: Being Human in the Age of Artificial Intelligence, by Max Tegmark.
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