Lifeless to Living
When does matter become life?
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We have now traversed a sweeping yet concise history of the universe, from its first moments to the formation of a young, fortuitous Earth, a planet uniquely suited for life. Along the way, we touched on the concept of entropy and how living organisms create pockets of order that raise net disorder. What we have not yet explored, however, is what "life" actually is in the first place. Everything alive is made of atoms, so how can one clump of atoms be alive while another is not? Surprisingly, the answer is far more elusive than it first appears. As physicist Chet Raymo aptly observed, “We recognize life when we see it, but it is devilishly hard to say what it is.” Here, we explore dissipation-driven adaptation and assembly theory; the final pieces of the puzzle that is life.
Life is what it does…
If one were to ask a biologist to define “life,” he or she would probably list the key characteristics of living organisms: they can grow, respire, excrete, reproduce, move, metabolize, and respond to the environment. Simple enough, right? The problem is that a characteristics-based approach always leaves us with troublesome outliers. A mule, for example, cannot reproduce, yet nobody would doubt that it’s alive. A virus, on the other hand, can reproduce, but most biologists don’t consider it to be alive because it needs another species to do so. Nobody argues that a crystal is alive, yet it still grows. Meanwhile, some bacteria enter dormant periods during which they don’t grow or metabolize, yet they are clearly living organisms. The more we attempt to tighten our grasp around a succinct definition of “life,” the more organisms slip through our fingers.
In physics, scientists approach this question differently. Instead of trying to list characteristics of life, they ask what it does on a more fundamental level. MIT physicist Jeremy England's ‘dissipation-driven adaptation’ theory, for instance, posits that the key differentiator between what is living and what is non-living is that the former is better at dissipating energy per the Second Law of Thermodynamics. Life, as I mentioned earlier, is an “open” system that captures energy, uses it to create pockets of low entropy, thereby increasing the entropy of the universe. Life accelerates entropy, and what better way to accelerate it further than by replicating (reproducing) itself over and over again? Darwinian evolution tells us that life’s ultimate goal is reproduction; the survival of the species depends on its ability to adapt to changing conditions. Biology tells us how (natural selection), physics tells us why (entropy).