What exactly is life

All living things age and die-my comment

I am not going to answer this question - J. B. S. Haldane (1)
The units of life are cells - Lynn Margulis (2)
Life Is Cells

People like to say, as if it were obvious, that life is hard to define. This is misleading. Life has properties that clearly distinguish it from everything else. Firstly, every living thing is cellular.

In other words, it is either a single-celled creature or a creature composed of biological cells. Every cell is bounded by its own outer membrane and contains a full set of instructions necessary for its operation and reproduction.

Furthermore, every cell uses the same operating system: "DNA makes RNA makes protein." DNA is a long complex molecule that contains the cell's instructions. It is transcribed into RNA, another long complex molecule similar to DNA; and then the RNA transcript is translated into protein. There are hundreds of billions of different proteins used by living things but all of them are made from the same twenty amino acids, the "building blocks of life."
Other Properties of Life

Living things reproduce themselves. Either individually or in sexual pairs, they have both the encoded instructions and the machinery necessary for self-reproduction. (Some creatures cannot reproduce, but every creature comes from reproduction.) Periodic crystals like sodium chloride (table salt) also undergo a kind of self-reproduction. In crystals however, the "instructions" are much simpler, they are not encoded, and they are not different from the "machinery."

Life uses processes collectively called metabolism to convert materials and energy for its needs. Metabolism creates waste products. When metabolism ceases with no prospect of starting again, we call it death. Machines also convert materials and energy for their needs, create waste, and could be said to die.


Life undergoes evolution. Notably, simpler forms are succeeded by forms with greater organization. Cars evolve also, in their way. Computers do, too. And computers even contain their own encoded instruction sets.

These latter properties of life are sometimes used to make the point that life is hard to define. But nothing else has all of these latter properties except cellular life using life's DNA-RNA-protein operating system. Another kind of life, entirely different from ours, is conceivable, yes. But the only kind we have ever seen is the one we are part of here on Earth.

As biologist and philosopher Harold J. Morowitz says, "The only life we know for certain is cellular..."Viruses and prions are not alive; they lie on the fringe of life. Viruses contain instructions encoded in DNA or RNA. (Prions don't.) Both are reproduced.

Viruses certainly and prions probably can evolve. But neither can reproduce itself; each needs the machinery of a living cell to carry out its reproduction. Without a cell, viruses and prions are merely inert, complicated particles which do nothing. Do they make it hard to define life? No, just as trailers don't make it hard to define motor vehicle traffic. We know what motor vehicle traffic is. And we know what life is