What Is “Life”?
Biology is the study of life yet “life” is not particularly well defined.
A common definition of a “living” thing is a thing that metabolizes and replicates. However, while “replication” means the production of similar things, our definition doesn’t quite allow us to objectively say what is and is-not “living” because we have to draw additional lines in the sand on what sorts of reactions constitute “metabolism”.
Some people, for example, think that viruses are not alive because of viral metabolism.
Sure, viruses replicate, but they don’t “metabolize” in the sense that viruses don’t consume matter and digest it, but rather they convert the mouths & bellies of cells to their own metabolic factories. I don’t particularly agree - I think viruses are alive in every meaningful sense. They even have DNA or RNA, and they encode enzymes that ‘metabolize’ small packets of energy (ATP) to catalyze the most basic acts of replication - copying their genetic material and packaging it in another bundle of proteins, maybe even dressing it in a lipid ball if they feel like it. Isn’t that “metabolism”?
There are weirder things than viruses which test the limits of our definition of life. Zoom inside every cell of your body, swim through the crowded cytoplasm of the cell to the nucleus where the genome lives, and scan your genomic DNA closely. There, you will find small sequences of DNA that appear to have copied and pasted themselves throughout your genome. These “transposable elements” or “transposons” vary in exactly how they copy & paste themselves, but at the end of the day these sequences can replicate, and the only way they replicate is by a series of metabolic reactions that catalyze their copying and pasting in a genome.
Transposons evolve. Up to 50% of the genome of plants can be transposable elements. As an old growth tree grows, cells at the tip of the tree divide, and as cells divide they copy the unique pattern of transposable elements. The transposable elements transpose themselves some more, and the cells divide, and along an entire branch of a tree you can have an evolutionary story of transposable elements, a Russian Doll of life within life. Hundreds to thousands of years in the life of a single tree will require vast numbers of cell divisions and many more generations of transposable elements copying and pasting themselves. The next tree you see, imagine if the genome in every cell of the tree was a color - every branch of the tree would look like a branching rainbow colored by mutations, replications, and even the metabolism of transposable elements.
How about a meme? Is a meme alive?
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