I forget sometimes that not everyone has the same background I do. A while back, I had a conversation about fat that made it clear the speaker had no clue what she was talking about. For the record, you breathe out the fat you burn, which gives you a sense of how densely energy is stored. Your fat leaves your body through your lungs, which is why burning it almost always involves doing something that makes you breathe harder. Fat does not exit through the intestine.
In fact, technically nothing exits through the intestine. Measured by weight, human solid waste is a) water b) dead bacteria c) undigested foodstuffs d) trace bits of junk, such as epithelial cells sloughed off from the intestinal lining and maybe trace amounts of digestive enzymes. But other than that, unless you are very sick, your solid waste is just leftovers, whatever your body didn’t absorb from what you ate.
The net carbon in your long-chain fatty acids, broken off two at a time, is oxidized in your mitochondria to release energy. From there it diffuses through the cell wall and into the blood stream and thence out the lungs as carbon dioxide. But a pound of fatty tissue includes more than just fatty acids. There is some water a little bit of connective stuff, which is why the calorie deficit required to lose a pound of weight (~3500 kcal) is less than that stored in a pound of pure human fat (~4100 kcal).
But that’s a technicality, really. In the interests of keeping things simple, we can say the fat in your body leaves — if it leaves at all — through your lungs, which function exactly like the smokestack on a factory.
But your breath is not the only way things leave your body. In fatty acid metabolism, for example, some metabolic byproducts are processed by the liver, released into the blood stream, filtered by the kidneys, and excreted in the urine. It’s fascinating, not least because your kidneys have to know what to keep and what to throw away. If they took out the bad stuff, they’d have to know what was bad, which means they would have to keep a record of every possible chemical compound — not just everything that does exist, but everything that could.
That’s impossible, of course, so they don’t even try. Rather, your kidneys squeeze everything out. And I mean everything. That’s what the renal glomerulus is for. It wrings the blood, removing everything right down to vital proteins and energy. Then it pumps back into the blood what it knows is supposed to be there. It’s a bit fascist, really. Whatever it doesn’t know, it bounces.
That record of what’s supposed to be in the blood, versus everything else, is (ultimately) stored in an encyclopedia. And as I mentioned in a comment yesterday, this encyclopedia is arguably the most amazing thing on earth, and it’s written in a language “spoken” by every living thing on the planet.
All creatures great and small share the same genetic code, which reduces to a sequence of four nucleic acids symbolized by their first letter: A, T, G, and C. Those four measly letters combine to make three-letter “words” called codons. Four letters expressed three ways means there are a total of 64 possible combinations. The genetic code is a language with only 64 words.
Combinations of three-letter words run together to make a sentence, which is a gene. Sentences form paragraphs, called operons, made up of genes and their regulatory units — genes and non-coding sequences that control the expression of other genes, one paragraph in the story, an elemental point and its explanation.
All those paragraphs are wrapped up in a series of books, complete with binding and stitching (called nucleosomes). We call these books chromosomes. Bacteria have a short story. Your body on the other hand is the expression of your own 46-volume encyclopedia completely unique to you but written with the same 64-word dictionary and the same four-letter alphabet as everything else in the world because all living things, from bacteria to birds to banana trees, share them.
Think about that for a moment.
This is what biologists mean when they speak of “The Unity and Diversity of Life on Earth.” We all share a common ancestor. We’re all sprouts from the same tree of life. And yet every species — and every individual — is wonderfully distinct, from mangroves to manatees.
If there was one lesson from biology I tried to instill in my students during my very brief teaching career, it was this: diversity isn’t a threat. It’s a secret reservoir of fitness.
In fact, the Nazis had the science completely backwards, as does your average ignorant racist. This is because the deep history of the universe — of geology, of life — is the history of change. It’s the only constant. So the best strategy for long-term survival is to stock rainy-day skills and abilities, even ones that may not seem helpful in the current climate.
Because you never know.
Our species’ greatest achievement is to develop an organ that can bypass this common language of life to create a flexible reservoir of adaptability — culture — stored in the brain and transmitted via descent with modification from parent to offspring. And it includes this very post.
Now, there is such a thing as being too diverse. Without shared values to bind us — toleration, freedom, respect — the center cannot hold, which is why you see so many otherwise tolerant-minded folks intolerant of intolerance.
But our species is young, so young in fact that we’re one of the least diverse species we know. Chimpanzees, for example, are millions of years older and so have accumulated more genetic variance than humans. So while there is a conceivable limit to how diverse we would want to be, the science suggests we haven’t even come close to approaching it. Not even close.
And that’s an empirically derivable fact of the world.
cover image “Dimlight Forest” by Ferdinand Ladera