You’re an amazing bag of bio-chemical reactions.
You have something in the order of a 1000 trillion cells. Of these, only a small fraction, 1/10th – or about 100 trillion – are actual human cells. The other 1000 trillion are mostly various kinds of bacteria, but also some micro-scopic, primitive eukaryotic”animals” like protozoa, and fungi such as yeasts. Many of these play a critical, symbiotic role in the functioning of your body, particularly in helping you digest your food. Others may not appear directly beneficial, but still their presence helps control the growth of less benign microbes, by out-competing them for food. These microbial cells make up 1 to 3% of your mass!
At some point early on in the evolution of life, some larger, single-celled organism managed to evolve that included some smaller, archaic bacteria and “farming” them for energy, possibly by the larger cell having “swallowed” the smaller cells. These archaic-bacteria components are called mitochondria, and they can turn sugar into energy. They contain a number of tiny molecules , the ATP synthases, which spins around like a little motor, driven by protons, to help drive the final part of the sugar→energy conversion process!
These more complex, larger and higher-powered “eukaryote” cells, with their “farms” of symbiotic mitochondria became the basis for all animal life, from the single-celled protozoa, to moulds, to plants, and all the way to you. These mitochondria have their own DNA, and replicate themselves as and when their cells do. There are some exceptions, such as human sperm cells, which do not have any mitochondria, and so the mitochondria in your human cells generally came only from your mother, which came from her mother, etc.
You posses one of the most complex objects known on earth. The human brain. A circa 1.4 to 1.5 kg gelatinous, fatty mass of about 1.3 litres in volume. Our deceased close cousins, Homo Neanderthalensis, actually had a slightly larger brain than us, at 1.5 litres in volume. Your brain consumes a significant amount of your body’s energy, 20 to 25%, about 10 times more than the proportion of its mass to your body’s total mass. The energy your brain consumes is at least equivalent to the energy needed to power a 7 to 10W CFL or LED light bulb¹, 24×7. The brain is still a lot more efficient than any computer though, for what it does.
The human brain contains on the order of 100 billion special cells called neurons. Each of which receives inputs from other neurons through junctions called synapses on spindly arms called dendrites. Additionally a neuron can have an axon, a potentially very long nerve fibre, which can reach from the neuron to muscles, glands, or to other neurons. The longest axons in your body are in the neurons of your nervous system, running the length from the base of your spine to your toes! There are estimated to be an order of 100 trillion synapses in the human brain, for an average of thousands of connections per neuron. This would likely make the network in your brain at least an order of magnitude more complex than the entire Internet!
As if that wasn’t enough, you have a second “brain” in your abdomen, by your stomach – the enteric nervous system. It’s a lot simpler than the brain in your head, having only 100 million neurons or so, but that’s still a lot of neurons – comparable to the brains of lower mammals, at least in terms of number of neurons! The enteric nervous system has a number of functions, in particular co-ordinating the muscles of your gastric system.
So your body is this giant bag of cells, most of them not human, all of them working away and, mostly, co-operating in a huge variety of, often complex, ways. If you dig into the cells that are you, you’ll find even they contain a number of primitive cells within them. Sitting on top of this super-colony of cells is a brain, directing things at a high-level and using the equivalent of a super-internet to do so! With a 2nd mini-brain helping out lower in your body! You are a mobile mini-eco-system.
As if that wasn’t amazing enough, you are related to essentially all other life on this planet. You are the direct descendent of some of the earliest life on this planet, if not the first. As is pretty much everything else on this planet. From the cats and dogs you might see on the street, and all other mammals, to the green plants and trees, to the black mould in your shower, or even bacterial slime you might find around a tap, you share some DNA with all of these – these are your cousins, to greater and lesser degrees. We are all bound together in the massive eco-system that is Earth. This eco-system is effectively all but closed, receiving only energy from the star it orbits – relative to which we hurtle through space at an average of about 107000 km/h, or 30 Mm/s. Life, simple bacterial forms particularly, inhabit near every nook and cranny of it that we have managed to visit. Including places where we had perhaps expected to find none, such as in the greatest ocean depths, even deep underground.
Your continued existence depends greatly on many of these cousins of yours, both animal, plant and bacterial.
Many of the species of life on earth have quite specific needs of their local environment, and are quite sensitive to even small changes to it. However life overall on this planet has proven to be quite robust. Any individual species could easily die out, indeed in catastrophic changes many species could die out, but life tends to cling on in some form – particularly in simpler forms. Life is both precarious, from the point of view of any single species, and yet also robust overall.
Do not think your species is an exception.
When the processes that keep your body-eco-system working together stop, when your own cells cease to function as such, life will continue. Your many cousins and their descendants, maybe even some closely related or even direct descendants, will go on. Some of the bacterial cells that helped maintain you almost certainly will outlive you and perhaps even help maintain another body for a while. It is almost certain that much of the DNA you contain will continue to help build further life on earth, through these many cousins. The molecules and atoms you contain will be recycled, they will become part of the air, the sea, the earth. In time, they will be taken up by other life and become part of those plants and animals, perhaps even people. Your body-eco-system is but a small, transient, component of a much more massive eco-system that has been developing for billions of years, and may well continue to be around for billions to come.
As you wonder about the magnificence of all this, you might look up to the stars. All the atoms in your body were originally forged inside stars, other than the very lightest (hydrogen, which is very common in the universe and your body, helium, and very small proportions of lithium and beryllium). You are, literally, made of star-dust. Complex organic compounds, precursors to life, seem to occur quite naturally, and we know they’re floating around on asteroids in the Solar system, and so likely they float around a great proportion of other star systems too. There are in the order of 100 billion visible galaxies, with a galaxy typically having in the order of 100 billion stars, meaning there are in the order of 100 billion × 100 billion, or 1022 = 10,000,000,000,000,000,000,000 stars out there. Even if the chance of life arising were a tiny 1 in a billion, that’d still mean that 1013 = 10,000,000,000,000 of the visible star systems had life on them. It’s very likely that the Universe is full of at least simple microbial life.
Life is amazing, and it is precious.
1. Base calorific requirement of, say, 1200 kcal / day = 58W, 20% = 11W.