I am always thinking about something.
One thing I’ve been thinking about is how common it is to hear people state that each of us is unique and unrepeatable; we usually hear this within a “motivational” context or just for the sake of it. Well, it just happens to be true, you ARE unique, there is no question about that, and I will use biology to make my point. Let’s briefly explore this from a few biological perspectives. I guarantee that you will be astonished.
Sex cells/reproduction. On average, a woman ovulates between 300-400 eggs in her lifetime, and under normal circumstances, only one is “released” per month. A typical woman is born with all the eggs that she’ll have “available” in her lifetime, and the actual number of potentially “fertile” eggs in a typical woman is close to 3,000 (this number will slightly change every month, of course, but the point still stands). So, to make a conservative estimate, let’s say that the probability of a particular egg being available to make a baby each month is about 1/3,000.
When you were conceived, an average of 200 million sperm (the exact number depends on many factors) competed to be the one that fertilized the one egg in your mom’s womb. Therefore, “spermwise”, we are talking about a chance of approximately 1/200,000,000.
Right there, the probability of you is 1/3,000 x 1/200,000,000 = 1/600,000,000,000. That’s 1 in 600 billion!
This fact is surprising enough, especially in light that there are “only” about 8 billion humans in existence right now. And ever since we humans have been around, some estimates indicate that there have been about 100 billion people in the last 200,000 years or so.
However, we do not have to stop there. You see, similar probabilities apply to your biological mother and father; therefore, we are talking about a probability of 1/600,000,000,000 x 1/600,000,000,000, which translates to 1/a-whole-freaking-lot!
Just by taking into account eggs and sperm, we can get a very rough idea of the immense improbability of each of us. Then of course, similar figures apply to each of your four grandparents, and so on. Moreover, imagine all the reshuffling that your chromosomes have to “endure” to (correctly) end up in a particular gamete (sex cell). The numbers boggle the mind, as in the following example.
Genes. There are about 25,000 identified genes in the human genome, give or take (the exact numbers change virtually every month as new information is available). Since humans are diploid, sexually reproducing organisms, it means that, on average, we have two versions of each gene. In practice, we will have more than two. These, by the way, are called alleles. This fact means that except for sex cells, each cell in us has up to 50,000 genes, separated into two sets, one from mom and one from dad. Developmentally, those 50,000 genes randomly recombine into one set of about 25,000, which is not the same set that we received from either mom or dad but an entirely new combination. What is the probability of any specific combination, you say? (Of course, for simplicity’s sake, we are not taking into consideration phenomena like genetic linkage, etc.).
Well, the probability of each combination is roughly one over 2 to the power of 38,000. Talk about your “hyperlottery”!
An alert reader may say: “…Well, I got a loophole. How about identical twins? They have identical genomes!”
That is true, but it turns out that what we call **the** genome is not the only game in town.
It gets better.
More genomes. Did you know that in addition to your “standard” genome, you have at least three more genomes? Well, you do! You possess an “epigenome”, which includes a variety of compounds or environmental conditions that, without changing a single letter of your own genome, can influence its functional capabilities. For example, usually, you would not grow an eyeball on a fingertip. All cells in your body have a complete gene complement, but those genes are differentially activated depending on the specific tissues. Twins most likely will not have identical epigenomes.
It gets weirder. You have at least four additional genomes that are an integral part of who you are: The “bacteriome”, which is the collected genome of all bacterial species that normally live in your body; the “mycobiome”, the collected genome of all fungal species that normally live in your body, and the “virome”, the collected genome of all viral species that normally live in your body. Sometimes these three are labeled together as the “microbiome”. Then, there is your mitochondrial genome. Mitochondria are the organelles in charge of generating most chemical energy in eukaryotic organisms. They possess their own DNA and are generally transmitted from the maternal line, meaning that you will likely have the same mitochondrial DNA as your siblings, all courtesy of your mom. Each of these can influence your physiology, sometimes all the way up to your behavior.
And these are some of the “-omes” we know about so far! There may be more surprises in store for future biology.
Anyway, since we mentioned behavior, what about the brain?
One of the factors that make you, well, “you”, is the specific arrangement and communication patterns of the roughly 86 billion cells in your brain. These cells are called neurons. Developmentally, any given organism (including its brain if it has it) is “constructed” based on the instructions encoded in its genes in collaboration with the environment (a story for some other post…). Right there, you surely noticed that the number of genes in an organism must not be the whole story. You see, as we saw above, humans have about 25,000 genes or so. Therefore, it is evident that there is not a simple relationship between the number of genes and the number of neurons, but let’s not even go there. This is a matter for a future post.
Your 86 billion nerve cells make connections with an average of 30,000 other neurons (I’ll let you calculate the number of possible connections). Many of those connections change in real-time when you are making memories, for example. Moreover, as I have said elsewhere, there is a special class of cells in your brain called glia that are an essential component of your brain and, in fact, are about ten times more numerous than neurons!
Oh, and the same argument about identical twins also applies with the statements above. In fact, most certainly, twins do not have identical brains.
I could go on, but I hope that I have convinced you that you are indeed a unique and unrepeatable individual.
So… In all likelihood, there has not, and never, ever will be, anyone like ***you*** in the known universe.
You are improbable. You are unique. Love yourself.
😊
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