The improbability of you

I am always thinking about something…

For example, it is very common to hear people stating that each of us is a unique and unrepeatable individual; we usually hear this within a “motivational” context. Well, it is true, no question about that.

But most of us do not usually know how **true** this is. Let’s briefly explore this from three biological perspectives, sex cells/reproduction, genes, and neurons (and associated cells). 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. In fact, women usually start with many more eggs during development, the actual number of potentially “fertile” eggs in a normal woman is close to 3,000 (this fact will change the upcoming calculations a little), but to make a conservative estimate, let’s say that the probability of a particular egg to be 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 fertilizes the one egg in your mom’s womb. Therefore, “spermwise”, we are taking 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 is surprising enough, especially in light that right now, there are “only” about 7 billion humans in existence. Moreover, some estimates indicate that there have been about 100 billion humans since the Earth has been around.

However, we do not have to stop there, you see, roughly the same probabilities apply to your biological mother and father; therefore we are talking about a probability of 1/160,000,000,000 x 1/160,000,000,000, which translates to 1/a-whole-freaking-lot!

Of course, the same numbers will apply to each of your 4 grandparents, and so on… Just by taking into account eggs and sperm we can get a very rough idea of the immense improbability of each of us, yourself included, of course.

But, what about genes? There are about 19,000 identified genes in the human genome, give or take. 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 means that with the exception of sex cells, each cell in us has about 38,000 genes, separated in two sets, one from mom, one from dad. Developmentally, those, 38,000 genes randomly recombine into one set of 19,000, which is not from mom or dad, but an entirely new combination. What is the probability of any specific combination? (Of course, for simplicity’s sake, we are not taking into consideration phenomena like linkage, etc.)

Well, the probability of each combinations is roughly 1 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.

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, are able to influence its functional capabilities, for example, normally you would not grow an eyeball on a fingertip, right? 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 three additional genomes that are an integral part of who you are: The “bacteriome” collected genome of all bacterial species that live normally in your body, the “mycobiome”, the collected genome of all fungal species that live normally in your body, and the “virome”, the collected genome of all viral species that live normally in your body. Sometimes these three are labeled together as the “microbiome”. Each of these is capable of influencing your physiology, in some cases, all the way to behavior. And this is what we know so far! There may be more surprises… As in the case of epigenomes, identical twins will most likely not have the same microbiome.

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 their 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 19,000 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…

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 apply with the statements above. In fact, most certainly twins do not have identical brains…

Moreover, I am a little embarrassed because I forgot something. At the same time I am very proud because the very first person who found out was a former student of mine… (:-)… What I forgot to mention is that we all have an additional “-ome” within our cells, the 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.

I could go on, but I hope that I have convinced you that you are indeed a unique and unrepeatable individual.

In all probability, there was never, there is not, and never will be anyone like you in the known universe.


However, infinity is a rather tricky concept; if there really is an infinite number of universes, as some theorists argue, there may be a copy of you “somewhere else”, but this is certainly a story for another day.

Nonetheless, I still think that even then, you are in no danger of running on yourself. Nope, not even a little bit.

You are improbable, you are unique.



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