Sunday, December 13, 2015

The Size of the Genome

A prevalent assumption is that everything that makes us human, from the shape of our bones to the shape of our personalities, comes from DNA.  Most accept that at some point the influence of DNA trickles out and other influences take over.  But where is that point?

I think that extending the realm of DNA very far past "mere form" exaggerates what is possible given the amount of information contained in the human genome.

The human genome contains "approximately 3 billion base pairs", according to the Human Genome Project.  Each base pair can be either GC, CG, AT or TA.  There are four combinations, so each base pair represents 2 bits of data.  That works out to 6 gigabits of data, or 715 megabytes (MB) in more familiar terms.  (One megabyte is equivalent to 1024x1024x8 bits.)

715 megabytes in total.  What can you do with that?

For comparison, one fairly high resolution picture, or one song, are on the order of 3-4MB.  This means that you could fit roughly 200 pictures or songs into your DNA.  Words can be stored much more efficiently than sound or vision, so you could fit about 25,000 compressed books.

That is a decent amount of information but it is hardly impressive by modern standards.  Video games can require as much as 60GB of storage.  Operating systems tend to need at least 10GB.  A single low res movie is about 2GB and high res movies start at 20GB.

We could start to imagine what we might be able to describe with that much information but we have a very important constraint to add.

Humans and fruit flies share about 50% of their genes.  That is, half of our genetic material is identical to that of a fruit fly.  This makes it highly implausible that this half of our genome is going to contribute to anything recognizably human, or for that matter, anything that is important to us.  We should be very impressed at how complicated of an engineering task it is to produce any kind of life at all, of course.  This is a very impressive foundation to be built on, even if it is "just a fruit fly".  It just happens not to contain anything vaguely resembling personality.

Let's move up from fruit flies, to chickens, with whom we share 2/3 of our genetic material.  Chickens, when young, are at least rather cute and fluffy.  But then, they also will descend into uncontrollable fits of cannibalism under certain conditions.  And the most tame chicken is probably more OCD than the most neurotic human.  I have nothing against chickens, but I think we have to discount a bit more of the genome from having anything to do with distinctly human characteristics.

Moving on to dogs.  These creatures share 84% of our genome.  Their "blueprint" is 5/6 identical to ours.  It is much easier to relate to dogs than fruit flies, so here there is some overlap.  We can vouch for the warmth and personality of dogs, and they have something akin to the "presence" that a human has, albeit a much wetter, slobberier version.  But still, I would hope that even this would be a level somewhat below what we would wish to be measured by.  I don't mean to denigrate dogs by listing such traits--dogs stand nicely on their own as potentially friendly mammalian friends--but let's consider how we, personally, would feel, without the capacity for human levels of language, creativity, critical thought, ethical reasoning, inward reflection and self control.  Certainly not all humans excel in all of these categories but it is wonderful to have the capacity to excel in any of them.

So far we have reduced this 715MB to 360MB that is not a fruit fly, and 240MB that is not a chicken, and then finally to 120MB that is not a dog.  This is getting pretty slim.  Hard drives weren't that small since about 1988.  We're down to 35 songs or pictures to illustrate your non-doglike humanity.  (To be fair, you could still fit 4000 books.)

But there is still one more step.  Humans are, according to wikipedia, 99.5% identical to one another.

So, though you may have an allotment of 4000 un-illustrated books to describe the essential differences between dogs and humans, you get only 125 to describe the differences among us.  Or just one song or picture.  We are down to 3.6MB.  And some portion of that 3.6MB has to describe purely physical characteristics that have no bearing on personality.  Let's split it down the middle and say that we have 1.8MB to describe what is unique about our personalities, habits, inclinations and aspirations.

This is a mighty thin soup.  You can't do much at all with 1.8MB.

To be fair, you could still fit 62 books into that space.  Or if you wanted to think about it in terms of questionnaires, you could answer a very large number of yes/no, multiple choice, or 'rate yourself on a scale from 1 to 10' questions.

But it takes over 300MB to produce a fruit fly, and another 120MB to produce a chicken.  I don't think the genome operates anything like a questionnaire or a book.  The language is different.  Words are impressionistic devices invented in the inner worlds of human experience, and founded on innumerable shared experiences.  They are quite different from blueprints or physical designs.  It only takes 32 bits to represent the word "word", but behind that word is a vast network of meanings and associations based on countless experiences.  I'll give nature credit for efficiency with respect to physical design but the type of design for the layout of an organism is a fundamentally different sort of design from what is required to describe a personality.

So I'm not impressed by a 1.8MB quota for everything that is unique about me as a person.

Another reference point comes from being a programmer.  Even in the most advanced, high level languages, 1.8MB of code may take a long time to type but it does not comprise a very large system.

What seems infinitely more reasonable to me as a description of the interaction between DNA and personality is that a fairly small section of the genome (somewhere between dog and man) causes the brain to grow enormously, and that the consequence of such an enlarged brain is that one can occupy it, learning and developing all the subtle inward elements of a complex human personality.  Rather than assuming that the same mechanism that builds the person also tells him or her what to think, I prefer to see the DNA as the hardware specification, the environment as the operating system and the individual as the programmer.

The realm of personality and consciousness is much too subtle to be carved into stone, regardless of bit depth.

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