# Digital brains beat biological ones because diffusion is too slow

I've spent quite a bit of time thinking about the possibility of genetically enhancing humans to be smarter, healthier, more likely to care about others, and just generally better in ways that most people would recognize as such.

As part of this research, I've often wondered whether biological systems could be competitive with digital systems in the long run.

My framework for thinking about this involved making a list of differences between digital systems and biological ones and trying to weigh the benefits of each. But the more I've thought about this question, the more I've realized most of the advantages of digital systems over biological ones stem from one key weakness of the latter: they are bottlenecked by the speed of diffusion.

I'll give a couple of examples to illustrate the point:

1.  To get oxygen into the bloodstream, the body passes air over a huge surface area in the lungs. Oxygen passively diffuses into the bloodstream through this surface where it binds to hemoglobin. The rate at which the body can absorb new oxygen and expel carbon dioxide waste is limited by the surface area of the lungs and the concentration gradient of both molecules.  
    ![21.1A: External Respiration - Medicine LibreTexts](https://med.libretexts.org/@api/deki/files/43955/jsel4j4bqhegxblreo4r.png?revision=1)
2.  Communication between neurons relies on the diffusion of neurotransmitters across the synaptic cleft. This process takes approximately [0.5-1ms](https://nba.uth.tmc.edu/neuroscience/m/s1/chapter06.html#:~:text=For%20chemical%20synapses%2C%20there%20is,change%20in%20the%20postsynaptic%20cell.). This imposes a fundamental limit on the speed at which the brain can operate.  
    ![How does a neurotransmitter move across the synaptic cleft, active  transport or diffusion? - Quora](https://qph.cf2.quoracdn.net/main-qimg-79757ed73562ec51e65f7006b4381612-lq)
3.  A signal propogates down the axon of a neuron at about 100 meters per second. You might wonder why this is so much slower than a wire; after all, both are transmitting a signal using electric potential, right?  
      
    It turns out the manner in which the electrical potential is transmitted is much different in a neuron. Signals are propagated down an axon via passive [diffusion of Na+ ions into the axon via an Na+ channel](https://www.youtube.com/watch?v=pbg5E9GCNVE). The signal speed is fundamentally limited by the speed at which sodium ions can diffuse into the cell. As a result, electrical signals travel through a wire about 2.7 million times faster than they travel through an axon.  
    ![File:Action potential propagation in unmyelinated axon.gif ...](https://upload.wikimedia.org/wikipedia/commons/f/f6/Action_potential_propagation_in_unmyelinated_axon.gif)
4.  Delivery of energy (mainly ATP) to different parts of the cell occurs via diffusion. The fastest rate of diffusion I found of any molecule within a cell was that of positively charged hydrogen ions, which diffuse at a blistering speed of [0.007 meters/second](http://book.bionumbers.org/what-are-the-time-scales-for-diffusion-in-cells/). ATP diffuses much slower. So energy can be transferred through a wire at more than 38 billion times the speed that ATP can diffuse through a cell.  
    ![](https://i.imgflip.com/7wz9aq.jpg)

Why hasn't evolution stumbled across a better method of doing things than passive diffusion?
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Here I am going to speculate. I think that evolution is basically stuck at a local maxima. Once diffusion provided a solution for "get information or energy from point A to point B", evolving a fundamentally different system requires a large number of changes, each of which individually makes the organism less well adapted to its environment.

We can see examples of the difficulty of evolving fundamentally new abilities in [Professor Richard Lenski's long-running evolution experiment](https://www.youtube.com/watch?v=w4sLAQvEH-M) using E. coli. which has been running since 1988. Lenski began growing E. coli in flasks full of a nutrient solution containing glucose, potassium phosphate, citrate, and a few other things.

The only carbon source for these bacteria is glucose, which is limited. Once per day, a small portion of the bacteria in each flask is transferred to another flask, at which point they grow and multiply again.

Each flask will contain a number of different strains of E. coli, all of which originate from a common ancestor.

To measure the rate of evolution, Lenski and his colleagues measure the proportion of each strain. The ratio of one strain compared to the others gives a clear idea of its "fitness advantage". Lenski's lab has historical samples frozen, which they occasionally use to benchmark the fitness of newer lineages.

After 15 years of running this experiment, something very surprising happened. Here's a quote from Dr. Lenski explaining the finding:

> In 2003, the bacteria started doing something remarkable. One of the 12 lineages suddenly began to consume a second carbon source, citrate, which had been present in our medium throughout the experiment. 
> 
> It's in the medium as what's called a chelating agent to bind metals in the medium. But E. coli, going back to its original definition as a species, is incapable of that. 
> 
> But one day we found one of our flasks had more turbidity. I thought we probably had a contaminant in there. Some bacterium had gotten in there that could eat the citrate and, therefore, had raised the turbidity. We went back into the freezer and restarted evolution. We also started checking those bacteria to see whether they really were E. coli. Yep, they were E. Coli. Were they really E. coli that had come from the ancestral strain? Yep. So we started doing genetics on it.
> 
> It was very clear that one of our bacteria lineages had essentially, I like to say, sort of woken up one day, eaten the glucose, and unlike any of the other lineages, discovered that there was this nice lemony dessert, and they'd begun consuming that and getting a second source of carbon and energy. 
> 
> Zack was interested in the question of why did it take so long to evolve this and has only one population evolved that ability? He went into the freezer and he picked bacterial individuals or clones from that lineage that eventually evolved that ability. And then he tried to evolve that ability again starting from different points. So in a sense, it's almost like, well, it's like rewinding the tape and starting let's go back to the minute five of the movie. Let's go back to a minute 10 of the movie, minute 20 of the movie and see if the result changes depending on when we did it, because this citrate phenotype there were essentially two competing explanations for why it was so difficult to evolve.
> 
> One was that it was just a really rare mutation. It wasn't like one of these just change one letter. It was something where maybe you had to flip a certain segment of DNA and you had to have exactly this break point and exactly that break point. And that was the only way to do it. So it was a rare event, but it could have happened at any point in time. The alternative hypothesis is that, well, what happened was a series of events that made something perfectly ordinary become possible that wasn't possible at the beginning because a mutation would only have this effect once other aspects of the organism had changed. 
> 
> To make a long story short, it turns out it's such a difficult trait to evolve because both of those hypotheses are true.

This is a bit like how I think of the evolution of the ability to transmit energy or information without diffusion. Evolution was never able to directly evolve a way to transmit signals or information faster than myelinated neurons. Instead it ended up evolving a species capable of controlling electricity and making computers. And that species is in the process of turning over control of the world to a brand-new type of life that doesn't use DNA at all and it not limited by the speed of diffusion in any sense.

It's difficult to imagine how we could overcome many of these diffusion-based limitations of brains without a major redesign of how neurons function. We can probably raise IQ into the low or mid 200s with genetic engineering. But 250 IQ humans are rapidly going to be surpassed by AI that can spawn a new generation of itself in a matter of weeks or months.

So I think in the long run, the only way biological brains win is if we simply do not build AGI.