Richard Dawkins used his Twitter account to ask some unanswered questions about biology that he finds fascinating, inviting others to share ideas about hypothetical life forms that may or may not have evolved. By nature, these questions can only be addressed using some degree of speculation, but I find this one particularly interesting and I will attempt to answer it. He asks:
Why do nervous systems use slow, chemically mediated pulses of voltage change rather than fast electric currents along wires?
Indeed, although neurons accomplish marvelous things, the way in which they are made seems awfully complicated. If nature was an engineer, it would get fired for designing a system as costly as the brain. Every time a burst of activity is generated by a neuron, a complex mechanism is triggered that involves the opening of channels present throughout the membrane of the neuron. The opening of these channels lets a flow of electrically-charged ions pass, which modifies the voltage difference between the inside and the outside of the neuron. This change in voltage then triggers the opening of other channels, located micrometers away, which results in a cascade of channel openings that makes the signal progress along the neuron, typically from the cell body (left on the figure below) to the pre-synaptic terminal (right on the figure below).
This mechanism is very energetically costly and far from optimal, if one looks at it from the engineering perspective. Why didn’t life find a more simple solution, like the metallic cables that carry our data signals and our electricity? It may seem far-fetched to think of a life form (other than humans) that would have evolved to process information using electrical cables, but it is certainly not impossible. Hemoglobin, which carries oxygen in our bloodstream, uses iron atoms. It is a good example of a protein that handles a chemical element, iron, which can take the form of a metal with a sufficient conductance to allow for electrical communication. The iron atom in hemoglobin is obviously not crystallized into a metal wire, but we can certainly conceive of a set of proteins that could deposit atoms next to each other, forming metallic wires that would then carry electrical signals. Carbon, a very common molecule in our body, can also be made into an acceptable conductor. The insulation of the cable would have been possible too. In fact, in our own brains, the axons of neurons are often wrapped by other cells called oligodendrocytes, which play a role in increasing the capacitance of neurons and reduce ion leakage. The idea that a life form may have evolved abilities to create metallic wires and insulate them is therefore not implausible.
Assuming the theoretical plausibility of such a life form, let us push further the logic laid out by Richard Dawkins and consider, from an engineering perspective, what would be the advantages of the two systems of transmission. Then, we will consider biology.
1. As indicated previously, electrical wires are faster than neurons at carrying signals. Nerve impulses in neurons travel at up to 120 meters per second. In contrast, electrical signals in wires travel at speeds close to the speed of light. Many factors in the composition of the wire influence the speed of an electrical signal, but we can safely say that they conduct at least a thousand folds faster than neurons.
2. Electrical wires require very little energy. The human brain would save enormous amounts of energy if it was running on a metallic wire system instead of neurons.
3. Electrical wires can be much smaller than axons. Modern microprocessors have elements that are 22 nanometers in width, while the diameter of the axon of a neuron is typically around 1000 nanometers. Smaller circuits would mean that a brain of the same size could contain many more connections, and potentially process more information.
Metallic cables are not perfect either; they come with a big disadvantage.
1. Metal is less malleable than neuronal membranes and harder to “undo”. The brain relies on neural connections that are constantly being reshaped as we learn stuff and develop. The membrane of a neuron can be reshaped by proteins because there are no powerful ionic links that bind atoms to each others in the membrane. Metals on the other hand are polycrystals, and breaking the link between two parts of that polycrystal requires physical strength or chemical reactions involving a product like nitric acid.
I could point out to a series of advantages that neurons seem to possess over metal wires. Neurons have a greater diversity of channels and neuromodulators that may influence their activity in more subtle manners than what a present-day electrical system can do. However this would be unfair to the electrical wires; they too could have a diversity of ways in which their signals could be modulated, we just tend to create them in a more straightforward manner because we choose to.
The biological reality
The thinking process we have engaged in up to now is essential for an engineer planning to produce a piece of electronic, but that is not how evolution works. Evolution works by very small mutations that occur in some genes of one animal, and these mutations may increase or decrease their presence in the next generations, depending on how much of an advantage or disadvantage they procure to the organisms carrying it. I believe that the best possible answer to Dawkins’ question lies in a principle of evolutionary biology called historical constraints. The idea is that because mutations are small and affect only a few genes in each generation, it is expected to be very rare for such mutations to induce what we could call a big jump, that is, a new complex innovation with many components that seem to work perfectly together. In The Extended Phenotype, Dawkins put it this way:
Not only must the end product get off the ground; so must every intermediate along the way, and each intermediate must be superior to its predecessor. When looked at in this light, far from expecting animals to be perfect we may wonder that anything about them works at all.
Neurons have two large sets of characteristics that make them different from other cells. The first is their wire-like appearance – the fact that they have branches that can extend over short or long distances. The many branches of neurons allow them to build complex networks and communicate quickly between different parts of the brain or the rest of the body.
The second set of features can be referred to as their modulatory abilities. I include in this category all channels that depolarize, hyperpolarize and modulate transmission within the branches of a neuron. I also include the ability to form synapses and modulate the activity of other cells through these connections.
Ultimately, these two sets of features are shared by all brains that we know of as well as by all electronics we have ever created. In the physical implementation of a network, there has to be some sort of branching that allows subsets of the network to communicate with distant elements – sometimes these connections may be very well organized in space, as is the case in a CPU, and sometimes they may have huge variation in their spacing and organization, as is the case with the brain. The modulatory abilities are also defining features for any nervous system or piece of electronics; if the branches of the network did not modulate each other, then there would be no processing of information.
We can now reformulate the principle of historical constraints to the particular question of evolving neural systems. Our formulation could look like this:
Not only must the end product have modulatory abilities and wire-like appearance; it must be that one of the two properties was there before the other and that it was adaptive in the absence of the other.
There, we have the beginning of an answer to Dawkins’ question. We have already discussed this question with Kenneth Kosik on NEURO.tv Episode 4. The modulatory abilities of biological cells, and the vast array of genes that is associated with it, was present millions of years before the appearance of any form of wire-like appearance. The brain as we know it today has evolved from an ancestral state in which molecules were being released by cells that had no branches and that did not build physical circuits. The sender-receiver interactions between cells is evolutionarily older than their wiring, and it was a necessary step above which the wiring could then evolve. The excitability of neurons through ion channels has a similar history; those ion channels exist in bacteria, plants and animals that have no brain. In these organisms, they play multiples roles such as propulsion, sensing and adaptation to local osmolarity.
Now consider the alternative hypothesis, that wire-like appearance would have evolved first, before modulatory abilities. This is in all practicality impossible, because wires have, by definition, no function other than to carry signal from one place to another. In electronic terms, wires are the least interesting bit of a system; they don’t change the signal, they don’t process information, they just relay it. For wire-like appearance to evolve, there has to be senders and receivers on the two sides of the wire to be able to send and process the signal; otherwise we have a useless wire, one that is unlikely to spread in an entire population during evolution.
The reason nervous systems do not operate on electrical wires, I argue, is that there are no life forms – or too few – that have evolved the ability to make metal filaments that would serve other functions, before they could be used as communication devices. In contrast, during most of our evolutionary history, Earth has been populated by organisms that use ion channels and chemical release, both of which have adaptive roles in the absence of wire-like connections.