Limit of Intelligence
This article was adapted from Scientific American July 2011 The Limits of Intelligence P22.
A honeybee has only a few neurons but gets the most out of them. On the other extreme an elephant has 5 million fold increase in neurons but suffers the inefficiency of neurons being spread out to the point where it takes 100 times longer, than in the bee's brain, for a signal to travel from one end of the brain to the other. Signals take so much longer to travel from the elephants brain to its foot that it has to plan its next step and therefore squander precious processing power.

One might think that evolutionary processes could increase human intelligence by packing more neurons into the brain space and increasing the rate at which signals are exchanged between neurons. However there are some physical limits that cannot be overcome. Limits that trace back to the nature of neurons and the way in which chemicals are exchanged between the intracellular and extracellular environments.

Increasingly the density of neurons in the brain has some obvious drawbacks. None more so than the energy demand of the brain and its ability to remove heat. At just 2 percent of the average human body weight the brain demands 20% of the body's energy consumption at rest.


AN average nerve cell has projections called dendrites which receive information and a long axon along which the signal is transmitted. As brains grow larger the length of nerve cells also grows longer. These long nerve cells pack into the brain less densely. With increased distance, lines of communication run slowly and nerves have to compensate by becoming thicker. Thick nerves transmit the signal faster than thin nerve fibres. Doubling the diameter of a nerve fibre increases the transit time of a signal by 40% but doubles the energy required to do so.



Having smaller, more densely packed neurons does seem to have a real impact on intelligence. In 2005 neurobiologists from the University of Bremen, Germany, reviewed several traits that predict intelligence and concluded that " The only tight correlation with intelligence is in the number of neurons in the cortex plus the speed of neural activity". Primates tend to have smaller neurons which can pack more densely in the cortex of the brain and transmit signals faster. Elephants and whales might be smart but their larger neurons lead to inefficiencies. As shown on the right, the larger whale brain has a lower packing density of neurons, as compared to the smaller human brain. The greater size of the whale brain causes a greater distance between neurons and leads to greater transmission times.


You would think that evolving smaller, more densely pact neurons is the key to greater intelligence. Similarly brains might become more efficient by evolving axons that can carry signals faster over longer distances without becoming thicker. But there is something that prevents the shrinkage of neurons beyond a certain limit. The mother of all limits is the inherently unreliable protein ion channels that generate an electrical pulse.

These ion channels allow for the flow of sodium, potassium and calcium ions in and out of the cell thus generating a pulse.



These ion channels are so small that they can flip open or closed by thermal vibration. Studies have shown that these ion gates are not switched on and off in a predictable manner. A voltage change, which is the sign that a signal has arrived, just increases the likelihood that the gates will open. It sounds like a horrible evolutionary design flaw, but it is a good compromise. If you make the gate too loose then it is easily opened and you get a lot of unwanted noise. If you make it too tight it takes a great deal of energy to open it but you don't get any noise. In other words neurons save energy by using hair-trigger gates.

Reliability is only achieved when a large number of gates are used to vote on whether an impulse will be generated by the neuron. Now this becomes problematic when the neuron is small as you reduce the number of gates used to carry the signal.
So have humans reached the limit of brain complexity? Well there does not appear to be any hard limit on brain function, but it appears more likely that it is a case of "diminishing returns". "It appears less and less worthwhile the more you invest in it." There is only so many neurons that can pack into our brain and those neurons can make only so many connections among themselves and carry only so many impulses per second.. A bigger brain will have costs in energy consumption, heat loss and greater times of transmission over longer distances within the brain. May be technology will help us achieve greater intelligence.

What determines intelligence?

Why are big brains in the animal kingdom not necessarily smarter?

If human brains were to evolve to be the size of a whale's brain then we would have serious issues with

Big brains require

As neurons evolve to become smaller we encounter a problem with

Increase in signal speed can be achieved by