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.

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.

What determines intelligence? Size of neurons, their density within the brain and the number of connections they make with other neurons The size of the brain, the bigger the brain the more intelligent the organism. The number of connection made by the neurons.

Why are big brains in the animal kingdom not necessarily smarter? The neurons in bigger brains are smaller and slower Big brains have longer, slower neurons Big brains form too many connections and slow the thinking process.

If human brains were to evolve to be the size of a whale's brain then we would have serious issues with random noise from accidental pulses heat loss too many signals

Big brains require no time to think more energy less time to think

As neurons evolve to become smaller we encounter a problem with heat loss strength of the signal noise

Increase in signal speed can be achieved by making axons thicker making axons longer decreasing the strength of the signal