What is it that gives us greater intellect than animals?
Is bigger better?
Watching Pinky and the Brain gives the impression that size of brain correlates to increased mental power, and given that our brains are bigger than a lot of animals, size is probably a contributing factor. However, this can’t quite be the full answer as there are other mammals with comparable brain size to humans that lack our humanoid mind-power.
Studies have examined the evolutionary development of the human brain with the theory that our intellectual prowess is linked to structural alterations in our brain cells, the neurons. However, this doesn’t seem to be the case, with the structure and variety of neuronal cell types having remained relatively constant through mammalian evolution.
However, astrocytes- key neuronal partners in the brain, have been shown to increase in size and complexity in human evolution. This implicates astrocytes as a key factor in the human brain’s unique functional power. (Read my first post on astrocytes to learn more about them)
Further studies comparing astrocytes in humans to astrocytes in other animals’ supports this theory. In humans there are a greater number of astrocytes per neuron than that found in smaller mammals. And structurally, human astrocytes are quite different to animal astrocytes, with rodent astrocytes limited to three or four core branches compared to the 40 branches per individual human astrocyte. The disparity in astrocyte complexity between rodent (A) and human (B) astrocytes is elegantly demonstrated in these two images.
Increased morphological complexity of human astrocytes means that they are approximately 2.6x larger in diameter than rodent astrocytes and with nearly 30x the volume. With greater size and number of branches it’s not too surprising that human astrocytes contact 2million neuronal junctions (synapses) each compared to the 90,000 contacts of a single rodent astrocyte. This gives human astrocytes nearly 10x more connections!
It is not just structurally that astrocytes are superior in humans; there is also evidence of human astrocytes being better at their job of supporting neuronal signalling. Neuronal signalling stimulates a chemical response in astrocytes, and more intelligent species, humans, are found to have quicker response speeds.
The power of astrocytes in determining brain capability was elegantly demonstrated in an experiment where rodent astrocytes were replaced with human astrocytes causing rodents to learn 3x faster!
In addition to all the human astrocytic advantages there is also a much greater variety of human astrocytic subtypes than the two identified rodent types.
With increased size, complexity, diversity and enhanced internal signalling, astrocytes make an attractive candidate to explain our unique neurological capabilities.