Transcript:

So, a neuron, if you remember basic biology, is responsible for transmitting electrical impulses. But specifically, the part of the neuron that does it is called the axon. And the axon has a sheath around it called the myelin sheath. The reason for the myelin sheath is that, without the sheath, the rate at which the signal will move is very slow. With the sheath, the rate at which the signal moves is quite fast. 

So now, it's interesting to figure out how did the sheath formed in the first place. So the evolutionary mechanism for forming the sheath is brilliant. So, this is the mechanism: What happened was, a virus infected an organism, and the gene that expresses the myelin sheath was part of the virus. And that gene was co-opted by the organism, which allowed it to form the sheath around its neurons. That gave it a huge evolutionary advantage. This is the beauty of evolution—genes can be co-opted from species that infect us. Interesting.