The oldest known minerals are 4.4 billion years old.
I believe it is the "myelin sheath." White matter = myelinated (has myelin sheath), while gray matter = unmyelinated (no myelin sheath)!
Because of their relatively simple genetic structure when compared to other organisms. Not to mention, they breed in extremely high numbers which is great for scientists looking to do many trials, as most are. Their life cycle is also very short which means we can observe their entire life span and witness the effects of testing at all stages.
