Olimar landed in a sublevel, one that was a barren tunnel. After he got his wits together, he gathered his pikmin and ventured into the darkness. Along the way, he encountered fifteen light-peach pikmin sprouts.
Perplexed, he plucked one. The result was... unexpected. The "pikmin" wasn't so much a pikmin as it was a pikmin seed with eyes, a longer stem and nubby feet. It was smaller than a white pikmin, and its stem was shorter than the average pikmin's.
"Krzzzzzt! This is quite peculiar! The pikmin you've plucked appears to be undeveloped! Could it be a primitive species...? My sensors aren't detecting anything noteworthy about this specimen. I don't see how it could be of much use..."
Olimar plucked the rest, and they fell in line with their "normal" brethren. As Olimar moved, the primitives had the easiest time keeping up with him. Soon, they encountered an orange bulborb. A fight ensued, and the primitives somehow managed to cling to their foe using only their feet. Whenever they were shaken off, the primitives would get up immediately, followed by the oranges and finally the other colors. Once the bulborb was dead, Olimar dismissed his pikmin to carry the corpse away, but the primitives merely stood there as the others did the work.
Primin are only found in caves and are meant to be nothing more than a convenience, like bulbmin. They're the fastest pikmin of all, even faster than whites. Their recovery time is also unrivaled, topping that of orange pikmin. These traits make them very useful in hectic battles. Their main weakness is that they're nearly useless outside of battle. Since they have nothing to grab with, they can't carry treasures/carcasses nor can they pull obstacles like boxes.