Ben, unbeknownst to his friends, was fighting his devolving urge to mate. Currently he was able to retain memory and social behavior but he was slowly losing it to his wild side.
As soon as the door shut with everyone inside, Bens caveman counterpart had chosen Samantha and Matt to be the first members of the tribe. Against Ben's true nature the caveman side elected to pressing the button on the app that devolved Samantha and Matt into Neanderthals like Ben.
Matt, feeling a tingling sensation, said "Hey is anyone else feeling strange right now?"
"Not at all" Shaniqua replied
"Must be the weather" said Tyler
"No it can't be the weather, maybe you are just sick" Suzy retorted
Then Samantha cut them off with a loud groan of pain and curled up on the floor. Then Matt did the same as Samantha and fell to the floor writhing in what appeared to be pain. Everyone but Ben rushed to see if their friends were alright when all of a sudden their muscles bulged and grew so big they tore their pants and shirts.
By now Matts underwear were highlighted by the impressive bulge Ben had when it finally snapped to reveal an enormous penis. Samantha on the other hand, her panty snapped when her legs grew muscular. Her bra snapped to show her muscular chest and her bare tits.
When both sat up an immense set of body hair covered their chests, genitalia, butts, arms, and hands. Finally a thick unibrow came to rest on their heads as they finished devolving to equivalents of Ben.