Lilly was with her husband and kids when the shift happened. Her and Dave were very happy with their small family with three kids, the twins Adam and Avery at ten years old, and little Alex at only a year old. They'd been enjoying a relaxing evening in their backyard up against the the nearby forest when it happened. The whole family suddenly dropped in pain, Lilly and Dave crawling to their kids to offer some measure of comfort as they feel their bodies bend and warp in shape.
They ended up crawling right out of their clothes, but didn't notice as their bodies shrank to ten inches in length, and a fourteen inch tail sprouted from Dave and Lilly's spines. Nor did they notice that's their hair had become more manelike, thick and pluming above their heads. Their feet too, had changed in mimic their hands as well, to better aid climbing trees.
When the moment passed, the family was forever changed, Dave walked over on all fours and picked up Adam and Avery, they followed their new instincts and climbed to his new mane, as Lilly carried little Alex in her arm.
The first ting that Lilly noticed, was that she (and her family) could no longer talk, but chatter and make calls more like monkeys than humans, and they were all naked. And hungry.
But as the small troop was just beginning to forage bugs to eat, and Alex nursing from Lilly's breasts, suddenly two bonobos hopped the fence. "There's a group of them!" One of them called, and the two rushed the family, who made a dash for the trees, trying to escape their pursuers.