So a group of behaviorally modern humans was cut off from the rest of the earth-like planet about four or five millennia ago. At the time of the separation, humankind had reached a technology level including cattle ranches, brass, writing, and cities. Due to a founder mutation, their descendants' legs do not develop, and the body ends at the hip. Otherwise, they stay close to normal human physiology, or at least as close as photographer Kevin Connolly and gymnast Jennifer Bricker do. They typically walk on their fists and bottom with a symmetric swing-through gait, placing both fists and swinging the torso between the arms at roughly 1 m/s.

Would it be plausible for these people to remain at the apex of the food chain or otherwise thrive long enough through hunter-gatherer to reestablish agriculture and develop industry? If so, how might they adapt? If not, what technological level would they have to reach before the separation for them to continue to thrive, and how would a founder population of four husband-and-wife pairs carry the knowledge of this technology?

I'm aware that they would need to solve at least the following problems:

• escaping danger or finding a meal when they can't run quite as fast as baseline humans
• carrying things, especially offspring

What other problems might be worth mentioning?

A real-world analog might be deafness in Martha's Vineyard, where people worked around the impairment by inventing a sign language.