"Oh, the post-humanity!" Meet the Blimpers (my fan-made descendants of the Striders from C.M. Kƶsemen's All Tomorrows) (tl;dr = Striders had airbags for safety, they evolved into Hindenburgs and avoided the dino-chickens.)
One might not see any family resemblance between these balloon creatures and their lanky ancestors, but the curious evolutionary path between the two was inadvertently put in place by the Qu themselves. The first Striders were notoriously fragile, often unable to recover from a fall, even on their low-gravity world. In response, the Qu made one final adjustment to the Striders' anatomy: air bags. These gas-filled chambers were not obvious at first glance, but their strategic placement at certain points on the Striders' body afforded a softer landing in the event of a fall. But evolution, through trial and error, found another purpose for these bladders. Over just a few million years, some lineages of Striders had been naturally selected to fill their more vertically-located air bags with hydrogen gas, giving them more stability and allowing them to grow to even greater heights.
Ā Although the flammability of hydrogen severely limited the biomes in which they could thrive, these balloon-assisted Striders kickstarted an ecosystem revolving around this feature. As each generation grew successively taller, predators evolving alongside the Striders also began to exploit the abundance of hydrogen gas in their prey for their own gravity-defying evolutionary features that gave them that predatory edge. Never enough of an edge to completely wipe out the Striders, but more than sufficient to deter all competition from the descendants of terrestrial poultry, when those predators began to spread across the planet. The ballooning Striders did not have to fend off the dino-chickens alone; they had an entire ecosystem resisting the outsiders.
Over the course of tens of millions of years, the Striders needed less sturdy appendages for walking, as their support increasingly came from above in the form of their huge gas bladders. Eventually they ceased walking entirely, becoming the Blimpers, drifting through the high treetops alongside many other organisms that followed suit in this evolutionary arms race of buoyancy. Although the Blimpers and their many evolutionary offshoots could never leave the treetops and rocky outcrops for long due to the risk of static electricity, they were in no danger from their ancient feathered foes that squawked far below.
The Blimpers never regained the sapience of their human ancestors; many evolutionary sacrifices were made to reduce their weight, including the size and complexity of their brains. In their competitive natural environment, having high intelligence was nowhere near as advantageous as being a high-flying airhead.
