Cielioids evolved from an ancestor similar to tumbleweeds on Earth.  These lightweight bushes would break off from their roots, and be carried by the wind to distribute their seeds. 

Some of these bushes evolved special leaves that could store water and nutrients after being separated from their roots in order to produce more seeds.  As time went on, some of the ancestor plants also developed specialized parts to digest insects, further extending their uprooted life.

In order to maximize their nutrition intake, and do some basic navigation, these plants evolved basic musculature and nervous systems that could move their branches and leaves a bit.  This proved so successful that the proto-Cielioids now spent most of their lives in an un-rooted state, after a short maturation.

Over a long span of time, the ancestors of the Cielioids lost much of their bushiness.  Their bodies gradually became large, single leaves that could flex to take advantage of the wind.  Some of these adapted to airborne life, while others developed basic limbs, torsos, and heads to live on land.

Gradually, the land-based ancestors of the Cielioids acquired an upright  posture in order to reach food sources that were off the ground and to see predators from a greater distance.

The first anatomically modern Cielioids appeared around half a million years ago.  Overall, their development of culture and technology has proceeded at a slower pace than that of the human race.