The field of Sidar biology remains a highly speculative one, as we are rather lacking in pure Sidar to study in order to make definitive statements. Such folk as the half-Sidar to be found in certain shadows provide some evidence, but not enough. Still, it is traditional for scholars to venture in where fools fear to tread, so I will stick my head out and hope it will not be bitten off.
Surviving evidence indicates that the best description of the Sidar is to call them meta-human. Through a combination of evolutionary pressures, divine meddling, and deliberate self-engineering, they gained capacities far beyond those of ordinary humans. Despite all the changes of later Sidar history, however, they remained sufficiently human to breed with ordinary humans without needing to resort to the use of special powers in order to do so.
What little evidence that survives from their earliest days indicates that they were fairly close to ordinary humans at one time, with perhaps a bit more talent for sorcery and elementalism than ordinary humans. Several things changed this.
The Sidar had a close relationship to the Elder Gods, who interbred with them, producing a strain of divine blood in the Sidar bloodlines. Certain families emerged who became great figures and leaders because of this heightened potential.
Eventually, the Sidar learned how to genetically engineer themselves, enhancing their lifespans and psychic potential and their bodies to superhuman levels. They learned how to create artificial gods and bred with them, heightening the divine power within certain bloodlines. Fuyutsuki and Anno appear to have come from one such bloodline, 'The Family of Design'.
In addition, the Sidar practiced many forms of body augmentation which granted enhanced abilities to those less lucky in bloodline, but worthy through merit or fortunate in their connections.
However, with the Fall of the Sidar, many of these powers ceased to function. Many gods perished who once had been a font of power. Many Sidar were slain during the chaos of the last days. The survivors seem to have found themselves cut off in various locations in shadow, stripped of the laws of nature on which their power rested, stripped of the divine strength they once had. Most of them died. The rest were lucky, and found themselves among peoples who knew how to live without the old powers. The Sidar seem to have bred with these populations, and then faded away as they died, one by one, leaving behind only some of the strength they once possessed.
It seems likely that the old blood of the Sidar lies dormant within many in Shadow and at the Poles, awaiting some day when the world changes and the old strengths return...
There are a variety of theories as to where all the peoples of shadow and the Primal Poles came from. It remains unclear which of them are true. Perhaps they all are in some measure.
Shorgaazz has theorized that some small remnant of the Sidar was saved by Anno and Fuyutsuki, and that these peoples, now fallen from their former power, are the ancestors of the peoples of the Poles, and that all other humans in Shadow are but reflections cast by them.
Miryan has theorized that many small pockets of the Sidar survived the cataclysm in various ways, and now their descendents are scattered across shadow, having lost their powers and forgotten their origins.
Dorcas has theorized that Fuyutsuki and Anno's creative vision included a vision of humanity spread across their universe, and that the very act of creating the Pattern created human beings across a very wide swathe of Shadow.
And Gorgas, because he seems to enjoy making theories that can't be proven, argues our entire universe is actually just a simulation inside some Sidar computer to see if this method of preserving the universe would actually be viable; in other words, he thinks that we are all just very sophisticated CGI and that the GodsWar? is actually raging outside the computer we're in, while Fuyutsuki and Anno run a simulation of a possible way to save the world...
It remains to be seen which of these is true.