Humans appear to be a young, energetic race. They burst onto the scene of the People of Ia with their First Sail about three hundred years ago, and have made colonies, friends, enemies, war, treaties, and discoveries more or less continuously since then.
But appearances are deceiving. Humans are odd, and have a mysterious deep past.
The first oddity is that humans come from three "home" archipelaga, not only one. The three clusters of islands were inter-navigable by well-known charts, the most ancient of which date back over two thousand years, and could be navigated by sculled ships. Minor civilizations rose and fell several times among these three archipelagi over the past few thousand years, before the first real understanding of marastronomy permitted a sail to any other arch.
The second oddity was noticed first by dwarfs: none of the three human home archipelagi appeared to provide a Gift - humans did not have an extra power on their home arch, unlike all other known races.
Extensive ruins, some unmistakeably human, have been found on islands in arches not far from the human home arches. Dwarfs knew some of these, as did eladrin. More such ruins have been found in areas reachable only via human arches, and thus discovered only after human First Sail - now known to be a misnomer.
Humans appear to be a very old race, and today's human civilization appears to be a revitalized, descended remnant of an older, fallen civilization, whose ancient ruins predate the oldest records of dwarfs (the oldest known race whose origin is near human worlds). (Eldar are likely to be older, but Eldar have not been in "local space" for very log. The same is true for some other races as well).
Occasional primitive, degenerate human tribes have been found in Ia; these form the basis of many creatures in D&D whose canonical origin is some offshoot of humanity (see, for example, shifters and shadar-kai).
Adding to the strangeness of humans, there appear to be ways to permit humans to interbreed with Eldar; while it is not "routinely" possible, events have happened at least twice that intermingled humans with Eldar, producing two viable new races (see half-elves and half-orcs).
There is even a small additional mystery in the relationship of halflings to humans, which appears in some ways to mirror the relationship between gnomes and eladrin.