Many of whom looked an awful lot like us: longish torso; hind legs almost half the total heel-to-head's-crown length, lined up with the torso; feet with a well-defined heel; two forelegs coming off well-defined shoulders, head at the end of a short neck.
Sure, some of them had bat-wings, funny-looking eyes, and weird complexions: but some makeup, an overcoat and an oversize hat, and they could walk around New York City without attracting all that much attention. Okay: maybe San Francisco or Los Angeles.
But, that could be the way it is. That exotic-but-familiar universe of earlier sci-fi may be just wishful thinking. We could be the only people who have ever been living in this solar system, this arm of the Milky Way galaxy, or anywhere in the universe.
Considering that we're not selling souvenirs to, and performing colorful native dances for, tourists from the more well-to-do worlds of the Galactic Empire; or slaving away in the lint-mines of Evil Overlord Squig's Unified Worlds; there is a distinct possibility that we're it.
On the other hand, there may be people who are a whole lot smarter than we are, and who don't bother to travel: physically and openly.
As a character in Walt Kelley's Pogo said, whether it's just us, or we're at the low end of the intelligence scale: it's a sobering thought.
Then, there's the possibility that we've already been to the stars. People who were essentially like us were around during the last major glaciation; the sea level was a lot lower then, exposing a great deal of the continental shelf.
Earth Was Not Always As It is NowThink about it: Many major cities are on or very near the coast. Civilizations - the ones that we know of - grew for the most part along major river valleys. During the last major glacial period, major rivers still ran down to the sea - but the shore wasn't where it is now.
Gone Without a TraceSomebody pointed out that a civilization equivalent to the one that bankrolled the Columbus expedition could have existed during the inter-glacial period before the one we're in - and left no obvious traces of its existence.
You can go further than that. There's been speculation that the last ice age ended fast, with continental glaciers melting and sliding into the ocean on a scale of weeks or months. If that happened, a thriving civilization's cities would have been flooded and sunk beneath hundreds of feet of water - with only a few survivors, who were near enough to boats and savvy enough to get out to relatively calm water while the getting was good.
Even if the seas rose slowly, it wouldn't have been good for any civilization. Think the worst-case global warming scenario, and multiply it by a few factors.
Atlantis? Sure: Makes a Good StoryNo, I don't think there was something like our stories of Atlantis. No fantasy-cities, wizards, and all the rest. Still, it would make a good story.
I do think, though, that we don't know all there is to know about what we call 'prehistory' - the period before the civilizations we're aware of started keeping records outside their memories. There's a detail or two about the Sphinx in Egypt, for example - but it's getting late, and I need my sleep.