You are a hominid. Australopithecus africanus, perhaps. Foraging for nuts and berries, perhaps claiming the occasional ancestor of modern squirrels or rats or mice.
Then, it happens. An octagonal shape descends through the cloud tops, descending gently, purposefully. Your tribe, surprised, crouches to the ground and looks up with fearful eyes.
The shape descends, a plume of dust rising from the earth around its landing site. Silence, for a few minutes, and then, a light beams out as a portion of the thing descends, making contact with the ground. Minutes pass. Hours. Finally, the shadow of some eight-legged... thing... is cast upon the dirty earth.
A gigantic spider.
Our bipedal ancestors know not how to react, other than with abject horror. With tremendous strides, the spiders leapt upon their humanoid prey, envenoming them quickly, then moving effortlessly to the next. The resident humanoid population is brought quickly under the arachnids' control.
But then... what's this?
A rogue among the spiders, a rebel, looks upon the scrawlings of animals scribbled upon cave walls with charcoal by humanoids not with indifference, not with pity, but with admiration.
Wait, what's that? In the shadows? Another of his ilk... female.
In the nearly lightless cave, they both gaze in awe at the crude glyphs. Days pass. Weeks. They, too, prey upon the indigent humanoids for sustenance. A few elude their predators with cleverness, savvy. The traits the biped survivors carry in their genetic heritage will be passed along to future generations.
Eventually, their comrades decide Earth is an unsuitable world. With their advanced technology, they can glimpse into the future of this planet, see that eventually these simple humanoids will evolve into creatures most dangerous to a strangely diminutive progeny of their kind.
The octagonal craft lifts off, punches through the cloud tops, and quickly vanishes into the depths of space, a faster-than-light drive hurling the aliens to parts unknown.
Meanwhile, the male and female mate. They begin their own brood of young, and as they emerge from their silken egg sac, they greedily feast upon their father's now headless carcass.
The female looks upon her children for a moment. This planet, she knows, will be less kind to them than her home world. The gravity, greater; the atmosphere, less oxygenated. Eventually, future offspring may reduce in size, though increase in numbers.
Survival, in whatever form, is not without cost.
. . .
Years pass. Centuries. Millenia. Millions of "years".
The mother's corpse and her children have long since been cast to the winds of this world as dust. Now, the varied descendants of this alien race lurk in the shadows. The cold equations revealed in their glimpse into the future have, apparently, come true. These humanoids are of tougher mettle than their brutish cousins left behind in the distant past.
Yet, what is that twinge of fear, of horror, that lurks in the minds of so many a human being? When those small, eight-legged creatures dash out from beneath a refrigerator, drop down onto them from a ceiling, or worse, capture them in their vulnerable nakedness upon emerging from their daily cleansing ritual?
In the end, this matters little, for they have mastered... fire.