What was it about big metal ships, anyway?  People always expected the alien visitors to come in them.  In reality, it was not practical to send large metallic ships nor was it feasible to send fragile biological life forms in them, as they are quite vulnerable to the stressors of space flight, and were possessed of a finite life span which made traversing the vast distances of space personally impossible.  But alien intelligences had evolved far beyond our own, making it possible for them to visit other civilizations by proxy.  Recognizing the brevity of their own existence, the aliens had first used nanobots to precisely duplicate their own neurology, and translate it in every nuance to an intelligent machine which in effect became themselves.  This guaranteed a functional form of immortality, with the software that represented an individual's consciousness transferred to a different machine when the software of the biological body wore out.

So comprehensive was the transcription of the nanobots that the intelligent machine which it was transferred to retained the individual's complete life experiences, their unique orientations and abilities, and all that was in essence their personality.  The resultant machine became, in effect, that specific individual in their totality without the annoyance of a high-maintenance, disease-prone body which deteriorated with aging and had a finite life span.  

Bigger is likewise not always better, and so for the purposes of interstellar travel the intelligent alien machines were quite small, not much larger than a Terran insect, really.  Each of these small but very sophisticated machines bore an individual consciousness which had once been a biological entity, and the unit was capable of sustaining, repairing, and even replicating itself.  With infinite patience and efficiency these tiny alien machines streamed across the vast reaches of space in diverse directions, with one penetrating the atmosphere of our system's third planet from the sun to visit what was deemed by the unfathomable alien mind to harbor conditions hospitable to the generation of life.  The tiny but sentient and durable machine readily passed undetected by the sensing devices of the global military establishment as well as by SETI; it was so small as to be inconsequential.  Everyone on Earth was programmed to detect missiles and large metallic saucers, or at least a decipherable message from an E.T.

The alien consciousness in the tiny but remarkable vessel was drawn by the abundance of chemicals in the vicinity of Africa, where human life itself may have had its genesis.  Sampling the atmosphere to its satisfaction, the tiny craft identified an indigenous life form by a waterway, a massive, herbivorous mammal that humans call a hippopotamus.  Excited at its discovery, the minute alien buzzed about the great head of the hippopotamus, anxious to further investigate the sensory apparatus that seemed to be centered there.  The hovering movements of the alien caused a buzzing, droning sound in the ears of the large mammal, which in turn caused the hippo to perceive the probing alien as a bothersome mosquito.  The hippo snapped its powerful jaws upon the interstellar visitor, ending its long journey and quite crushing the extraordinary device.  Days later, the remains of the advanced alien technology would be excreted in a large pile of dung, and draw no attention whatsoever except for that of dung beetles, who had no use for or comprehension of evidence of vastly superior intelligences.

This would prove most unfortunate for the inhabitants of Earth, who thereafter would be visited by a planet destroyer sent by the perturbed alien civilization...