Nachalian

The truth is that they are not a lost species but a bioweapon. Built by an alien empire known as the Xeltian Imperial Expanse. They are created as a near-blank slate genetic canvas. When they enter the orbit of the planet they were sent in, they scan a likely landing point. They would learn the language and moral and social structures of it. They would craft the weapon to look attractive to the societal structure it wanted to integrate itself into. In a planet with multiple local power centers, this would mean numerous pods forming different aesthetic standards to maximize their appeal.    They would then try to become hero's and celebrities in their new home, crafting their origins and source to maximise appeal where ever they are infiltraiting to garner praise and love. Once had integrated themselves they would signal to the empire. Once invasion force was near able to launch a surprise attack, the sleepers would launch the pre attack weakening the planetary defenses what ever they be before the main invasion force arrives.    They proved very successful in taking down multiple planets allowing the empire to slowly expand with out massive loss of life or supplies of their own forces.    These agents could be on a planet for decades or more, sometimes multiple generations of their host planets dominate life.    The pods containing these sleeper bio weapons would be launched and take massive journies to the target worlds. This is what lead to the complication for the ones sent to Earth.    After the bioweapons were launched in number to handle the earth and its Meta population and other defenses, but before they landed on Earth the Imperial Expanse was overthrown in a popular revolution. The new government sent a message to all sleeper bioweapons that they were to shut down, self-destruct and not try and take over the government.