Just like the various exams and tests on Earth a millennium ago, the alien invaders conduct various forms of tests to select even finer individuals among this group of already relatively superior humans and their descendants.
And those genetically modified humans that Shi Wanyun encountered on the world tree are people who have passed the second round of selection.
But the selection has not ended yet.
Humans who have passed the second round of selection are like students who passed the college entrance exam millennia ago and got into a university.
They still do not meet the standard of excellence.
Thus, on that world tree, akin to higher education institutions of old, a new round of selection takes place.
The third level of selection.
Based on the success of genetic mutations, the status of humans who have entered the world tree is differentiated by levels of superiority and inferiority.
The three levels of selection are interconnected.
