The history of evolution on earth may hold the answer for why the aliens are so secretive.  As demonstrated earlier, there have been inexplicable sudden appearances of diverse new body plans at certain periods of earth's history, and inexplicable movements of species across oceans.  But these events do not seem to have the markings of a well-designed plan that has been effected down through the ages.  Events like the Cambrian Explosion, the rapid evolution of snakes and birds, and the sudden appearance of seals and turtles – these rapid evolutionary events are spaced so haphazardly across such great stretches of time, that it is unlikely they are part a premeditated plan of extraterrestrial guidance across all time. 

     Thus, the bulk of alien involvement in earth's evolutionary history was not premeditated; rather, it was accidental, temporal, and non-directional.  It was not intelligent design; rather, it was intelligent interference. 

     A logical hypothesis, therefore, is that the aliens did not create or design new forms to put on earth.  Rather, they designed new forms for themselves, and occasionally they accidentally leaked DNA from their genetic engineering laboratories, which unintentionally came to earth. 

     From the aliens' perspective, why should they care about the course of evolution on earth?  Is there any profit in it for them?  If not, then why should they try to influence it?  If aliens have a stake in the genetic engineering of new forms, it would be in the genetic engineering of themselves, in order to improve themselves, or to diversify themselves in order to fulfill different tasks, or to create subordinate slave species to assist them.  They have no reason to cause genetic engineering on earth.  Hence, they don't intentionally do so.

          However, if some forms on earth occasionally produce a DNA sequence they find useful, they may abduct the individual who carries the DNA sequence, and copy that individual's DNA for usage in their own laboratories, for the improvement of their own species.  The aliens, in essence, create hybrid alien-earthling DNA.  Occasionally, every once in a long while, one of the hybrid DNA strings accidentally finds its way back into earth's general population, and breeds with other members of the original species, thus interjecting a new and nearly functional DNA sequence into the gene pool, which is adjusted by means of natural selection to produce a fully functional new form.  If several such events occur within a lineage in rapid succession, we have an explanation for the sudden appearance of new forms.  Thus, DNA leakage from the genetic engineering laboratories of aliens may very well be the reason for the abnormally sudden appearances of new forms that paleontologists regularly observe in the fossil record.  The rich biodiversity of earth, therefore, is made possible because we are the accidental DNA dumpster of space aliens.

     What causes such breaches of security in the alien laboratories?  How is it that individuals carrying hybrid DNA are allowed to roam free procreating in the general population?  If the book of Enoch and other ancient texts are to be believed, these breaches are the result of bestial perversion and lust, which drives "angels," that is aliens and hybrids, to engage in acts of intercourse with the species of earth.  This may explain why the angels who had sex with human women were thrown into the dungeons of hell, according to the ancient texts, because the government of the aliens considers it a crime to compromise the security of the genetic engineering laboratories, and so they lock them in some kind of prison, which they have revealed to us as "hell." 

     This also explains why sudden changes in body plan occur only sporadically in the fossil record, not all the time, because the security of the laboratories is tight enough, and the penalties deterrent enough, that breaches in security don't happen every day, but only happen once in a great while.  Therefore massive leaps in evolutionary diversity only happen once in a great while. 

     There is also the possibility that the more embarrassing mutants are absconded with, and thus never fossilized and found.  For example, although Bigfoot sightings and footprints are very common, the bones of Bigfoot have never been found.  Perhaps the UFO occupants who are responsible for the recent evolution of Bigfoot take precautions to hide the bones of such transitory species.  Also, the aliens may quickly annihilate the children of their comrades' indiscretions, as was the case in Noah's Flood, whereby the alien authorities destroyed the half-breed nephilim giants.  Because such monsters were short-lived, the opportunity for fossilization was greatly abbreviated, and this may explain why frankenstein monsters in the fossil record are the exception rather than the rule.  We may never know the true measure of how many monsters have come and gone on this planet, because the alien governments have destroyed them before they become common enough to be seen in the fossil record.

     Now that we have become an intelligent species, it has become even more necessary for the space alien gods to tighten security and conceal their activities, in order to continue their genetic engineering projects without arousing the suspicion of humans.  Consequently, we just don't see them very often, and when we do, they are secretive, illusive, and they promulgate disinformation campaigns to confuse us.

Another factor may be the periodic absence of alien intelligence from our solar system due to interstellar star wars between alien societies, which force aliens to leave the solar system.  Alien absence may explain why certain long periods of time passed with no lineages undergoing radical changes. 

Hence, macro-mutations caused by aliens are infrequent and sporadic, because there were long periods of time during which the extraterrestrial presence was non-existent or self-restrained.

A factor which may have retarded the aliens’ ability to effect evolutionary change might be a limited number of space aliens.