The aliens are a parasitic race

The aliens land in cities all over the world and demand that everyone
submit to one specific test - an anal probe. When asked why, all they
will answer is - "It is necessary for the survival of both our
species."

It's a veiled threat

The aliens are implying that if humans don't comply then they will wipe them out.

What they really mean is that they desperately need humanity for their survival.

The aliens

Since a disastrous disease killed off the hosts of their parasitism. They have been desperately searching the Galaxy for a new victim species.

The best they have come up with is humans but initially the eggs they implanted didn't survive.

After years of research they have developed a bacterium that will live in human bowels and make the environment suitable for their eggs and offspring.

When the larvae are ready to be 'born' they are about the size of a human stool and brownish in colour. They are excreted and swim off into the drainage system. Here they make their way to rivers and streams where they predate on other water creatures. Eventually they emerge from the water and take on their land-living form whereupon they seek out their kind and are integrated into the alien society.

The test

This is a lie. Now that the research has been done the aliens are ready to implant eggs. It's not a 'test' it is mass implantation into their new host species.

The probes are actually ovipositors. The victims never see this because they are held face down during the procedure.

Because humans are really stupid and overestimate the importance of brain because, no surprise, it's their brain who tell them he's the important one.

But there are only 100 millions neurons in brain. 5 time less than in guts.
Aliens know that (that's why they are so advanced) and after doing "catch and release" they concluded they need to do mass check-up on humanity. They are aliens so they have aliens reason to check us.

Some of them may include:

• A gut bacteria that is actually interplanetary threat to any more developed race. Aliens spotted our tries to escape solar system and want to stop humans from making everyone in the galaxy dumb (like we think that brain eating bacteria make us dumber).




• We are showing strange development in the gut region and it may make us go extinct. And for alien we are like white doves at Disneyland.



• Aliens are much more advanced technologically. Their MRI scan is much better when done from inside and the easiest way is through rectum.

There is another alien species that works in a "puppet master" fashion, it is ingested and grows from eggs before attaching itself to the gut lining like a tapeworm. It then sends filaments out into the body and gradually influences and then takes control of the host body.

The new aliens are cleansing the galaxy of this threat and perform random sampling of all inhabited planets on a regular basis. The most recent sampling discovered an infected individual on earth. They now need to screen the entire population and contain the threat.

It's a large scale health study

Every few years there's another breakthrough in what your gut bacteria say about your health. These aliens have taken this down to a fine art apart from one particular aspect, how to gather and identify the owner of the bacteria in question.

This particular aspect they're still doing the old fashioned way. Somebody once said that the only way to be absolutely sure where the bacteria came from was to take the sample directly, and it stuck.

Now they, as a culture, have become too homogenised. They no longer have the range of diets and environments to maintain the variety of bacteria that humans have. Something is slowly killing them and they've traced it to a lack of certain bacteria in the gut. If they can't get what they need, the next generation may be the last. And as for why it affects human survival? They'll kill us all if we don't cooperate. Corpses don't fight back.

Religion. That's really the easiest answer. Why would someone do something illogical at great personal expense? Because of an illogical motivation like faith guided by religious rules.

The most basic motivation of this kind is some supernatural punishment for not completing certain objectives like following a set of commandments for daily life. In this specific case the aliens belief refusal could perhaps trigger an armageddon like event.

If you want an explanation of why they started with the occasional abduction call it research. They needed to ensure we qualify. Once the clergy was satisfied we did an invasion was launched.

Now for as to what the aliens exactly belief, that's up to you.

They want to understand how a bipedal species could achieve the level of intelligence we did.

Source: https://www.smbc-comics.com/comic/evolution-3

The aliens are born from cloacas, and they can't understand the difference between cavities and have mistaken human asses for human birth channels.

I'm thinking there are a couple main threats that could potentially wipe out both species (and be solved by anal probes)

1. A disease spreading across the galaxy has devastated the alien species, and due to the genetic similarities between our two species, will likely do the same to us if we become exposed to it.




2. Some third race (potentially either some intergalactic locust or an incredibly aggressive military empire) has been going from world to world wiping out all life. The alien species identifies that they may be susceptible to earth born pathogens from the initial case study of anal probes, and devises a large scale study to cultivate the most deadly pathogens to these species and release them as bioweapons.

In case 1, they may be looking at what bacteria and viruses our body already has defences against- alternatively they may be taking samples of human DNA to reproduce in the lab, and test the disease on.

In case 2, the answer to their problems lies in the wide range of bacteria we have in our guts, viruses etc. Through this large scale study, they hope to get a large sample of microorganisms which they plan to breed and test on test subjects from race #3.

Humans have a better immune system than the aliens do.

The Aliens have a viral disease that will, in time, kill them all.

However, compared to the Aliens, humans have an awesome immune system. Primarily because we are a young and a new race, and not particularly bright, and have been living in rivers of our own sewage for millennia. Out of survival necessity we have phenomenally powerful immune systems compared to the Aliens.

So the Aliens decide to infect us with the virus, to see if we can kill it.

So now the clock is ticking. Both races are doomed to die. But if one human's immune system is able to manufacture an antibody that can kill the virus, the Aliens with their superior tech could take that antibody and synthesize it into a cure.

So immediately, all Humans must be probed! The Aliens have to hopefully find one human that is able to kill the virus, to save everyone. Please report to your local probing station at once.

1. They are slaves to a master race of sadist who demand sodomizing. The race doing the sodomizing is the master race's 'Silver Surfers' of prime booty holes. Those who have the butts, control the universe.




2. Different life forms absorb and process radiation differently. Species of a certain intelligence level or capability are tested for mineral, waste, and radiation concentrations in this area to determine long term longevity following random universal incidents (cosmic rays, background radiation, nuclear oopsies, etc.).




3. Similar to phrenology, buttology tells them the traits they can use to instantly judge us and destroy those that would cause the them/us or the entire universe harm.




4. Just kidding!