The Tardigrade is a microscopic eight-legged creature capable of surviving the vacuum of space. They are found everywhere: from mountain tops to the deep sea and mud volcanoes; from tropical rain forests to the Antarctic.
By Schokraie E, Warnken U, Hotz-Wagenblatt A, Grohme MA, Hengherr S, et al. (2012) – Schokraie E, Warnken U, Hotz-Wagenblatt A, Grohme MA, Hengherr S, et al. (2012) Comparative proteome analysis of Milnesium tardigradum in early embryonic state versus adults in active and anhydrobiotic state. PLoS ONE 7(9): e45682. doi:10.1371/journal.pone.0045682, CC BY 2.5, https://commons.wikimedia.org/w/index.php?curid=22716809
This tells us that if Earth were to be hit by a cataclysm in which a part of our crust was thrown out into space, some Tardigrades may find themselves as involuntary astronauts. A small portion of these may later find themselves on a moon of Jupiter or some other place not too hostile to life.
Assuming that these creatures can shut themselves down biologically for decades, or even thousands of years, they may find themselves alive and well at their destination where they resume their life functions.
These brave survivors of space travel can then be properly termed extraterrestrials.
This same logic can of course be reversed.
A cataclysm on a distant planet throws up a bunch of dirt and water full of primitive life forms. This muddy mess is sprayed everywhere into space, some of it directed towards Earth. Frozen solid in the vacuum of space, the organisms carried along for the ride survive for millions of years.
On arrival at Earth, the ecosystem of microorganisms that was blown into space comes to life once more.
In the hospitable climate of a young Earth there is an abundance of available space for the organisms, and we get an explosion of life. The Cambrian explosion may well have happened this way.
On a later date, when there is already an abundance of life on Earth, a similar arrival of newcomers may cause great trouble. The newcomers may kill off certain species in favor of their own survival. Viruses and bacteria may cause plagues that kill a large number of original inhabitants.
It is in this respect interesting to note that plagues and severe virus infections tend to happen when solar activity is low. In times when the magnetic shield around our planets is at its weakest, more stuff from outer space finds its way to Earth. Severe weather, volcanic eruptions, earth quakes, famines, disease and general misery tend to go hand in hand. History books are full of stories in which these things happen together.
The Black Death is a good example of this. It killed off 50 to 70% of humankind in the late middle ages. Interestingly, this disease was associated with obnoxious fumes, asteroids and comets. The chronicles would not have mentioned this if this was merely a general observation of conditions at the time. The mention of these things as harbingers of death and destruction must have been due to their appearance ahead of subsequent events.
The Black Death may well have been a strand of extraterrestrial bacteria for which the human immune system was particularly badly prepared.
It appears then that primitive organisms, from viruses and bacteria to the Tardigrade, permeate the universe. When raining down on young planets, they kick-start processes necessary for more complex life forms to appear. In established ecosystems, they can wreck havoc, killing off large portions of the population.
Assuming that Venus is a fairly recent newcomer to our solar system, we can expect the seeding process to start shortly, if it has not already started. The thick and hot atmosphere may be turned into something less lethal quicker than expected. The time required for Venus to become relatively hospitable to life may not be millions of years, but more like thousands of years.
With deliberate seeding from our part, we may even be able to influence the development of Venus in a direction that is particularly welcoming to us. Human colonies on Venus may appear in a not too distant future.
The fact that our own planet is expanding indicates that it may be getting close to the end of its optimal period. Moving to a new and fresh planet is then the intelligent thing to do. There’s certainly no harm in establishing a colony on Venus should conditions open for this.
However, there will eventually come a time when our whole region of the Milky Way galaxy will become inhospitable to life. Assuming that matter becomes increasingly heavy and radioactive over time, eventually collapsing into radiation, humankind will have to move far away in order to avoid the catastrophe.
We can therefore expect extraterrestrials with intelligence comparable or above our own to move from time to time. After a few thousand or million years, they move on to other places, establish colonies and branch out.
This opens for the possibility that intelligent extraterrestrials have in fact been visiting Earth in the past. They may have been here for a while, and then decided to go somewhere else. Chances are that Earth is past its prime when it comes to being hospitable to life. The visitors may have known this and decided to leave for somewhere better.