This page continues ETI Scenario I past the point where an extraterrestrial intelligent technological species first arises in the universe. Adopting the assumptions from ETI Scenario I, it is here assumed that in many important ways such a species would be "human-like". Such a species could be different in many unimportant ways such as having different chemical and anatomical details. This scenario also assumes that the Fermi paradox is relevant in that we naturally wonder why extraterrestrial species have not made their existence known to us here Earth. Particular attention is given here to the question of how a human-like species might spread through the universe from its home world.


Rare event hypothesis

In this scenario it is assumed that technological species are relatively rare. For example, maybe there are only about 20 such species in a galaxy like ours.

Rapid extinction

In this scenario "rapid extinction" means that a newly evolved technological species becomes extinct before ever making contact with another species on another planet. For example, maybe there are 20 species in our galaxy with human-like intelligence and 19 of those will become extinct before contact is made between any two species.

Speed of light

If we assume that the speed of light is an absolute barrier (no Faster-than-light travel is possible) then how would a human-like species spread through the universe?

Generation ship

A generation ship is a spaceship that travels slowly between stars. The original occupants of such a ship might be biological entities who leave their home planet and grow old and die on the spaceship during the journey to another star. However, their descendants continue traveling to a distant star and reproducing. Eventually some descendants reach a new star system and a planet where they can live, possibly a planet that has its own form of intelligent life.

Suspended animation

Some science fiction writers have used suspended animation as a plot device. It is assumed that future medical technology will allow the physiological processes of an intelligent species to be suspended. Thus, several members of an intelligent species could be loaded aboard a spaceship while in a "suspended" state, take thousands of years to reach a distant star system, then have their physiological processes reactivated. Thus, even normally short-lived members of an intelligent species might be able to travel to distant planets.

Artificial life

It is a theoretical possibility that an intelligent species could create an artificial life form that is particularly well adapted to interstellar travel. For example, some form of computer-based artificial intelligence or robotic system might be devised that can easily survive a thousands-of-years-long trip by spaceship to a distant star system.

Strategies for Colonization

How might an intelligent species spread from its home world to distant star systems? Below, it is assumed that a traditional "generation ship" is less efficient than a smaller spaceship that would only carry artificial life forms and/or "suspended" or otherwise stored forms of biological life.

Symbiotic colonization

In a "symbiotic colonization" scenario, frozen gametes might be transported from a planet like Earth to a distant planet. The spaceship might be piloted by an artificial intelligence that can perform maintenance on the spaceship during the long flight and nurture new members of an intelligent species that are created from the frozen gametes upon arrival in a new star system.

Colonization by artificial life

In this scenario, no effort is made to carry biological organisms between planets and only some form of artificial intelligence makes the interstellar journey.


In this scenario, the "code" for an artificial intelligence is transmitted by means of a signal (probably electromagnetic) that travels between the stars. It is assumed that a distant intelligent species will detect the signal and build the equipment that is required to "instantiate" the artificial intelligence that is coded within the transmitted signal.

Space flight

In this scenario, the artificial intelligence actually travels by means of a spaceship. What would be the most efficient form for such an artificial intelligence to take? Assume that there is a period of a million years during which artificial life evolves on a planet before an attempt is made to travel to another star.

One of the dramatic aspects of the development of machine intelligence on Earth is the process by which computing devices become smaller. We can imagine the development of nanorobotics and some type of artificial intelligence that might involve swarms of nanites that assemble in a way analogous to how cells make up a human body.

If such a nanite-based artificial life form reached a distant planet that was home to its own evolving biological species, what would the space travelers do? Now, if we were dealing with Hollywood, then it would be important to show the alien invaders taking over the defenseless planet for their own evil purposes. However, is it likely that an artificial life form would evolve for a million years and then decide that it needs to make Hollywood movies?

In most cases, no species with human-like intelligence would exist on the newly reached planet. The arriving nanites would be free to start studying the forms of life on the planet. They might even want to start performing genetic engineering on the planet's life. In most cases, no species with human-like intelligence would exist on the newly reached planet. The goal of the nanorobotic colonizers might be to help a form of life evolve on the planet that is similar to the biological species that previously evolved on the home world of the nanites.

Assume that it takes a few million years for the nanites to artificially select and evolve a new species with human-like intelligence. Their goal is to be able to communicate with such a new species. Assume that before the nanites left their home world they had devised a way to link into the nervous system of intelligent biological species in order to facilitate communication. See VirileMail for an example of this scenario.

If we assume that it is possible for an artificial life form to spread in this way, then after many millions of years it might be possible for all of the Earth-like planets in a galaxy to be colonized and populated with biological intelligences and artificial life forms. It then becomes of interest to speculate about the possible directions that would be taken in the development of millions-of-years-old space faring cultures. See The Start of Eternity and Exodemic Universe.