The extraterrestrial lifeforms can be so similar and so different than we are.

(Image by Gemini)

Extraterrestrial lifeforms can be more usual than previously thought. There are models that the extraterrestrial lifeforms must be like us. They must have cells with a nucleus that protects them against UV radiation. That is one way to close the complicated problem. But those lifeforms can live in oceans where they are protected against radiation. The problem with this model is that. 

Before, eukaryotes formed there were prokaryotes. Those primitive cells have no nucleus. And the very first lifeforms on Earth had no mitochondria. They got their energy from volcanic temperatures. Before genetic code was written in the DNA the RNA molecule was the thing that stored the data that controlled life. But then we can look at life on Earth. All lifeforms on our planet are formed of cells. The viruses are not living, because they have no metabolism. So in this text life means cells or other organisms that can form their energy. Viruses also have their protein shells. That protects their DNA or RNA against radiation. 

But even if we think that all lifeforms on Earth include cells we face another reality. That is if we think about cats and corals both organisms include cells. They both live on the same planet. But they look very different. In the same way, we can look at medusas and birds. Even if those animals look different they live the same way on the same planet. The water protects the amoeba and the feathers protect birds against UV radiation. 

Those examples show that DNA, RNA, and cells can form organisms that are far away from each other. There is a very large-scale diversity in lifeforms on Earth. That means that even if alien lifeforms have skin, two arms, and two legs they can be far from the thing that we see on our planet. If those lifeforms have green skin they can form oxygen in their body and that means they can live in carbon dioxide.

The original Kardashev scale. 

A Type I civilization is able to access all the energy available on its planet and store it for consumption. Hypothetically, it should also be able to control natural events such as earthquakes, volcanic eruptions, etc.

A Type II civilization can directly consume a star's energy, most likely through the use of a Dyson sphere.

A Type III civilization is able to capture all the energy emitted by its galaxy, and every object within it, such as every star, black hole, etc.

(Wikipedia, KardashevScale)

Image: Pinterest

The intelligent lifeforms in the universe can be rare.  And before we can make contact with them we are alone. The reason for that is open. There are billions of reasons why we are not getting answers. There is the possibility that there is no intelligent civilization at a distance of 200 ly. It's possible. That there are no intelligent civilizations in the distance of billion ly. Or maybe the Earth is the laboratory for alien civilizations and they follow the advance of primitive intelligence. Or maybe they follow the same rules as the UN follows with primitive tribes. 

The Kardashev scale 3+ civilization might spend their entire life in spacecraft in zero gravity conditions. That idea is taken from the hypothesis called the Faber hypothesis. The idea in that hypothesis is that when humans make space colonies into our solar system finally they are born children. And if the journey from the Kuiper Belt to Earth takes years. There are humans. That ever see Earth. If those space humans live in zero-gravity conditions. That makes those human's body turn very weak. And that means they cannot even land on planets. In some evolution models, those creatures are called "Fabers". And those stations where they live as "Solar Wardens"

Researchers made models about the Kardashev scale to level 7. That civilization can control space and time in their body. 

But then we can go to alien civilizations and their models. Many models make the Karedashev Scale 3+ civilizations impossible. But then we must realize that the planet-scale civilization must have a different culture than we have. One of the reasons why the Kardashev Scale 3+ civilizations thought being impossible is the long distances in the galaxy. Sad but true, that limit is in our culture. The Kardashev Scale 3+ civilizations can have millenium ships their generations change. 

The Millennium or generation ships are the futuristic visions of the large O'Neill cylinders. Those extremely large space stations include artificial villages and Earth-looking environments. Those creatures can live in planet-looking conditions. The travel time between stars takes thousands of years. And generations change in the craft. That means the crew in those extremely large crafts can advance to different species than the species that left them to journey. 

That kind of vision includes models that the Kardashev scale 3+ civilizations don't need to step on planets. Those creatures can also live their entire life in zero-gravity conditions. That space race cannot land on planets because their body is fully adapted to the zero gravity environment. 

https://bigthink.com/13-8/how-a-popular-model-of-cosmic-life-and-intelligence-got-it-wrong/

https://scitechdaily.com/unlocking-the-universes-secret-code-the-quest-to-understand-alien-life/

https://en.wikipedia.org/wiki/Eukaryote

https://en.wikipedia.org/wiki/Kardashev_scale

https://en.wikipedia.org/wiki/Prokaryote