There is certainly a case to be made for extraterrestrials; especially since they can be a small as a microbe or as big (or bigger) than Yeti.
There are many ways to view this question and scientists have expressed differing opinions. Interesting to note is that noscientist says they don’t exist. There are some who don’t say they must exist, but with the 460 planetary systems in our galaxy times the number of galaxies (likely in the billions), that gives a significant number of chances for life.
If one believes that water is necessary for life, consider Deuterium, a plentiful substance in the universe. Deuterium combined with oxygen gives one heavy water. Mars is believed to have once had lakes of Deuterium. These lakes then evaporated and left the Deuterium sediment we have found.
But if one doesn’t subscribe to that line of thought then understand that here on Earth there are numerous Extremophiles which seem to confound many with living in acid or areas of extreme heat, or extreme pressure - Life always finds a way.
Dr. Godfrey Louis, a solid-state physicist at Mahatma Gandhi University, believes that ET has arrived. He published a paper in a peer reviewed journal back in 2006 regarding samples that came from the blood red showers experienced in the Indian state of Kerala in 2001. His hypothesis; that these showers contained microbes from outer space.
Odd & intriguing at the same time are the facts: Dr. Louis was able to isolate strange, thick walled, red tinted cell-like structures that were 10 microns in size. While he did not find any DNA, they reproduced in great number and with ease, even in water superheated to 600 degrees Fahrenheit. Keep in mind that these microbes also survived the cold temperatures of space, not to mention the radiation.
These ‘visitors’ are believed to have hitched a ride on a comet. So if that occurs, then life gets spread all around the universe through comets; Life that can survive conditions beyond the usual expectations. There is no reason to not further hypothesize that this life might experience evolution. A Sci Fi writer could get ahold of this thought and offer some amazing likelihoods….oh wait, I am a Sci Fi writer.
And as a Sci Fi writer I suggest that often Sci Fi is capable of explaining these sorts of things because science feels it must wait a hundred years and exhaust every test whereas Sci Fi isn’t quite as strict.
If some doubt remains, as this is only one occurrence. Then I offer the Nakhla meteor that fell in Egypt about 95 or so years ago. It also gave clues to ET. It is thought to have originated from Mars and contains a large number of microscopic tunnels which mimic the size, shape and distribution of a bacteria feeding frenzy. While no DNA was found, based on the Indian example above, that may not be necessary. When the rock came into debate as to its origin; the fact that the isotopes of carbon it contains are not common to Earth was evidence that it may indeed be from from Mars.
Food for thought.
“I think the universe must be teeming with sentient life... sooner or later we will make contact with extra-terrestrial intelligence.” - Prof Wickramasinghe