Despite decades of looking there is still no substantial evidence for extra-terrestrial life of any kind, despite several claims of fossilized microbes of different kinds (usually in the same Journal of Cosmology). Regardless many prominent scientists believe in the existence of extra-terrestrial life simply due to the vastness of space. Many also believe in panspermia and an extra-terrestrial origin of life on Earth, again without any substantial evidence.
One of the problems is the difficulty is finding persuasive evidence. Proving that something is both extra-terrestrial and a life-form is very difficult. To be convincing that whatever is found was created by a life-form scientists compare it to things known to be caused by Earth-based life. But the more similar it is to something found on Earth the more likely it is to have originated on Earth.
I find the popularity of panspermia very strange. It is after all the biggest, most blatant violation of Occam's Razor I've ever seen. Panspermia hypothesizes that:
1) life originated from inorganic materials on another planet,
2) it ends up on comets and meteors following some kind of large collision with this planet of origin,
3) it was then transported to Earth (surviving the harshness of space in the process), survives reentry,
4) despite evolving on some other planet for an unknown amount of time is able to survive upon arrival on this new planet.
Compared to the alternative hypothesis:
1) life originated from inorganic materials on Earth.
Clearly the alternative is much simpler and explains all the known facts just as well as panspermia.
The newest alien-life hypothesis is that alien-life is continuously raining down from space through the upper atmosphere and down to earth. This hypothesis is actually contradicted by many facts of biology which clearly indicate a single origin for all life on Earth ~3.5 billion years ago. A few examples of this evidence:
1) all the major types of biological molecules come in two forms: left and right. Chemists have synthesized these from raw inorganic materials and in all cases left and right forms are produced in equal quantities. However, all known life exclusively uses the left-forms.
2) The genetic code which is used to translate nucleic acids into proteins uses 3-nucleotide codes to encode 20 amino-acids, some features of this code reduces the effect of translating errors but many features are completely arbitrary. However, the exact same code is used by nearly all known life (a few exceptions living in extreme environments with one or two differences exist but there are no examples of significantly different codes being used).
3) Likewise the same set of nucleic acids and amino acids are the building blocks of all known life.
4) ATP (adenosine triphosphate) is the primary energy molecule used by all life, despite no clear reason why GTP (guanine triphosphate) or CTP or TTP couldn't be used instead.
The current fad of looking for diatom shells to prove extra-terrestrial life is even more misguided since diatoms are eukaryotic algea which emerged just a few hundreds of millions of years ago, meaning the 'alien diatoms' must have somehow followed the same 3 billion years of random mutations and evolution as Earth-based diatoms despite evolving on a different planet with a different chemical make-up and different sunlight. Clearly, the chance the findings are the result of contamination from Earth-based diatoms (which are commonly found in most bodies of water) is much much higher than diatoms evolving independently on another planet and being transported here.
