I believe in aliens but some people says that aliens does not exists

We don't have a definitive answer, or a 'true' answer. The only answers we have are a) we don't know and b) it is likely that aliens exist.

Our ancestors from a Billion+ years ago are now living across the universe.

They probably do exists. But chances are we will never find out.

The Universe is wide and technically speaking an alien is an unknown living thing and the possibility of mutated humans may be true technically classifying them as aliens

I believe that there is almost certainly life elsewhere in the universe, though I daresay contact with said life appears rather unlikely, at least at this point.

"Aliens do not exist" is less sensible a viewpoint than "Gods do not exist."

The two claims are fundamentally different.

On the alien side of things, we lack any conclusive evidence of life elsewhere in the universe. That's more or less it; belief in aliens doesn't necessarily mean belief that there are intelligent aliens close enough to earth to establish contact with. The anti-alien claims "There is no evidence for aliens, therefore non exist." We have an absence of evidence, but not evidence of absence.

On the god side of things, however, it goes a bit beyond a simple lack of evidence. One of the most basic qualities attributed to the concept of a god is interference with earth; this ranges from said interference only involving the creation of the earth (the great clockmaker) on one end of the spectrum, to regular and frequent direct interference in the live of humans by the alleged god or gods on the other. The entity can't simply exist somewhere in the universe, but must be able to interact with the earth in some way. The specifics of this claim allow us to make observations and experimentations about portions of the various claims; and therein lies the rub. Under the premise of naturalism, reasonably accurate predictions are made quite frequently- not so much for those operating under the premise of theistic creationism.

We can test some of those scenarios where people claim to have been visited by actual aliens. We can further test some of those scenarios where people claim the existence of a god or gods. None of these tests result in accurate predictions based on the premise that the alien encounter was real, or the act of god was real. The difference is that the very concept of a god or gods fundamentally entails involvement with the earth on some level, meaning that we can reasonably expect to find evidence of such here, on earth. In that respect, while hardly conclusive, an absence of evidence is effectively somewhat an evidence of absence. The very concept of alien life does not necessarily entail any involvement or evidence on earth whatsoever; we can freely disprove most claims of alien encounters, but aliens need not come anywhere near earth- unlike gods.