Atheists: Is it safe to assume that all atheists ...?

I'd not be quick to assume anything about all atheists other than a lack of belief in gods.

No it isn't fair to say. The only thing you can say is atheists don't believe in God or gods.

Not believing in any God is the only qualification for being an atheist. Some atheists won't even engage the question "where did the universe come from?"

The three main responses are:

1. Scientific (cosmological) answer "Big Bang" or "Inflation theory"

2. It was always here (now less popular "Solid State Universe theory" it was a popular scientific theory in the 1950s just another example why scientific theories are NOT the same thing as facts.) Stephen Hawking made this theory less popular. Another variation of this theory is adapted from Hinduism and I think may be held by a lot of Buddhists (who are also atheists) who argue that time itself is circular and the universe is a self creating entity.

3. Don't know where it came from and don't care either, it exists why ask where it came from?

Those are the three main answers you're likely to get from atheists.

Oh sorry, I misread the question. Yes Evolution is the theory that is most popular, however, I have heard of other theories like Gaia theory. This is the theory that the planet is a self maintaining living organism and that all life on earth are like specialised cells in our own bodies. We were "created" not by a god or God but by the Earth itself. I think there are other theories but the point is not all atheists are scientific or secular atheists. Again not all atheists feel compelled to answer the question "how did we get here" it is enough for them to know we are here.

"Atheists: Is it safe to assume that all atheists ...?"

-Unless this sentence end with "don't believe in God," the answer is no.

"believe in the evolution theory as the main reason we exist today?"

-Most, but not all, of the atheists you are likely to meet will believe in evolution, but it's likely they won't view it as "the main reason we exist" depending on how much you like to split hairs. Evolution explains the diversity of life, not how it started.

"And for those who say "No", then how do you explain our existence in that case?"

-I can't, so I don't.

It is safe to assume that many atheist understand that evolution is not a belief system and are more likely to treat it as any other scientific theory.

Edit:

You're missing the two major points: 1. Atheists share no common belief structure or "dogma", the only thing we have in common is a lack of belief in all gods (and often we can not agree on how to word that simple statement).

2. Evolution is not a belief. It's sad that your approach to knowledge is based in belief rather than knowing.

And the third: We do not have to explain how the universe formed or how life diversified to realize that there are no gods. You have done this as well for the majority of all gods, we've just gone one god further than you.

It is very likely that an atheist would believe in the theory of evolution but the two things are not connected so maybe he or she as another theory. Not that there is one as far as I know but I don't know everything any more than anyone else

No. Atheists don't believe in God because He's unproven. Evolution is also unproven. It can be endlessly discussed and referenced and credited and discredited, but ultimately the only 100% true answer is that we don't know. Just because we exist doesn't indicate that we can explain our existence or that of the Universe, and in an infinite Universe guided by the balance of probability and chaos, the only thing that we can be sure of is our own uncertainty. "The more I know, the more I know that I don't know." Unfortunately, too many people know little enough to think they know everything.

Edit- Also, no atheist would freely admit to assuming anything, I doubt that they'd appreciate you assuming things about them. In fact, the word "all" just about sent shivers down my spine. It's just so un-scientific.

All atheists? Not necessarily. Ayn Rand, for example, said the following in her essay, 'Philosophy: Who Neesd It?':

"I am not a student of the theory of evolution and, therefore, I am neither its supporter nor its opponent."

How an atheist who doesn't accept evolution would explain our existence is something you would need to ask each one individually.

Personally, I think it's part of the process that led to us, along with the Big Bang, nucleogenesis, accretion, abiogenesis, etc. Evolution was just one step on a very long ladder.

As the main reason we exist as a species, yes. But as the main reason why life exists in the first place, that would be a fallacy. Most of us disagree in one way or another on how life began.

If you're an atheist, you shouldn't believe in deities. That's all that ties us together.

No.

Atheism is just the lack of belief in a god or deity.

It's true that many of us came to that conclusion through reason and evidence, making it very likely that we follow scientific evidence and facts but that's only a few of us. Anyone not brought up believing in a god is atheist yet that doesn't say anything at all about how scientifically literate they are.

No, it's safe to assume that all atheists have a disbelief in the existence of a god, or gods. That's the ONLY thing that is safe to assume about all atheists.