Today I'm going to talk about the prehistorical civilisations.

Since you're talking about the concept of prehistorical civilisations rather than a specific set of civilizations, there's no "the" used.

Indeed, we all know about the prehistorical men and the dinosaurs but someone thinks there were other civilisations we don't know about.

He thinks there could behave been other advanced civilisations who lived one hundrerd million years ago.

It's in the past, so "have been" should be used

I always have been interestinged by life on Earth and even ion other plantets.

"Interested" is the feeling of interest, "interesting" is the property of something that causes interest. So something interesting makes you feel interested.

I fthindk it's so cool.

But a lot of people aren't convinced by this hypothesis because there aren't any fossils or traces.

For example, future civilisations will know about the Human presence because of the concrete they leave in the Earth's crust.

The scientist answerreplies to this objection by saying that a thousand years are a blink of an eye, it's a very short time if we compare to the Earth's age (roughly four billion and a half billion years).

But he is still open to the idea and would like to discover evidence that confirms or not confirmdisproves this hypothesis.

Tell me how you found my eEnglish.

Indeed, we all know (of) the prehistorical men and the dinosaurs but someon people thinks there were other civilisations we don't know (about).

"to know of" or "to know about" is to have deeper knowledge in a particular topic. You could also say "we all know the history of..." but the first two are more common and grammar based.

HeThey thinks there could be other advanced civilisations who livthat existed one hundrerd million years ago.

"who" and "to live" is for people and animals. Civilizations are inanimate, so using "existed" rather than lived is more correct, though I can understand what you mean

I always have been interesting byed in life on Earth and even ion other plantets.

The "always have been" implies you've felt this in the past and still do now, so you do not need to put the -ing part in. Because the verb phrase is referencing the past as well, you should use the past tense form of the verb and modify from there.

I find it's so cool.

Grammatically correct, but flows better this way

For example, future civilisations will know about the Hhuman presence because of the concrete they leave in the Earth crust.

Human doesn't need capitalization here. Only for names of specific people.

The scientist answers to this objection by saying that thousand years are a blink of an eye, it's a very short time if we, compared to the Earth's age (roughly four billion and a half) years), are like a blink of the eye.

Great use of simile/metaphor! The wording felt a little off but the message was clear. This is a very common metaphor so you don't need to explain it, but I like that you did because it shows your understanding of the phrase.

But the scientific community is still open and would like to discover evidence that confirm or not confirms this hypothesis (or not).

By saying "he", we are assigning a gender to the scientist that was not clear in the previous sentence, so it is unnecessary. If you want to talk about the opinions of scientists as people, say "scientific community". If you want to talk about science as a discipline of study, say "But science is still open...". Either way, the noun will be a they/it, so the tense for "to confirm" should be "confirms". "or not" is more natural when added at the end of a verb phrase.