Can someone tell me the difference between the following two sentences

1) Recent fossil discoveries in United States show that birds are more closely related to dinasours than any other ....

2) Recent fossil discoveries in United States show birds are more closely related to dinasaurs than any other ....

What role does "that" in the first sentence play?
In the second sentence, without "that" does it incorrectly mean "United States is showing birds are related to dinasaurs?" and not the fossil discoveries?

Please explain

Anybody?

the "that" indicates that the stuff before the "that" is talking about the entire clause coming after the "that." Without "that," the preceding words are only talking about the noun immediately following "that."

So:
is it the case that the discoveries show birds?
OR
is it the case that the discoveries show that (birds are more closely related to dinosaurs...)?

If the latter (as in thise case), you need the "that" to indicate that what is being shown is the entire clause, not just "birds."

As opposed to, say:
Recent fossil discoveries show extensive damage. I don't need to say "recent fossil discoveries show that extensive damage" because I really am only talking about that one noun - damage.

If I wanted to say:
Recent fossil dicoveries show that extensive damage occurred as a result of floods and earthquakes
then I need the "that" again.

Thank You very much for the explanation. That really helped

You're welcome!