Recently I've bumped into a research concerning molecular clocks and I'm really struggling with understanding the way relative rate test works, specifically the role of "outgroup sequence" in it, I don't understand the way outgroup helps with determining whether molecular clocks of two compared species are the same. If I get the way clock works correctly, we compare proteins of the two species that diverged from common ancestor, thus we can find out the approximate time species diverged from common ancestor. Relative rate test is used to determine neutral mutation rate without knowing the exact time of species divergence using outgroup. So, what's exactly the role of an outgroup in this process? Do we use it as some kind of a replacement for a common ancestor or what? I re read the article a bunch of times and yet I understand everything despite this test, which certainly feels frustating, that's why I'm asking about it here. I have attached a paragraph from an article, I hope it helps with understanding what I was trying to say.
Thank you for devoting your time to read this question and maybe even answering it.
UPD: I accidentally added one file 3 times in a row, sorry for that.
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