Why could we infer a physical time scale (e.g. a billion year) from a phylogenetic analysis?

Question

[johnnytam100]

I have been wondering this question for a while:

Why could we infer a physical time scale (e.g. billion years) from a phylogenetic analysis?

or

What is the basis that we correlate the sequence similarity of two sequences with the time of evolution?

or

How do we deduce the physical time scale from a phylogenetic tree?

Any idea?

Thank you very much!

[dsull]

To get time (in years), remember, in addition to mutation rate, we also have the fossil record.

Read into the concept of "molecular clock".

[gb]

Also as far as I know it has to do with fossils. And I thought the timeline is partly estimated by the depth of the fossils or the amount of earth or rock layers or something (like counting the rings of a tree). This is what I vaguely remember if you want to know for sure look up a real source =)

[dsull]

Yep! And the strata (rock layers) themselves can be dated by radioactive age dating (isotope half life decay).

[Anima Mundi]

Yes, these are called "calibration fossils".

[WouterDeCoster]

Genetic distance and known or expected mutation rates, I guess.