NOTE: Someone else asked this same question a few years back, but it was incorrectly marked as a duplicate and so the question may not have been answered satisfactorily. I'm asking it again, in hopes of getting a stronger answer (if there is one). Please do not mark as a duplicate; I really appreciate it!
QUESTION: How many species have ever existed on Earth, throughout the entirety of Earth's history? For clarity, I'm not asking about the number of individual organisms (answered in 2nd link above), nor about the number of living species estimated to exist today (also cited in an answer in the 2nd link above). All species, ever (never answered in any link, to my knowledge).