Following on from this question: How many humans have been in my lineage? Is it almost the same for every human currently living?

How many generations back do my ancestors go? Not just human ancestors, but all ancestors, right back to the first lifeform.

I recognise that there are some impossible unknowns here, so I’m looking for some kind of Fermi estimate.

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    $\begingroup$ sexual reproduction changed the generational thing so you have perhaps 1 - 4 billion sexually reproductive generations since your precambrian times, if you have an average 1 to 4 generations per year, seasonally, on average. since 4 billion years, zircons from australia have isotopes of oxygen that suggest life, and stromatolites date from <3.7, so we are talking about 4-400 billion prokariote and eurkariote zygote meioses. $\endgroup$ Oct 18 '17 at 19:41

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