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According to what we've learnt :

  • we can approximate the number of generations since the first human pretty much like us (depending what / who we talk about).
  • we have an idea of the series of life forms we come from.

So is it possible to give it a very rough number ? Or it's not, because of some issues - but which ones ? (I only think about the many, short generations of the first forms, which moreover we don't really know about, as a limit to counting)

What if we stop counting at "unicellular" (or else) ?

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    $\begingroup$ Interesting question. Using the tree of life, we could get back as far as the last universal common ancestor (LUCA). I don't know if we can estimate generation times for early prokaryotes within an order of magnitude, though. Things get even harder before LUCA, because we don't know what life was like then. For example, how long did a generation last in the RNA world (if there was one)? Is the concept of a generation even meaningful in that world? $\endgroup$ – Corvus Apr 29 '15 at 23:40
  • $\begingroup$ Perhaps we stick to an organism, that has been around a while, get its generation time, and calculate back from that. But as Corvus said, the more back you go, the more inaccurate you get. $\endgroup$ – Rover Eye Apr 29 '15 at 23:43
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    $\begingroup$ shouldn't be too hard. see this biology.stackexchange.com/a/19569/8290 $\endgroup$ – r2d2 Apr 30 '15 at 5:27
  • $\begingroup$ Not an answer to your question, but see here for a most interesting article How many people have ever lived on Earth? by Carl Haub of the Population Reference Bureau. He estimates 106 billion, assuming H.sapiens arose about 50,000 bc, and states Life expectancy at birth probably averaged only about 10 years for most of human history. At the dawn of agriculture (8000 bc), the estimate for world population is 5 million, and 6.5% of all people ever born are alive today. $\endgroup$ – user1136 Apr 30 '15 at 14:55
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If you're willing to accept many orders of magnitude and define life as the Last Universal Common Ancestor.

For the rest of this answer, life begins 3.5 Gya with cyanobacterial mats and stromatolites and so on. Genetically the LUCA is dated to around this time, which matches the fossil record and everything's great. The LUCA can't have sprung from nothingness, so there may(will) have been (many)generations before the LUCA of things we would recognize as life, but I'm ignoring them. If they wanted to be considered they should have left descendants.

The fastest doubling times for modern bacteria are about 7-9 minutes but most are longer. E. coli takes about 20 minutes, and yeast takes hours. Given the 3.5 billion year clock, that gives an upper bound of about 1x10^14 generations. (Green sea turtles take 20-30 years to reach sexual maturity, or 1.1x10^9 generations). Something a bit representative is probably Pelagobacter ubique, which is vastly successful and takes about 29 hours to divide. Averaging out ice ages and so on and skipping the slowing of generations multicellularity probably implies(the Cambrian explosion is 82% of the way to the present day, being only 543 million years ago. We could account for it, but our guesses on generation time are way more inaccurate than any possible effect that could have) would estimate about 1x10^12 generations.

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  • $\begingroup$ If they wanted to be considered they should have left descendants :-) $\endgroup$ – Oct18 is day of silence on SE Jun 10 '15 at 23:04
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    $\begingroup$ Averaging out ice ages and so on and skipping the slowing of generations Is it just hand-waving (I say that respectfully)? If not, I would be interested in math behind these statements $\endgroup$ – Oct18 is day of silence on SE Jun 10 '15 at 23:06
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    $\begingroup$ Nope, it's hand-waving. :( It would be nice to take into account but the largest source of error is the average generation time pre-cambrian explosion. Estimates of that need to be within 15% or so to outweigh the error introduced by assuming there has been one generation since the Cambrian explosion, and even tighter to justify accounting for a yearly spawn(starting in the devonian, maybe? I'd have to review the evolution of land animals) instead of just random growth. It's basically a significant figures issue. $\endgroup$ – Resonating Jun 11 '15 at 14:18
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This question has actually been covered (although not to the complete extent of evolutionary history) by Richard Dawkins in his book The Ancestor's Tale.

He gives the following estimates of LUCA time and generations:

Monkeys and apes: 40MY(3M)

Mammals: 180MY(120M)

Reptiles: 310MY(170M)

Ray finned fish: 440MY(195M)

Sharks, hagfish and lampreys: 530MY(240M)

Lancelets and all chordates: 560MY(270M)

However, he ceases to provide any generation estimates from here onwards, due to the error and general uncertainties being too large for a meaningful answer.

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    $\begingroup$ I like that this answer and my answer are both answers to the question but consider exclusively different creatures. $\endgroup$ – Resonating Jun 12 '15 at 15:05

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