How many generations of humans have there been?
When you say “humans” are you just talking about Homo sapiens or about any species in the genus Homo? It will make a huge difference to the answer. Which will be approximate in any case.
Depends on how you define “human”.
Technically, that would be the rise of the genus Homo which is generally thought to have evolved about 2.5M years ago. If 20 yrs = 1 generation, that would be 125,000 generations.
If you mean Homo sapiens, then that was probably 150k yrs ago, meaning about 7,500 generations.
Ooh, ooh. Sounds a bit like a thread I started awhile back.
A ‘generation’, I think, is considered to be a 20 year period. So, if you assume the human race has been around in its present form for x number of years, the answer is 20x.
When I was a kid, I tried to figure something out, doubling myself every 20 years to see how many people in the past were in some way responsible for my existence. Well, when I got to 500 B.C., I was at 10 billion, so I guess I did something wrong. I gave up and haven’t given it much thought since.
And of course it depends quite a bit on what assumptions one uses for the average time length of a generation. For modern humans, 20 - 25 yrs is the convention. For our early Homo ancestors, it was probably a bit shorter.
So, make your assumptions and plug in the numbers. And make sure you realize that species generally don’t evolve in one generation, so that it would be impossible to look at one group of animals and say: That generation is “human”, but the generation preceeding it is not.
What you did wrong was made the assumption that all your great, great,… grandparents were unique individuals. Once you go back 10 or so generations, it is likely you start getting some inbreeding, and it gets more significant for each generation you go back in time.
I’m sure you meant x divided by 20, not 20x. 
The 20 year estimate for generations is quite low. Most sources give 30-33 years. In my particular case, going back 1000 years is 32 generations in one branch giving a little over 31 years per generation.
Based on personal experience, 20+ years of geneology, I’d say 10 generations is rather generous. Most people start to “double up” after 4 or 5, and if you have cousins marrying, it’s only 3. The more sedentary your ancestors were, the more likely this is to happen.
My guess is that in the US, due to a high rate of ‘movage’ and a 300+ year history of immigration, our rate of “doubling up” is lower than in Europe. In strongly tribal areas of Africa it is probably higher yet. Just my thoughts.
I don’t think so. Even in modern industrial societies where women typically delay childbirth, that would probably be an upper end estimate. For the timeframes we are talking about, a lower number (20-25) is appropriate. I picked 20 just to make the math easier, but I think for pre-sapiens poplulation, that lower number makes sense.
In his book Genes, Peoples, and Languages, Cavalli-Sforza uses 25 years/generation to estimate the number of generations back into prehistoric times for Homo sapiens. If it’s good enough for ol’ C-S, it’s good enough for me!