I would probably use a variation of the case "How many babies are born in the world per year", that is presented in one of the videos on this platform.
a) # of people ever living = number of people ever dying on earth
b) Assumption a) = # of people on earth at any given time (7bn) / avg. lifetime (70 yrs.)
Now, here comes the variation. Since the number of inhabitants on earth has grown exponentially to reach its current size of 7bn. and since people have not always gotten to the age of 70, I would come up with assumptions for how long 7bn people have become 70yrs. Let's maybe assume for 100 yrs. Thus:
c) Result of calculation b) x 100
That's how many people died in the past hundred years.
Next, you come up with a list of guesses, for how many years a certain number of people has gotten how old, repeat the steps above and add all these numbers up.
Your avg. number of people on earth and their age should decrease drastically in the early past and then become more stable and cover longer time spans (e.g. UNO assumes that there were only 500m people on earth 500 years ago, down to about 1,000-10,000 people 75,000 yrs. ago - *Source: Wikipedia*). However, generally mankind may have been around for about 2.8 million years.
Hope this helps.