We had an interesting anonymous solution to this problem, giving rise to some sensible numbers. We think that similar investigations would make for interesting data-handling project work.

We used the internet to research lifespans. We were surprised to see that fishes and reptiles live for a very long time. A modern day bony fish is the sturgeon, from which caviar is taken. Lizards seem to have quite a wide range of lifespans (between 10 and 50), so we picked 30 as an average. The numbers for dingos and lemurs seemed typical for these sorts of creatures. We also changed the human numbers because in the past both would be smaller. Our table became

@

Creature	Timespan	Time to maturity	Lifespan	Min number	Max number	Average number
Fishes	50,000,000	20	100	500,000	2,500,000	833,333
Amphibians	50,000,000	20	100	500,000	2,500,000	833,333
Reptiles	100,000,000	2	30	3,333,333	50,000,000	6,250,000
Early mammals	125,000,000	2	10	12,500,000	62,500,000	20,833,333
Mammals	60,000,000	2	25	2,400,000	30,000,000	4,444,444
Apes	14,000,000	2	35	400,000	7,000,000	756,757
Humans	1,000,000	13	40	25,000	76,923	37,736

To get the minimum number of ancestors for each row we divided the time in years by the lifespan. To get the largest numbers of ancestors we divded the time in years by the time to maturity.

This gave grand totals of between $19,658,333$ and $154,576,923$ ancestors.

To get the average age of an ancestor when giving birth we found an average age = (Lifespan + time to maturity)/2 and timesed this by the timespan.

Assuming that ancestors were born at an average point in their parents' lives, we get $33,988,937$ ancestors.

Rounding up to sensible values, we think that there were at most $150$ million ancestors, at least $20$ million ancestors and probably about $35$ million ancestors.