evolution - How many humans have been in my lineage? Is it almost the same for every human currently living?

If I were to count my father, my grandfather, my great-grandfather, and so on up till, say chimps, or the most common ancestor, or whatever that suits the more accurate answer, how many humans would there have been in my direct lineage?

And would it be almost the same for every human being currently living?

Answer

A quick back-of-the-envelope answer to the number of generations that have passed since the estimated human-chimp split would be to divide the the split, approximately 7 million years ago (Langergraber et al. 2012), by the human generation time. The human generation time can be tricky to estimate, but 20 years is often used. However, the average number is likely to be higher. Research has shown that the great apes (chimps, gorilla, orangutan) have generation times comparatble to humans, in the range of 18-29 years (Langergraber et al. 2012).

Using 7 million years and 20 years yields an estimated 350000 ancestral generations for each living human. A more conservative estimate, using an average generation time of 28, would result in 250000 generations. However, some have argued that the human-chimp split is closer to 13 million years old, which would mean that approximately 650000 generations have passed (using a generation time of 20 years).

The exact number of ancestral generations for each human will naturally differ a bit, and some populations might have higher or lower numbers on average due to chance events or historical reasons (colonizations patterns etc). However, due to the law of large numbers my guess would be that discrepancies are likely to have averaged out. In any case, the current estimates of the human-chimp split and average historical generation times are so uncertain, so that they will swamp any other effects when trying to calculate the number of ancestoral generations.

However, this is only answering the number of ancestral generations. The number of ancestors in your full pedigree is something completely different. Since every ancestor has 2 parents, the number of ancestors will grow exponentially. Theoretically, the full pedigree of ancestors can be calculated using:

$$N_\text{ancestors} = \sum_{i=1}^t 2^i \hspace{1em} \text{or} \hspace{1em} 2^{t+1}-2$$

where t is the number of generations. However, this will yield an unreasonably large number of ancestors (~$2.3*10^{105}$ over just 350 generations), since it assumes that all of your ancestors are unrelated. Basically, you run into something called pedigree collapse, which means that your pedigree will have many overlaps (due to inbreeding, overlapping generations etc), so all of your ancestors cannot be seen as unrelated. In practice, this means that the number of ancestors in successive generations will stop doubling for each generation you go back, and will eventually start shrinking, which will drastically reduce the number of unique ancestors. For more on this, see answers to the question Initial population when i count backwards?. That question is basically asking about the same issue, but less specifically. There might be studies that have tried to calculate the average number of ancestors to living humans, but I haven't seen any. Therefore, I cannot say anything more specific about the average number of unique ancestors for a living human individual.