How far back into your ancestry do you want to go? The following table gives a count of how many ancestors you have at each level back. The far right column gives the percentage of genetic makeup you have from each individual from that generation. For example line 4 is your great-grandparents. Normally there are 8 individuals of that generation. That means that each of those individuals of that generation contributed 12.5% of the genetic makeup that is you and your full siblings. You can see that after the 7th generation back, the individual contribution to your gene pool drops below 1 percent, and the further back you go the thinner the genetic contribution becomes per individual of that generation.
But that said, you will say to me that though it is true that at Generation 20, the blood relation per individual is pretty thin, yet the 540,672 individuals of that generation are an absolutely vital link in who we are genetically today. And that assessment is entirely correct! As an aside, I estimate that Generation 20 takes you back somewhere around 800 years.
How far back you go in your ancestry is a matter of personal choice influenced by time and resources available, as well as availability to reliable genealogical sources. I also realize how I am speaking about this chart suffers from a mathematical simplicity that does not necessarily accord with actual genealogy; cousins marry cousins and so forth. That said, I trust the chart will give you some things to think about. After all, we are the descendants of survivors, and as you think of all the ups and downs of history over the last several thousand years, there is something to be said for that.