Which isn't that hard to achieve really... It can theoretically be possible in just a few short generations. Take you for example. You have two parents. Your parents have two parents each. Their parents have two parents each, so on. Let's count it out from you:
Generation 1 - 1 <== You
Generation 2 - 2 <== Your mummy and daddy
Generation 3 - 4 <== Your grandmummies and granddaddies
Generation 4 - 8 <== Your great grandmummies and great granddaddies
Generation 5 - 16 <== etc.
Generation 6 - 32
Generation 7 - 64
Generation 8 - 128
Generation 9 - 256
Generation 10 - 512
Generation 11 - 1,024
Generation 12 - 2,048
Generation 13 - 4,096
Generation 14 - 8,192
Generation 15 - 16,384
Generation 16 - 32,768
Generation 17 - 65,536
Generation 18 - 131,072
Generation 19 - 262,144
Generation 20 - 524,288
Generation 21 - 1,048,576
Generation 22 - 2,097,152
Generation 23 - 4,194,304
Generation 24 - 8,388,608
Generation 25 - 16,777,216
Generation 26 - 33,554,432
Generation 27 - 67,108,864
Generation 28 - 134,217,728
Generation 29 - 268,435,456
Generation 30 - 536,870,912
Generation 31 - 1,073,741,824
Generation 32 - 2,147,483,648
Generation 33 - 4,294,967,296
Generation 34 - 8,589,934,592
So, by going back only 34 generations, you would very possibly have every mutation from every generation prior to this in your genome at todays population - now That's fixation, and that's also only around 680 years ago if you take average age of family life to begin around 20 years. Obviously this is overly simplistic and doesn't properly model the actual rate of fixation since it isn't possible to have around 16 billion unique ancestors in your lineage, but this certainly is indicative of how soon and how easily beneficial mutations can fixate in a population, even ours at 7.5 billion people.