This is known as the Texas sharpshooter fallacy. We observe one out of a billion unlikely events and – after the fact – draw our bullseye around that observation and say "what are the odds the particular thing under our bullseye happened?"

Of course, it's probabilistically invalid to decide on the target after you've observed which event actually occurred. Feynman exemplified this as

> You know, the most amazing thing happened to me tonight... I saw a car with the license plate ARW 357. Can you imagine? Of all the millions of license plates in the state, what was the chance that I would see that particular one tonight? Amazing!

If Feynman had walked into the parking lot with no prior knowledge of it and said "car number 5 on my right will have the license plate ARW 357" then this truly would have been amazing. But given that he saw the license plate first, it's not as amazing anymore.

The same principle applies whenever someone says "Of all the mates my ancestors could have met, they met the specific configuration of mates which produced me as an offspring, several generations down the line. What are the odds?" Well... that was kind of bound to happen. There were billions of possible offspring that didn't happen! One of them had to happen for anyone to be able to say that.

See also: weak anthropic principle.