Unknown, but you google it and it turns up nothing. In that case unknown usually means me.
Conceptually, it just isn't possible for each challenge to be one that a person is capable of overcoming (really, one overcomes obstacles, and accepts or completes challenges) because if 10000 kids want to be an NFL qb, it just isn't possible that they all can if they try. If two people run for president, only one will win.
Now, grammatically, it is poor writing to say each challenge--singular--and then conclude with the plural them. Perhaps that is why this has stayed unknown.