According to the Bible, who was the fifth human on Earth?

If we're going by the Adam and Eve story, then the answer is in Genesis 4:

http://www.biblegateway.com/passage/?search=Genesis+4&version=NIV

The first four persons were, in order, Adam, Eve, Cain, and Abel.

It's a little unclear who number 5 is. The next person mentioned is Cain's wife (Genesis 4:17), but she isn't named; the next named person is their son Enoch (mentioned in the same verse).

In verse 25, we see the birth of Adam and Eve's third son, Seth. Though mentioned after all of Cain's descendants, Seth would have to be older than almost all of them if you want to read this as a history book.

Within the story, the answer would have to be either Seth or Cain's wife, but there isn't enough information to be sure.

Of course, in Genesis 1 it's a different story; in 1:27 God creates men and women at the same time, and the number of men and women is not specified. The "fifth person" could easily have been among that number.

And there is the myth of Lilith (p/o #1&2) to add to the mix. She is said to have been created at the same time as Adam (p/o #1&2) but didn't like him and left Eden. God then created Eve(#3) to keep Adam company. After expulsion from the Garden Eve gave birth to Cain (#4) and then Abel (#5).
So, if the Lilith myth is accepted, Abel was the fifth.

https://en.wikipedia.org/wiki/Lilith