Is that maybe "Hancock"? I haven't seen that movie, but I read a description, and it sounds kind of like what you described.
I'm not sure if what I saw, or read, before was actually about the first human, but there was definitely something about a person having been around longer than anyone else with amazing healing powers.
This sounds like an interesting myth.
I cannot think of any particular mythology with this particular origins story.
It reminds me of a strange movie I saw years ago.
It was called No Such Thing.
It is about an alcoholic monster that has been around since before humans.
He is rather inert, in that he is indestructable and immortal, although he can feel pain.
He is tired of life and is looking for a way to die. He is envious of humans because we can adapt, whereas he is always unchanging.
There are some Native American stories that tell how humans lost the chance to be immortal; although this is different than the hard-to-kill man that you are looking for.
One story tells how First Man and First Woman decided the fate of the first humans.
First Man wanted humans to die, then after four days, rise again and live forever.
First Woman said that man should remain dead. Since First Woman always has the last word, we stay dead.
When they then had a little girl, she sadly died while still very young.
First Woman wanted to reconsider her stance on staying dead, but First Man said she had the final say, and it had to remain that way.
I will keep my eyes open for the hard to kill first human story.