Riddle Me This. #2

I am the ever-changing one,
My origins lost, my journey begun.
Through countless shifts, I have evolved,
My forms and features all resolved.

From single cells to complex beings,
My transformation never ceasing.
Adaptation is my greatest skill,
My survival always on the thrill.

Though some may view me as mere chance,
My existence is not happenstance.
For in the struggle to survive,
I am the force that will always thrive.

What am I?