When did the iPhone start tracking steps?

I was looking at my step count history, and out of curiosity, I scrolled back to see how far it went. The first day I have step count data for is March 18, 2015, and this seems like a very random day.

Does anyone know when the iPhone started tracking steps and why this might be my first day with data? I've had iPhones with up to date software since 2009.