I really can't say what it "earliest." I have a mishmash of memories from my life in the house we lived in in Cheyenne (where I was born and we lived until I was six) -- scenes, sensations, feelings, mostly -- I remember sunflowers growing over the neighbor's fence; I remember snow; I remember the incinerator in the backyard where Dad burned the trash; I remember sitting on the steps to the upper story and Mom telling us someone had died and crying, but can't remember who; I remember a Christmas ornament that hung on the arch into the lower floor living room, it was a set of green-and-red velvet covered bells that played carols; I remember watching television in my parents' room upstairs, specifically Jonny Quest; I remember riding my Big Wheel on the sidewalk outside and up the driveway; I remember laying in my bed and thinking that my door and my brother's door, there at the end of the hallway, looked like a Giant's book and I was part of a story; I remember my best friend moving away; I remember another friend on the other side of the backyard fence (there was no alley, hence the incinerator); I remember teaching my little brother how to tie his shoes.
I don't know in what order these memories come, so I can't say for sure what is "earliest." mostly I remember sensations and feelings, music and the layout of the house, like the back of my hand. Those sunflowers may not have been as gigantically tall as I remember them being, or as large.
And I remember one year when the neighbor's dog stole my chocolate Easter bunny right out of my hand. That was a tragedy in those days -- you only ever got ONE per year. :)