The Bible tells us when we became sinners.
The first ones to become sinners was Adam and Eve, or whatever name the humans went by that God reached out too.
New American Standard Bible
12 Therefore, just as through one man sin entered into the world, and death through sin, and so death spread to all mankind, because all sinned— 13 for until the Law sin was in the world, but sin is not counted against anyone when there is no law.
So sin has always been here. But until there was a line drawn in the sand by God, there was no accountability to it. So the neanderthals that were here before sapiens committed sins. They had a flesh like us and they had to choose to do what they want, or choose to do what was good by their conscience. But there was no accountability to sin for them because god did not yet reach out them, and if he did, it’s in a way lost to us. Eventually sapiens were here and God called a couple out to a special promised land. Something we see God doing again and again. Repeatedly throughout scripture God called out to various people and blessed them snd told them of a promised land and a promised king.
There mortals, though they could die physically, they could not die spiritually. There was no second death for them at this time. He gave them a few laws. They disobeyed the laws and committed sin that they were held accountable to.
Becsuse God had reached out to them, and continued to reach out to humanity such as with a Cain and so on, the laws begin to spread and mankind repeatedly and consistently chose to pursue their flesh. So sin spread to all mankind and as the laws kept coming through Noah, Mosses and eventually even laws passed by the apostles and God eventually send how holy power, which is the Holy Spirit into the world writing the laws on all hearts. As humanity rejects the fruit of the spirit, and chooses the fruit of the flesh, the are held accountable to their conscience and to God.