According to Wikipedia's Timeline of scientific experiments the first such experiment was conducted by Empedocles in the 5th century BC some 2.500 years ago. Personally i do not share this view of history after having read The Dawn of Everything which outlines how much human activity of the past is being ignored by todays accounts just because people did not have the tools to communicate their findings in a way that preserved it for future generations in the original code.

So lets imagine some Human being of the far past say 100.000 ore more years ago conducting something we would call a scientific experiment these days, e.g. trying out different ways of coloring clay for body painting and later wall murals.

The "scienfific communication" in this case would have been based on oral/vocal/language which is thesed day is assumed to be well established in the human community at the time.