Steve Blank and Bob Dorf define a startup as an "organisation formed to search for a repeatable and scalable business model". According to Alex Osterwalder, a business model "describes the rationale of how an organisation creates, delivers and captures value". The Oxford Dictionary defines value as "the regard that something is held to deserve; the importance, worth, or usefulness of something".
So the question is: what was the earliest organisation formed to search for a repeatable and scalable way of creating, delivering and capturing something of importance, worth or usefulness?
Scientists estimate the first humans appeared on earth between 1–2 million years ago. But lots was going on before then. The earth is estimated to be around 4.5 billion years old, with life beginning 700 million years later. The first life forms were single-celled organisms called prokaryotes, which are chemoautotrophs. Chemoautotrophs obtain energy from the oxidation of inorganic (non-carbon) compounds, like hydrogen sulfide and methane. The energy stored in chemical bonds is used to create food from carbon dioxide. Later on, prokaryotes evolved the ability to free the energy within organic molecules, like glucose, and use it to create ATP: the primary energy currency of cells. Today, ATP is critical for almost on life on earth.
Prokaryotes searched for and ultimately found (probably unwittingly) a way to generate energy for use in cellular processes in order to sustain life; or a "repeatable and scalable way of creating, delivering and capturing something of importance, worth or usefulness".
This could be a little tenuous... but prokaryotes might be the first and most important startup in history!