The original script indicates that the Engineers seeded their DNA into the world in the year 12,000 BC
Life was already present in abundance and their contribution was (presumably) mutagenic, transforming existing life rather than creating it from scratch.
FADE IN: EXT. EARTH - DAY (12,000 B.C.)
The world turns below us, vast and slow.
A RUMBLE. A shadow sweeps over the land. We move with the shadow. We
cast the shadow. Landscapes slide by. Reduced by altitude to
abstractions: river deltas, forests and flood plains. A raw natural
world. No trace of civilization.
The shadow glides over mountains and glaciers. Across an ocean and a
pale beach. Over lowland plain at the foot of a VOLCANIC MOUNTAIN it
EXT. LOWLAND PLAIN - DAY
THREE FIGURES walk out of the shadow.
They are men - and yet not men. Their skin is snow-white. Their
features heavy and classical - as if Rodin's Thinker had risen from
his seat. Their smooth heads are earless and hairless. Their
glittering eyes entirely black.
Against the stark land their height is impossible to judge.
They are ENGINEERS.