What is meant by 'human intrusion' (Line 9)?

Answer the question based on the following passage.

            Discovered during December 1994 at Vallon – Pont – d’Arc,
            the cave composed of several spacious gallerias and dens,
            is adorned with almost 300 paleolithical paintings & engravings
            (dating 18000 – 20000 years before present time). The focus
(5)        is on a wide variety of animals including bears, owls, mammoths,
            rhino’s and felines. The cave has also retained several
            vestiges of human activities: fireplaces, entailed flints and
            other clues which denote an evolution in tools and habits.
            Totally left intact and untouched by any human intrusion,
(10)      the cave represents an exceptional source of studies for
           archeologists.