During the 1980s, the age of the Petralona hominid estimated by Poulianos was challenged by an article in "Nature". The scientists involved used electron spin resonance measurements and dated the age of the skull to between 160,000 and 240,000 years old (research based on the detailed analysis of the Cave's stratigraphy has proven that the original chronography of the skull is ultimately closer to the age of 700 000 years claimed by Dr. Poulianos). However, Poulianos states that his excavations in the cave since 1968 provide evidence of human occupation from the Pleistocene era. The Petralona hominid, specifically, was located in a stratigraphic layer containing the most amount of tools and traces of habitation. Poulianos states that the age of the overall layer is approximately 670,000 years old, based on electron spin resonance measurements. Further excavations at Petralona revealed two human skeletons that were dated to be 800,000 years old.