Original sentence: A shell glyph -- -- was used as a zero symbol for these Long Count dates, the earliest of which (on Stela 2 at Chiapa de Corzo, Chiapas) has a date of 36 BCE.However, the Olmec civilization had come to an end by the 4th century BCE, several centuries before the earliest known Long Count dates--which suggests that zero was not an Olmec discovery
Edited sentence: A shell glyph -- -- was used as a zero symbol for these Long Count dates, the earliest of which (on Stela 2 at Chiapa de Corzo, Chiapas) has a date of 36 BC.However, the Olmec civilization had come to an end by the 4th century BC, several centuries before the earliest known Long Count dates--which suggests that zero was not an Olmec discovery