The civilisation now known as The Olmecs are believed to be the first Mesoamerican civilization, living in the region that is now Mexican states of Veracruz and Tabasco. The Olmecs are generally considered as the forerunner of all subsequent Mesoamerican cultures including the Maya and Aztecs.
The existed from (around) 1,400 to 400 BCE, and reached a population of [not known]
An extensive range of sophisticated stone carvings have been recovered - particularly so-called Colossal Heads.
The origin of the Olmecs was for many years suggested as Africa, but mitochondrial DNA studies carried out in 2018, showed (conclusively according to the authors) that they were not African, but American.
The reason for decline and disappearance of the Olmecs (around 400 - 300 BCE) is unknown, but environmental and/or climatic factors have been suggested.
Further reading Wikipedia