Populations began to rise after 3500 BCE, with further dips and rises occurring between 3000 and 2500 BCE but varying in date between regions. Around this time is the Neolithic decline, when populations collapsed across most of Europe, possibly caused by climatic conditions, plague, or mass migration. A study of twelve European regions found most experienced boom and bust patterns and suggested an "endogenous, not climatic cause". Recent archaeological evidence suggests the possibility of plague causing this population collapse, as mass graves dating from around 2900 BCE were discovered containing fragments of "Yersinia pestis" genetic material consistent with pneumonic plague.