During the Precambrian, stromatolite communities of microorganisms grew in most marine and non-marine environments in the photic zone. After the Cambrian explosion of marine animals, grazing on the stromatolite mats by herbivores greatly reduced the occurrence of the stromatolites in marine environments. Since then, they are found mostly in hypersaline conditions where grazing invertebrates cannot live (e.g. Shark Bay, Western Australia). Stromatolites provide ancient records of life on Earth by fossil remains which date from 3.5 Ga ago. the oldest undisputed evidence of cyanobacteria is from 2.1 Ga ago, but there is some evidence for them as far back as 2.7 Ga ago. Oxygen concentrations in the atmosphere remained around or below 1% of today's level until 2.4 Ga ago (the Great Oxygenation Event). The rise in oxygen may have caused a fall in the concentration of atmospheric methane, and triggered the Huronian glaciation from around 2.4 to 2.1 Ga ago. In this way, cyanobacteria may have killed off much of the other bacteria of the time.