Precambrian cyanobacteria were land-dwelling organisms produced oxygen needed oxygen were tiny plants that used nitrogen

Precambrian cyanobacteria produced oxygen.

Cyanobacteria have been amazingly significant in forming the course of progression and environmental change during earth's history. The oxygen environment that we depend on was created by various cyanobacteria photosynthesizing throughout the Archaean and Proterozoic Period. The earth's climate before that time was of a mixed chemistry and unfit for survival as we know it.  Cyanobacteria were the leading organisms that practised H2O preferably of H2S or other mixtures as an origin of electrons and hydrogen for settling CO2.  They were capable to manufacture organic aggregates from water and CO2  By freeing Oxygen as a by-product about their photosynthesis they in influence altered the earth's climate.

hi there!

question - how does mountainous terrain affect settlements in a region? choose all answers that are correct.

groups trade with each other regularly.

people travel from settlement to settlement.

why: since they travel to go trade or visit it may be harder to get there because of the mountainous terrain.

hope this : )