Stromatolites are sedimentary rocks in the form of layered mounds, sheet like structures, columns and dome shaped structures. The primary composition of these sedimentary rocks is the limestone. These are found in the shallow waters and are usually formed by the cynobacteria and other single celled organisms such as algae and lichen.
A way a disease that is fatal can stay in the population is if its hereditary. Such as, it allows the carrier to live long enough to have children, which have the chance to either get it, or pass it on to their children.
