Endospores

The bacteria, Bacillus, Clostridium, and Sporosarcina have a two-step life cycle; a vegetative cell, and an endospore. The vegetative cell is the phase of the life cycle where the bacteria are metabolically active. After a process called sporulation, the bacteria turn into metabolically dormant endospores. This spore is extremely resistant and capable of surviving harsh conditions including freezing, radiation, chemicals, and heat.  In a couple cases, there have been viable Bacillus endospores that are millions of years old. Testing shows that these spores are from a genetically different species of Bacillus than the species that are currently known (Talaro 105). This suggests that the species evolved or branched at some point in time. These ancient spores never came into contact with a host to infect and have since stayed spores.



Spores under an EM microscope
http://molmedicaltidbits.wordpress.com/2011/05/03/anthrax-as-biological-weapon/

Sporulation is a very complex process, as many genes (up to one-third of its total genes) are involved in the spore production. It takes only five hours to become a spore because the vegetative cells need protection, so sporulation must occur quickly. Each anthrax spore has approximately 750 in proteins in it.
The whole process of sporulation can be an evolutionary advantage. The bacteria that change into a spore the quickest will not die from environmental factors that normally will kill bacteria. Selection favors the bacteria that morphed into spores quickly. Also, the composition of spores must be fairly complex to provide such protection for the bacteria. Spores have a core adn cortex surrounded by a spore coat and an exosporium. This spore structure also could be credited to evolution by selection. The spores with better protection would have survived as opposed to any with minimal or insufficient protection from the environment. Spores are very necessary for the survival of Bacillus anthracis.

Structure of a spore
https://www.llnl.gov/str/June02/Balhorn.html

Yay!

http://www.medicalnewstoday.com/releases/4906.php
Chess, Barry. "4." Foundations in Microbiology. By Kathleen P. Talaro. 8th ed. New York: McGraw Hill, 2008. 105. Print.