There are four basic elements or part as you say on earth which are natural that cannot be change or controlled that are water, mud , air and fire so fire can be called as essential part of universe.it is not an element ,compound or a mixture but it is basic.the reason is that man cannot controlled sometime these things could be a blessing and at the same time they apear as storm flood or calamity.the temprature of earth is maintained by a gret ball of fire called sun.