Ice and snow covered most of the northern part of the world. The people made fires to keep warm and to cook, they hunted and gathered food, and used stone tools to mine, chop, and dig. They lived in caves to get out of the weather elements
Cyanobacteria, also known as "blue-green algae", are the unicellular organisms which are able to produce oxygen.
Approximately 540 years ago, the atmosphere was very poor on oxygen. Cyanobacteria are thought to be responsible for the oxidization of the atmosphere back then, in an event known as <em>"The Great Oxygenation Event"</em>. As a result, Earth's life forms suffered drastic changes in their composition, as they evolved from being <em>anaerobic</em> to becoming organisms <em>highly dependant</em> on oxygen. The oxygenation of the atmosphere would eventually provide the adequate environment in order for the <em>Cambrian Explosion</em>, or the first diversification of living organisms, to happen.
All of the following were United States presidents except "<span>C.William Seward," since although he did indeed run for the office of President, he instead ended up serving under President Lincoln. </span>
<span>it allowed people to settle in one place, and instead of focusing on all aspects of survival, specialization and trade.</span>
The vice president presides over the senate,but president pro tempore is the highest ranking senator.The president pro tempore preside senate only the vice president is absent.The other junior senators preside the senate when the president and vice president are absent
