Current theories calculate that the “Big Bang” was several billion years ago, and the universe has been cooling down ever since then. Humans are thought to have evolved a few hundred thousand years ago. That’s mere blip in the lifetime of the universe, and the temperature of the universe then would have been almost the same as now.
James is right, the theory of the “Big bang” says that the universe was created around 13.7 billion years ago, but the Earth is younger. Humans appeared around 100,000 to 200,000 years ago, when the universe and the earth was a lot cooler.
The boys have already answered this one. The Earth gradually cooled over time. Atmospheres were created, and the gases in the air changed over time. These lead to changes in the very basic living things that existed at the time. Gradually, as these creatures have become more complicated, they have grown and developed into humans.
Humans are able to deal with a lot of extremes, and the various crazy climates around us, because we have changed slowly over time to suit our surroundings. This process is called evolution, and it means that living things best suited to their normal living conditions survive, while living things without these specific responses to their surroundings don’t manage to survive and reproduce. The genes best suited to the environment carry on living and are passed on to the next generations, which over time become well adapted to their surroundings.