First, some perspective

Before we start exploring the different events of Earth's history, it is important to try to understand just how old the Earth is. I say try, because for most of us it is a bit hard to understand a timescale of a million years, and even worse, a billion years!

The Earth is estimated to be between 4.51 to 4.55 billion years old (Dalrymple, 2001). This chart by the International Commission on Stratigraphy shows the units of time in which the geological time scale of our planet is divided and will be useful once we start dealing with past events.

 Cohen, K.M., Finney, S.C., Gibbard, P.L. & Fan, J.-X. (2013; updated) 
The ICS International Chronostratigraphic Chart. Episodes 36: 199-204.

However, to get a better understanding of the massive timescale we are talking about, this "Cosmic Calendar" is quite useful. On it, the Big Bang happened on the first second of January 1st. Considering the universe is around 13.8 billion years old, each month is about 1 billion years on this calendar. Our planet was formed in August (this would be beginning of the Precambrian eon on the stratigraphic chart). Dinosaurs roamed the Earth from about December 24 to 29, and it is only in the last few minutes of the year that we, humans, appear. This already tells us that a lot has happened on our planet before we appeared.

From: www.nobelprize.org (adapted from 'Cosmic Calendar' by Carl Sagan)

It is very interesting to consider that in the comparatively small amount of time that humans have existed, about 200,000 years, we have been able to transform the whole planet, develop incredible technology and even go beyond Earth. This has come at a price: pollution, higher levels of greenhouse gases in the atmosphere, deforestation, etc. Despite the evidence, many people refuse to believe that we are responsible for the present changes in the climate. As we will see in the following weeks, climate changes have happened before and will happen again - however we cannot say that our actions are insignificant to the global system, for smaller beings than us have changed the planet before.