If you wanted to squeeze the 3.5 billion years of the history of life on Earth into a single minute, you would have to wait about 50 seconds for multicellular life to evolve, another four seconds for vertebrates to invade the land, and another four seconds for flowers to evolve- and only in the last 0.002 seconds would “modern” humans arise. It can take many generations to produce the changes over time that we see in plants and animals. Most, but not all, of this change takes place over thousands of years. It is sometimes hard to grasp these long time spans of change because of our own limited time on Earth.