WATCH: Threshold 5 — Life on Earth Video

NARRATOR: Threshold five brings us life. What is life? Though there may be many other life forms in the Universe, we could only study those that emerged on our planet. So, from now on, we focus on our home planet, the Earth. All living things on Earth share important qualities. First, they take in energy from outside to maintain themselves. This is metabolism. Second, they all seem to constantly try to balance the conditions inside and outside themselves in order to survive as long as possible. This is called homeostasis. Third, they can make copies of themselves through reproduction because they all contain huge molecules of DNA or RNA that record how they were made. And fourth, over time, the information in those molecules can change so that later generations may be slightly different. And in this way, living things slowly diversify. Let's consider the ingredients and the Goldilocks conditions needed for life to form. The ingredients are a great variety of complex chemical compounds including DNA and RNA. Two Goldilocks conditions provide ideal environments for these chemical compounds to combine in many different ways. First, you need energy to fuel those chemical reactions but not too much since that would just blast molecules apart. And second, you need a liquid. That's because it's hard for atoms to link up in gases, where they're too far apart, or in solids, where they can't move very much. So where could we find these Goldilocks conditions? Well, liquids can only exist in a narrow range of temperatures. If the temperature is too high, like right next to a star, then liquids turn into gases. If the temperatures are too cold, like in deep space, liquids turn into solids. Some rocky planets or moons are just the right distance from their suns to have lots of liquid water. They also have the right amount of energy to encourage chemical reactions. Here, on our Earth, four billion years ago, you would have found almost perfect conditions for some of the most complex chemistry you can imagine. That chemistry led to life.