What makes life so special? Is there life elsewhere in the Universe? And how exactly did life emerge and diversify? In many ways these remain mysteries, but we do have enough evidence to explore some possible answers.

With the appearance of the first planets, the Universe became much more complex. Planets, which formed from clouds of chemicals created during the death of stars, are more chemically diverse than the stars from which they came. Planets also differ from stars because they don’t generate huge amounts of energy at their centers. As a result, their surface temperatures are much cooler. This combination of diverse chemical ingredients and less violent conditions enabled planets to become the setting for life. When life emerged on Earth, it had characteristics that made it very different from nonliving things. Many people will tell you they “know” the difference between living and nonliving things. However, biologists – the experts – have struggled to agree on a single definition of life, even though many of the proposed definitions share similarities. In this lesson, we’ll focus on understanding the characteristics all living things share.

For centuries, scientists have searched for the answer to the question, how did life begin? Some have argued it began in a shallow pool of water. Others have argued that it began deep below the surface of the ocean. Still others believe that a meteor from some distant corner of the Universe brought to the Earth the ingredients for life. How life appeared and how it changed over time are some of the most interesting questions you’ll tackle in this course.

The biosphere is an amazing place, serving as home for humans and many other species. The atmosphere provides the air we need to breathe, but the atmosphere also helps to protect us from the Sun’s radiation. The water, plants, and animals of the biosphere provide humans with many of the resources we need for survival. But as amazing the biosphere is in creating a “comfort zone” for humans, it can also be quite fragile, and from time to time has been subject to significant threats from various astronomical, geological, and biological forces. Changes in these forces can result in both mild and extreme impacts on the biosphere.

Life first appeared on Earth around 3.8 billion years ago—not long after the Earth itself —and life forms have been changing and diversifying ever since. How is it possible for scientists to know about the history of life? What methods do they use to study life and how it has evolved over time? The methods scientists use to reconstruct the story of living things and how they change over time also change over time. New techniques and instruments are constantly being developed to look more deeply into the world of cells and DNA.

Review Big History vocabulary.

Download the teacher resources from the Big History Project classroom version of this course. Text transcripts for videos are also available.