In looking for life elsewhere, it’s helpful to know how it got started on Earth.

1. In looking for life elsewhere, it’s helpful to know how it got started on Earth. All matter, whether alive or not, tends to use energy to form larger and more complex structures and molecules. Two examples are the formation of crystals and photosynthesis.

2. Early Earth When life arose on Earth, environmental conditions were very different than they are today, with different atmospheric composition, no ozone layer to block the Sun’s ultraviolet radiation, many meteorites, violentcomet and asteroid impacts, and lots of volcanic activity and electrical storms.

3. Building Blocks Miller and Urey’s experimental setup The 1953 Miller-Urey experiment was designed to find out if lightning energy could have formed the building blocks of life, such as amino acids, carbohydrates, and components of RNA, in the Earth’s early atmosphere. Ultraviolet radiation, volcanic activity, and even shock waves from impacts also can cause organic molecules to form.

4. The First Living Things Modern bacteria that may resemble primitive life The building blocks of life on Earth probably arose right here, though some think they might have been “seeded” from elsewhere by the arrival of a comet, asteroid, or space dust (an idea called “panspermia”). In any case, the first living organisms occurred only after millions of years of chemical trial and error.

5. The Tree Of Life New species evolved as life adapted to new environments. Over time, species changed genetically and physiologically to better deal with environmental challenges and opportunities for survival and reproduction, resulting in the fantastic biological diversity of modernlife on Earth.