Life on Earth began about 3.5 billion years ago. At that point in the development of the Earth, the atmosphere was very different from what it is today. As opposed to the current atmosphere, which is mostly nitrogen and oxygen, the early Earth atmosphere contained mostly hydrogen, water, ammonia, and methane.

 

    In experiments, scientists have showed that the electrical discharges of lightning, radioactivity, and ultraviolet light caused the elements in the early Earth atmosphere to form the basic molecules of biological chemistry, such as nucleotides, simple proteins, and ATP. It seems likely, then, that the Earth was covered in a hot, thin soup of water and organic materials. Over time, the molecules became more complex and began to collaborate to run metabolic processes. Eventually, the first cells came into being. These cells were heterotrophs, which could not produce their own food and instead fed on the organic material from the primordial soup. (These heterotrophs give this theory its name.)

 

    The anaerobic metabolic processes of the heterotrophs released carbon dioxide into the atmosphere, which allowed for the evolution of photosynthetic autotrophs, which could use light and CO₂ to produce their own food. The autotrophs released oxygen into the atmosphere. For most of the original anaerobic heterotrophs, oxygen proved poisonous. The few heterotrophs that survived the change in environment generally evolved the capacity to carry out aerobic respiration. Over the subsequent billions of years, the aerobic autotrophs and heterotrophs became the dominant life-forms on the planet and evolved into all of the diversity of life now visible on Earth.

 

 

    Humankind has always wondered about its origins and the origins of the life around it. Many cultures have ancient creation myths that explain the origin of the Earth and its life. In Western cultures, ideas about evolution were originally based on the Bible. The book of Genesis relates how God created all life on Earth about 6,000 years ago in a mass creation event. Proponents of creationism support the Genesis account and state that species were created exactly as they are currently found in nature. This oldest formal conception of the origin of life still has proponents today.

 

    However, about 200 years ago, scientific evidence began to cast doubt on creationism. This evidence comes in a variety of forms.

 

    Rock and Fossil Formation

 

    Fossils provide the only direct evidence of the history of evolution. Fossil formation occurs when sediment covers some material or fills an impression. Very gradually, heat and pressure harden the sediment and surrounding minerals replace it, creating fossils. Fossils of prehistoric life can be bones, shells, or teeth that are buried in rock, and they can also be traces of leaves or footprints left behind by organisms.