Peroxisomes contain a variety of enzymes, which primarily function together to rid the cell of toxic substances, and in particular, hydrogen peroxide (a common byproduct of cellular metabolism). These organelles contain enzymes that convert the hydrogen peroxide to water, rendering the potentially toxic substance safe for release back into the cell. Some types of peroxisomes, such as those in liver cells, detoxify alcohol and other harmful compounds by transferring hydrogen from the poisons to molecules of oxygen (a process termed oxidation). Others are more important for their ability to initiate the production of phospholipids, which are typically used in the formation of membranes.
In order to carry out their activities, peroxisomes use significant amounts of oxygen. This characteristic of the organelles would have been extremely important millions of years ago, before cells contained mitochondria, when the Earth's atmosphere first began to amass large amounts of oxygen due to the actions of photosynthetic bacteria. Peroxisomes would have been primarily responsible at that time for detoxifying cells by decreasing their levels of oxygen, which was then poisonous to most forms of life. The organelles would have provided the cellular benefit of carrying out a number of advantageous reactions as well. Later, when mitochondria eventually evolved, peroxisomes became less important (in some ways) to the cell since mitochondria also utilize oxygen to carry out many of the same reactions, but with the additional benefit of generating energy in the form of adenosine triphosphate (ATP) at the same time.