The earth was formed about 4.6 billion years ago. For a very long period of the earth's history, there was a very hostile and volcanic environment. It is difficult to imagine any life being viable in those types of conditions. It wasn't until the end of the Precambrian Era of the Geologic Time Scale when life began to form.
There are several theories about how life first came to be on Earth. These theories include the formation of organic molecules within what is known as the "Primordial Soup", life coming to Earth on asteroids (Panspermia Theory), or the first primitive cells forming in hydrothermal vents.
The simplest type of cells were most likely the first type of cells that formed on Earth. These are called prokaryotic cells. All prokaryotic cells have a cell membrane surrounding the cell, cytoplasm where all of the metabolic processes happen, ribosomes that make proteins, and a circular DNA molecule called a nucleoid where the genetic information is held. The majority of prokaryotic cells also have a rigid cell wall that is used for protection. All prokaryotic organisms are unicellular, meaning the entire organism is only one cell.
Prokaryotic organisms are asexual, meaning they do not need a partner to reproduce. Most reproduce through a process called binary fission where basically the cell just splits in half after copying its DNA. This means that without mutations within the DNA, offspring are identical to their parent.
All organisms in the taxonomic domains Archaea and Bacteria are prokaryotic organisms. In fact, many of the species within the Archaea domain are found within hydrothermal vents. It is possible they were the first living organisms on Earth when life was first forming.
The other, much more complex, type of cell is called the eukaryotic cell. Like prokaryotic cells, eukaryotic cells have cell membranes, cytoplasm, ribosomes, and DNA. However, there are many more organelles within eukaryotic cells. These include a nucleus to house the DNA, a nucleolus where ribosomes are made, rough endoplasmic reticulum for protein assembly, smooth endoplasmic reticulum for making lipids, Golgi apparatus for sorting and exporting proteins, mitochondria for creating energy, a cytoskeleton for structure and transporting information, and vesicles to move proteins around the cell. Some eukaryotic cells also have lysosomes or peroxisomes to digest waste, vacuoles for storing water or other things, chloroplasts for photosynthesis, and centrioles for splitting the cell during mitosis. Cell walls can also be found surrounding some types of eukarotyic cells.
Most eukaryotic organisms are multicellular. This allows the eukaryotic cells within the organism to become specialized. Through a process called differentiation, these cells take on characteristics and jobs that can work with other types of cells to create an entire organism. There are a few unicellular eukaryotes as well. These sometimes have tiny hair-like projections called cilia to brush away debris and may also have a long thread-like tail called a flagellum for locomotion.
The third taxonomic domain is called the Eukarya Domain. All eukaryotic organisms fall under this domain. This domain includes all animals, plants, protists, and fungi. Eukaryotes may use either asexual or sexual reproduction depending on the organism's complexity. Sexual reproduction allows more diversity in offspring by mixing the genes of the parents to form a new combination and hopefully a more favorable adaptation for the environment.
The Evolution of Cells
Since prokaryotic cells are simpler than eukaryotic cells, it is thought they came into existence first. The currently accepted theory of cell evolution is called the Endosymbiotic Theory. It asserts that some of the organelles, namely the mitochondria and chloroplast, were originally smaller prokaryotic cells engulfed by larger prokaryotic cells.