Evolutionary biology is a branch of biology that is primarily concerned with the evolution of species. It encompasses other fields of biology such as genetics, ecology, systematics, and paleontology. Evolution pertains to the sequence of events depicting the gradual progression of changes in the genetic composition of a biological population over successive generations. These processes resulted in the diversity of life on earth and one of the fundamental tenets is that all life on earth evolved from a common ancestor (i.e. referred to as the last universal common ancestor). At present, research in evolutionary biology includes studies in molecular evolution, genetic drift, biogeography, etc. Some of the topics currently being studied are the evolution of sexual reproduction, the evolution of cooperation, speciation, evolvability, and so on. Recently, a sub-field emerged called evolutionary developmental biology. It is a field concerned with understanding the development of an organism from a single cell to its adulthood and finding out about the phylogeny among organisms. A person studying evolutionary biology is known as an evolutionary biologist.