The human evolutionary tree has many branches. One of these branches includes all species in the Paranthropus genus, including Paranthropus boisei who lived 2.3 to 1.2 million years ago in eastern Africa. Fossils of Paranthropus boisei are characterized by a skull that is adapted for heavy chewing with a large sagittal crest on the top of the braincase. There is evidence of large chewing muscles and massive teeth: 4 times the size of our own teeth! They also had flaring cheekbones with a wide and dish-shaped face. Although their anatomy suggests they would survive on tough leaves, stems, and roots, patterns of wear on their teeth indicate that their diet was mostly composed of fruit and less tough plant material. Their fossils have been found alongside other early human species such as Homo erectus. This coexistence indicates that the pattern of human evolution was like a tree with many branches, rather than a single lineage of one species evolving into another.

Evolution occurs when a species adapts and changes over time through descent with modification. This is based on selective pressures within their environment. Human evolution is no different. Early humans walked on four legs, ate mostly plants, and had small brains. Over millions of years, they adapted to take advantage of the world around them. For example, they began to walk upright on two legs, became predators and skilled hunters, and developed large brains that allowed for sophisticated communication, a higher degree of forethought and planning, complex social interactions, and other traits that define them as a species. Often these adaptations were in response to a change in their environment, such as temperature change in the beginning or ending of an ice age or when a new species of predator moved into the area. Like all species, modern humans continue to change today and further adapt to their environment.

Humans are part of a large taxonomic group known as Primates that falls within the phylogenic class that contains all mammals, Mammalia. This group includes our closest living relatives, the great apes, as well as living monkeys, and all extinct early human and non-human primate species. Because we are related, humans and non-human primates share many anatomical and behavioral characteristics. For example, our hands and feet have five digits, we have large brains relative to other mammals, and we have binocular vision in which our eyes face forward on the front of our skulls. Also, some non-human primates can communicate with symbols, occasionally use tools, walk on two legs for short distances, and have complex social interactions. By studying non-human primates, researchers can learn about the ancestry of modern humans and more about the lives of early human ancestors.