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Why do scientists describe human evolution as a branching bush rather than a straight line?

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Illustration showing side-by-side reconstructions of five hominins in the human lineage: Australopithecus afarensis, Homo habilis, Homo erectus, Homo neanderthalensis, and Homo sapiens, with each figure labeled and depicted in a walking pose to highlight evolutionary changes in posture and body structure. A scale bar at the top right indicates relative height. — human evolution An artist's depiction of five species of the human lineage.

Scientists describe human evolution as a "branching bush" rather than a straight line because the fossil record reveals a complex history of experimentation and diversification, rather than a single, direct path of improvement leading to Homo sapiens.12

Historically, human evolution was often incorrectly viewed as a linear progression toward modern humans. However, paleoanthropological evidence shows that for most of human history, multiple hominin species lived contemporaneously. Evolution has been a process of trial and error, where new species were frequently spawned, competed in the ecological arena, and often went extinct.12

Rather than being the sole occupant of a summit earned through a linear climb, Homo sapiens is simply the last surviving twig on a vast and intricately branching tree. This model accounts for the significant anatomic diversity found in the fossil record, which represents a variety of distinct species rather than a single evolving lineage. Consequently, it is impossible to connect a full, universally agreed-upon chronological series of species leading directly to modern humans.132