Simpson, George Gaylord (1902-1984) was an American paleontologist, a scientist who studies animals, plants, and other organisms that lived in prehistoric times (more than 5,500 years ago). Simpson helped advance the study of evolution, a process of change over a long period. He led several expeditions that found numerous prehistoric animal fossils, remains of organisms that lived thousands or millions of years ago. Based on his studies of these and other fossils, Simpson made important contributions to the theory of evolution, the idea that all living things developed from simple organisms and changed through the ages to produce millions of species (kinds). Simpson wrote more than 40 books and over 700 scientific articles and papers.

Simpson was born on June 16, 1902, in Chicago, Illinois. While he was a baby, his family moved to Denver, Colorado. Simpson entered the University of Colorado at Boulder in 1918. He later transferred to Yale University, where he received a bachelor's degree in geology in 1923.

In the summer of 1924, Simpson collected fossils of vertebrates (animals with a backbone and cranium) in Texas and New Mexico as a field assistant for the American Museum of Natural History in New York City. He returned to Yale for graduate work and received his doctoral degree in geology in 1926. He studied at the British Museum in London from 1926 to 1927 on a research fellowship.

In 1927, Simpson joined the staff of the American Museum of Natural History as assistant curator of vertebrate paleontology. He was promoted to associate curator in 1928, a post he held until 1942. During that time, Simpson made several research expeditions to South America.

Simpson and his second wife, Ann Roe, a psychologist, published the textbook Quantitative Zoology (1939).

From 1942 to 1944, during World War II (1939–1945), Simpson served in the U.S. Army in Italy and northern Africa as an officer on the staff of General George S. Patton. After his military service ended, Simpson returned to the American Museum of Natural History as curator of fossil mammals and birds and chairman of the museum's department of geology and paleontology.

In his scientific book, Tempo and Mode in Evolution (1944), widely considered his most important work, Simpson argued that living things evolve (gradually change) in three ways. He labeled these ways speciation, phyletic evolution, and quantum evolution. In speciation, individual members of a species leave the main population and evolve into a new species. In phyletic evolution, an entire species slowly evolves into a new species. In quantum evolution, isolated members of a species evolve rapidly into a new species. Simpson's ideas led to the development of the synthetic theory of evolution. This theory synthesizes (combines) evolutionary theory with paleontology and other sciences. Simpson later explained evolution in terms that would be easier for readers without a scientific background to understand in The Meaning of Evolution (1949).

In 1945, in addition to his museum position, Simpson became professor of vertebrate paleontology at Columbia University in New York City. From the mid-1940's through the 1950's, he made many expeditions to fossil sites in New Mexico, Wyoming, and Colorado. Among his more exciting finds was his 1953 discovery of eight skulls and other bones of “dawn horses,” small, prehistoric ancestors of the modern horse. During a 1956 expedition to South America, Simpson suffered multiple leg fractures and other injuries when an assistant who was clearing a campsite accidentally felled a tree that landed on him. Even after his recovery two years later, the museum administration refused to allow him to travel on any more expeditions.

In 1959, Simpson resigned his positions at the museum and the university in New York City and accepted a post as Alexander Agassiz Professor of Vertebrate Paleontology at Harvard University Museum of Comparative Zoology, in Cambridge, Massachusetts. Simpson received the National Medal of Science in 1965.

Simpson held his Harvard position until 1970. He was professor of geosciences at the University of Arizona in Tucson from 1967 until his retirement in 1982. In the early 1970's, Simpson made three trips to Antarctica to observe penguins. He reported his experiences in the book, Penguins: Past and Present, Here and There (1976). His book Concession to the Improbable: An Unconventional Autobiography was published in 1978.