Sir Grafton Elliot Smith (15 August 1871 – 1 January 1937) was an Australian-British anatomist, Egyptologist and a proponent of the hyperdiffusionist view of prehistory. He believed in the idea that cultural innovations occur only once and that they spread geographically. Based on this, he traced the origins of many cultural and traditional practices across the world, including the New World, to ideas that he believed came from Egypt and in some instances from Asia. An expert on brain anatomy, he was one of the first to study Egyptian mummies using radiological techniques. He took an interest in extinct humanoids and was embroiled in controversy over the authenticity of the Piltdown Man.

Smith was born in Grafton, New South Wales to Stephen Sheldrick Smith who had moved to Australia from London in 1860 and Mary Jane, née Evans. He received his early education from Grafton Public School where his father was headmaster. When the family moved to Sydney in 1883 he went to Darlington Public School before joining Sydney Boys High School. He attended evening classes on physiology by Thomas Anderson Stuart and took an early interest in biology. In an autobiographical note Smith noted that Stuart had shown them the convolutions of the human brain and declared that nobody understood them fully. Smith decided at that point that he would work towards understanding them. Accordingly, he went to study medicine at the University of Sydney in 1888 and received a Doctor of Medicine in 1895, with a dissertation on the fore-brain of the monotremes) and developed an interest in the anatomy of the human brain. He received a James King travelling scholarship and went to St John's College, Cambridge, graduating BA in 1896. Afterwards he catalogued the human brain-collection of the British Museum.

Smith obtained an appointment at the Cairo School of Medicine in 1900 on the suggestion of his anthropologist friend Alexander Macalister. Smith became archaeological advisor to the archaeological survey of Nubia in the wake of plans to construct the Aswan Dam which threatened to drown numerous archaeological sites. Smith conducted investigations on the brains of Egyptian mummies. He was one of the first to non-destructively use x-rays to study mummies. Smith took a special interest in the pathologies indicated in ancient skeletal remains. He noted for instance that many Egyptian skulls had biparietal thinning which had been common in European aristocrats in the past. Smith hypothesized this was the result of wearing heavy wigs or headgear. From 1909 to 1919 he was Professor in anatomy in Manchester, 1919–1937 he held the chair of Anatomy at the University College London. He was elected President of the Anatomical Society of Great Britain and Ireland for 1924 to 1927. During World War I he took an interest in the neurology of shell shock, visiting military hospitals and serving on the British General Medical Council.

Smith was the leading specialist on the evolution of the brain of his day. Many of his ideas on the evolution of the primate brain still form the core of present scholarship. He proposed the following stages of development:

Smith was decorated by the Khedive of Egypt, Abbas Hilmy in 1909 with an Insignia of the Third Class of the Imperial Ottoman Order of the Osmaniah. He became Fellow of the Royal Society in 1907, FRCP, elected to membership of the Manchester Literary and Philosophical Society on 4 October 1910, President of the Society 1919, cross of the French Legion of Honour, and was knighted in the 1934 Birthday Honours. In 1912 he received the Royal Medal of the Royal Society, in 1930 the Honorary Gold Medal of the Royal College of Surgeons, in 1936 the Huxley Memorial Medal from the Royal Anthropological Institute of Great Britain and Ireland.

British anthropologists Arthur Keith and Grafton Elliot Smith both supported the European origin of humankind as opposed to models of Asian and African origin. In several of his works, Smith argued that Europe was the cradle of humanity, identifying a European Mediterranean race as the occupants of the original home of modern humans. His cradle was large, as he claimed the Mediterranean race had occupied the Levant, Egypt and western Europe, including the British Isles. He especially linked the Mediterranean race to the civilisation of Egypt. Smith's arguments later became known as his theory of diffusionism. According to Smith and William James Perry, Egypt was the source of all cultural innovations and the ultimate source of human civilisation.

According to Smith, "Man did not become truly erect until his brain had developed in a very particular way to make it possible for him to use his hands". That line of reasoning reinforced the European origin of human, which Smith and Keith supported, as the mostly large brained specimens such as the Cro-Magnon had been found in Europe.

The term 'hyperdiffusionism' seems to have been coined by the British archaeologist Glyn Daniel in his book The Idea of Prehistory (1962) with a somewhat derogatory intention. It was intended to represent extremes of diffusionism, a theme popular in early 20th century archaeology that itself has been subject to criticism. Smith believed that all megalithic phenomena, whether in Northwestern Europe, India, Japan or Mesoamerica, had originated in Ancient Egypt. "Small groups of people, moving mainly by sea, settled at certain places and there made rude imitations of the Egyptian monuments of the Pyramid Age." (Smith 1911, ix). Smith believed in a direct diffusion to Syria, Crete, East Africa, Southern Arabia and Sumer, and other areas were influenced by secondary diffusion. The Neolithic culture of Europe was derived from Egypt as well, according to Smith. He even interpreted a mummy from a Torres Strait island as definitely being indicative of an Egyptian influence. The concept of hyperdiffusionism is now referred to by more neutral terms (when referring to the Americas) such as Pre-Columbian trans-oceanic contact.