Modern archeologists have found two artificial big toes in Egyptian tombs: the linen and plaster "Greville Chester toe," which dates back past 600 BC (below); and the wood and leather "Cairo toe," which was built between 950-710 BC and was unearthed affixed to a female mummy. Researchers were unsure if these devices were functional or merely aesthetic, but after some sure-footed testing, they appear to be the earliest prosthetics discovered yet.
To test whether these tomb toes were used as prostheses, researchers at the University of Manchester built facsimiles and asked volunteers (who were missing their right big toes) to give the recreations a test run. Sure enough, the false toes offered greater mobility and comfort. Said Manchester study leader Dr. Jacky Finch in a recent article in The Lancet:
To be classed as true prosthetic devices any replacement must satisfy several criteria. The material must withstand bodily forces so that it does not snap or crack with use. Proportion is important and the appearance must be sufficiently lifelike as to be acceptable to both the wearer and those around them. The stump must also be kept clean, so it must be easy to take on and off. But most importantly, it must assist walking. The big toe is thought to carry some 40% of the bodyweight and is responsible for forward propulsion although those without it can adapt well [...]
My own research used two volunteers with similar amputation sites and suggested that replicas of both ancient Egyptian false toes performed extremely well. Neither design should be expected to be completely efficient in emulating the ﬂexion of the normal left big toe when pushing oﬀ . However, high eﬃciency was recorded by one volunteer when wearing the replica cartonnage prothesis and also when wearing the wooden one (both worn with replica Egyptian sandals). More importantly, no signiﬁcant elevation in pressure under the sole was recorded although both volunteers found the articulated wooden design to be especially comfortable
And here's Dr. Finch posing with the original Cairo toe and her model.
Photos via The University of Manchester