Erasmus was born in 1731 at Elston Hall in Nottinghamshire, the youngest of seven children.  In 1756 Erasmus moved to Lichfield to establish a medical practice. He continued to live in the city until 1781.

Erasmus came up with the theory of evolution long before his grandson Charles did, this he present in his book Zoonomia or the Laws of Organic Life which he started to write while living here. The book was extremely well-received when it was first published in 1794.