Born in Leicester and having attended local schools, Hallam won an exhibition to St John's College, Cambridge, graduating with a double first class degree in geology in 1955.
He remained at Cambridge University as researcher under the supervision of the late William Joscelyn Arkell, widely regarded as the expert on the Jurassic Period at that time.
His thesis involved the study of the alternating limestone – shale rhythms of the Lower Jurassic of Southern England.
It was during this time that he continued his research into the controversial evolution of Gryphaea, publishing several papers with the late Stephen Jay Gould.
He has written over 200 research papers and is the author or editor of more than twelve books, including Jurassic Environments, Great Geological Controversies and Catastrophes and Lesser Calamities: The Causes of Mass Extinctions.