It appears that Dougal Dixon may have not been as original as it seems when conceiving of some of the creatures in this work. I refer you to The Alien Life of Wayne Barlowe. Read what Mr. Barlowe has to say on the subject and draw your own conclusions.
From The Alien Life of Wayne Barlowe by Wayne Douglas Barlowe:
These preliminary drawings were created as part of a proposal for a book project in 1984. The book would have dealt, in a psuedoscientific manner, with the possible evolution of Mankind in the distant future. Unfortunately, it's a book that will never come to be, as these sketches were plagiarized and utilized in another author's work.