From the title and cover we can infer that the people in the book will be evolving.
A chrysalis is the pupil stage of a butterfly. In other words, it is the butterfly before it becomes a butterfly. The original title of the book shows a human footprint with six toes. This seems to depict Sophie’s footprint.
On the flat rock was a footprint, still undried. Sophie had rested one foot there as she bent over to tip her catch into the jar. The mark was still damp enough to show the print of all six toes clearly. (ch 5)
This is significant because Sophie was the catalyst for David realizing that some people are different. She is an example of the humans in their not-quite-ready state.