Darwin's Blueprint

Two maverick biologists assess planets for possible human settlement following the abandonment of the Earth. Known as First Footers, Murphy and Greenspan operate far from any central authority and have to be self-reliant and resourceful. They discover they have to battle not only alien environments, but corrupt officials too who will exterminate entire species to make money. Once every twenty years they must return to the planet Home to undergo review and relicensing procedures. It's a process they hate, but without a license the Guild of Exobiologists would stop giving them assignments. They return to Home and submit to a series of humiliating tests and procedures encountering symbionts - human/machine hybrids -
and a new technique that allows them to visit the past. It's an adventure which turns out to be more dangerous than anything they've faced before.