Genesis

Astronaut Christian Brannock has lived to see artificial intelligence develop to a point where a human personality can be uploaded into a computer, achieving a sort of hybrid immortality. He welcomes that because the technology will make it possible for him to achieve his dream and explore the stars.
A billion years later, Brannock is dispatched to Earth to check on some strange anomalies. While there, he meets Laurinda Ashcroft, another hybrid upload. Brannock and Laurinda join forces and investigate Gaia, the supermind dominating the planet, and learn the truth of her shocking and terrifying secret plans for Earth.
In classic Poul Anderson fashion, Genesis investigates the subject of human immortality as well as another possibility for what our future as humans may hold, subjects that he has been investigating for fifty-two years.