It's been a long time since I read it, but in my youth I read one of his Gaia books. possibly not the first, since it had a long "rebuttal" part and also includ

It's been a long time since I read it, but in my youth I read one of his Gaia books. possibly not the first, since it had a long "rebuttal" part and also included stuff on endosymbiosis (which I believe came slightly later). It did address these sort of questions at length. I remember one of the concrete predictions was that Mars was completely lifeless. Since life begets conditions for more life according to the theory, if there ever was significant life on Mars it would have kicked off the extreme makeover process that we see on Earth. Not necessarily exactly the same of course, but just like it's extremely apparent from space that there's life on Earth, so it would be on Mars. And he made that prediction (according to the book, according to my memory!) before the Viking sondes. It was a bold prediction. They hoped to find life. You find evidence of life even in the driest, coldest, harshest places on Earth, and even deep into the Earth's crust, so why wouldn't there at least be some primitive archaea-like organisms in Martian soil? But they still haven't found any, fifty years, 18 orbiters and ten landings later.