Well, they do not quite say it. But get this: From Charles Q. Choi at Astrobiology:
Assuming that early life adapted to survive in a checkerboard of many different kinds of environments, “the complex ecological relationships between different species may have been a part of life on Earth since very near its beginning, and LUCA is only one example of the life that may have been extant at that time,” says Fournier.
“Perhaps a rapid establishment of complex environmental and ecological relationships was even necessary for early life to persist,” adds Cantine.
The picture painted by Cantine and Fournier of the early evolution of life on Earth is just one plausible scenario. “Our interpretation, like others, relies on a limited [genetic] record and is one contribution to a lively debate,” says Cantine.
The researchers note that, when hunting for possible signs of life on Mars, searches should not be constrained to probable environments in which life could have originated, since life on Mars may have moved to other environments since it emerged. Instead, Cantine and Fournier propose that the search for life on Mars should focus on those environments that are most likely to have captured and preserved traces of life. More.
In short, it would not be possible without some kind of pre-existing intelligence.
See also: Maverick theory: Cambrian animals remade the environment by generating oxygen. The trouble with maverick theories today is that they usually aren’t really mavericks. They are merely theories that stretch reality past the shredding point. Note the use of “would have.”
But yes, interesting, anyway. The Cambrian would make great theatre if it weren’t just all about animals.