Good novel by Michael Crichton
What was the cause of the Cambrian explosion then? Lots of oxygen. Earth was once an ice-ball from pole to pole. Volcanoes eventually pumped out enough carbon dioxide to warm up the planet. That in turn made plant growth go wild. Plants make oxygen so they created it in vast amounts. Suddenly animals are given a boost of energy and that is why evolution happened so quickly after so many years under the ice.
Comets are not a trivial thing to study up close and scientists have only landed on one so far. It doesn’t appear scanning for lifeforms was part of the mission (Rosetta).
They did prove it does contain pockets of cryogenic water ice below the surface, however.
Cryogenic ice enclosed in thick rock is as ideal a carrier of life through the cosmos you can get. It would block radiation better than any man made spacecraft. See my last post above.
Good novel by Michael Crichton