Yes, all true. The mass extinctions have been a source of wonderment. Amazing that something always survives, and that something then expands to occupy all the newly available ecological niches when conditions improve. We might wonder why life shows so much variation (I mean, do we really need 300,000 species of beetles?) but evolutionary history has demonstrated that their is value in have many variant organisms available when a mass extinction occurs. One of them might be just what is needed for Life to survive.

In my cited statement, I was referring to lines of descent, not Life in general. "You can't go back again" means that as we evolve as a species, we can develop new adaptations but we can't return to the previous adaptations. We may return to the sea, but we will not re-develop gills, we will evolve some new way to breath.

In one of the most amazing events after the last mass extinction, for example, some mammals managed to return to the sea, adapting to ecological niches unoccupied by fish - that is, feeding in high volume on plankton and other micro-organisms. The closest the fish have come is the whale shark, which is a mass feeder but has gotten no larger than the smaller whales. Meanwhile, the whales have become the largest organisms on the planet. They have moved their nostrils to the top of their heads, but they haven't re-invented gills. Gills probably can't capture enough oxygen to maintain such massive bodies. The whales have managed to combine the best of two worlds, the high-energy oxygen breathing of land and the ability to exploit an almost limitless (until now) food supply found in the ocean.