Justify the argument that today's organisms would not exist if mass exticntions had not occurred?

Answer 1

There is certainly no way to prove it definitively. However, it is a reasonable conclusion to come to.

We could compare, for example, the creatures that occupy a particular niche before and after a mass-extinction. I'm sure in many (if not most) cases you would find that the prior creatures were MUCH better adapted to survive in that niche.

And you know what that means - if the pre-extinction creatures had still been there, the post-extinction ones wouldn't have gotten the food and space they needed to survive. Perhaps they wouldn't have. Arguably such new lifeforms were crushed before the mass extinction so they probably would have KEPT getting crushed.

But that's just a probably. Less food isn't NO food, so it is conceivable that some of the organisms that arose after a mass extinction would have showed up anyway. A massive coincidence. Yet stranger things have happened.

Since the laws of probability argue against it, most biologists do too. Judging from the fossil record, mass extinction events provide opportunities for new life that otherwise wouldn't have existed.

Just keep in mind that 99.9% of all species died out through normal competition. The amount of species wiped out in mass-extinctions is large for a small point in time, but small compared to all of evolutionary history.

Answer 2

Mass extinctions allow for a reduction in competition for available resources, this allows for a rapid increase in the remaining organisms, and an obvious complete change in the population, any evolutionary changes up to that point would be swept away. In effect, the population starts again, with the remaining organisms as the base line.
For instance, if a cloud suddenly blocked out the Sun, killing of 99% of all plant and animal life on Earth, then there would be a rapid growth of the 1% of remaining plants that could survive with minimal sunlight, and an increase in animal species that could survive on these plants. Without the mass extinctions, a steady state of existence would ensue, and the rapid growth phases would not occur.
The Earth has had 5 known mass extinctions.

Answer 3

99% of all species that ever were have gone extinct. If there had been no mass extictions, then there would have been no mass radiations; mammals, for instance.