The Largest Extinction Event
There is evidence that the largest extinction event, which occurred 252 million years ago, was caused by the moon bouncing off the earth. It would have destroyed all large animals, because they could not tolerate the extreme acceleration which would be similar to falling off a building. All animals would have been thrown into the air some distance. Only the smallest animals would have survived.
This result appears to have occurred, as all large, four legged animals apparently disappeared at that time. The reptiles which survived evolved into dinosaurs without much competition due to the elimination of large animals. Dinosaurs became large, as they needed much size to walk through the heavy brush created by the nonwoody plants which they used for food.
The only mammals which survived were the smallest ones which could tolerate the acceleration force of the impact. These mammals had to stay small to move under and through the nonwoody brush.
Dinosaurs would have benefitted from fat production which would have facilitated moving through brush, but fat and other storage molecules did not exist until after the modernization of biology after dinosaurs died out 66 million years ago.
Circumstantial evidence indicates that yeasts evolved fat metabolism, as their physiology was closely related to fat production. But yeasts did not exist until they evolved on sugary solutions which did not exist until modern biology produced the sugary solutions. The earliest fossil evidence for yeast only goes back 55 million years. After fat production as a storage molecule evolved in yeasts, the genes would have been transferred to other species through horizontal gene transfer.