"Student Body", a novelette by F. L. Wallace, available at Project Gutenberg, also my answer to the old question Short story about human colonists vs. alien animals that evolve very rapidly. It doesn't start with bacteria, the progression of forms the "omnimals" take is squirrels, mice, rats, tigers, men. And we don't see what comes after men:
"Don't you still see? There is a progression. After the tiger, it bred this. If this evolution fails, if we shoot it down, what will it create next? This creature I think we can compete with. It's the one after this that I do not want to face."
It heard them. It raised its head and looked around. Slowly it edged away and backed toward a nearby grove.
The biologist stood up and called softly. The creature scurried in the trees and stopped just inside the shadows among them.
The two men laid down their rifles. Together they approached the grove, hands spread open to show they carried no weapons.
It came out to meet them. Naked, it had had no time to learn about clothing. Neither did it have weapons. It plucked a large white flower from a tree and extended this mutely in a sign of peace.
"I wonder what it's like," said Marin. "It seems adult, but can it be, all the way through? What's inside that body?
"I wonder what's in his head," Hafner said worriedly.
It looked very much like a man.