There are plenty of critical events in evolution that occurred only one time or a couple of times.
Eukaryotes only appeared one time. Eukaryotes split into animals and plants only one time. Animals and insects diverged only one time. Photosynthesis evolved only one time. Mitochondria appeared only one time. There was only one last universal common ancestor.
Actually, not. Eukaryotes exist because, hundreds of millions of years ago, some of the single cell organisms found a survival advantage in living inside other single cell organisms. Similarly, mitochondria are bacteria that found an advantage to living inside other cells; there are still pathogenic species of the same bacteria (rickettsia) that became mitochondria. Chloroplasts are also ancient bacteria that found an advantage in living inside other cells. Plants arose when cells that contained both mitochondria and chloroplasts gained advantages by living in colonies, and the colonies in which some cells took on specialized functions gained advantages that other colonies did not have. Animals developed through a similar process.
In the study of microbiology, you can examine examples of microorganisms that are at different stages of the process of becoming true multi celled organisms. Euglena, for example, are single celled plants that form spheres. Inside the spheres, smaller spheres form. Dictyostelium live as single celled organisms until food becomes scarce. At that point, they send signals into the environment to communicate with each other, and they move together and form a small slug-like organism. And the cells specialize; eventually some of the cells become a stalk and others become a fruiting body, within which new Dictyostelium cells grow, waiting for food to become available again. Then they live as single cells.
And so on. Not only are there no "critical events" that only happened one time, the key developments in the progression of multi celled life are ongoing and observable; there are examples of organisms at all stages of their evolutionary progress.