The concept of evolution does not involve the replacement of one group by another in a linear progression. Instead, it is better understood as a branching process resembling a bush rather than a ladder of progress. Populations within a species diverge and branch off from their ancestors, following their own unique evolutionary paths.

To illustrate this, consider a simplified phylogenetic tree. It demonstrates how different groups evolve in parallel, rather than in a linear sequence. Reptiles and mammals, for example, share a common ancestor from the distant past. However, instead of mammals replacing reptiles, these two groups diverged and evolved separately along their own distinct trajectories.

There is no inherent reason why reptiles and mammals cannot coexist simultaneously. Evolution allows for the existence of diverse species across different lineages and time periods.