Here's a simplified explanation:
* Mammals are a class of animals that share certain characteristics, like having hair, mammary glands, and giving birth to live young.
* Placental mammals are a sub-group of mammals that develop a placenta, which allows for nutrient exchange between the mother and the developing fetus.
* Marsupials are another sub-group of mammals, known for their pouch where their young complete development.
* Reptiles are a distinct class of animals, including lizards, snakes, turtles, and crocodiles, that have scales and lay eggs.
The evolutionary tree shows that:
1. Reptiles and Mammals diverged from a common ancestor millions of years ago.
2. Within the mammalian lineage, a further divergence occurred, leading to the marsupial and placental lineages.
Therefore, placental mammals did not evolve from marsupials; instead, they share a common ancestor with marsupials. Similarly, placental mammals did not evolve from reptiles, but they share a more distant common ancestor.