What are the four main evolutionary lines of placental mammals?

It's not accurate to say there are four main evolutionary lines of placental mammals. It's more complex than that. While there are many different ways to classify placental mammals, the most common and widely accepted approach uses four superorders:

1. Afrotheria: This group includes animals like elephants, manatees, aardvarks, and tenrecs. They are thought to have originated in Africa.

2. Xenarthra: This group consists of sloths, anteaters, and armadillos. They are found mainly in the Americas.

3. Laurasiatheria: This diverse group includes animals like bats, whales, horses, dogs, cats, and many others. They are thought to have originated in Laurasia, the northern supercontinent that existed millions of years ago.

4. Euarchontoglires: This group consists of primates, rodents, rabbits, and a few other orders. They are also thought to have originated in Laurasia.

It's important to note that this is a simplified representation. There are many other classifications and subgroups within these superorders. For instance, within Laurasiatheria, there are many orders like Artiodactyla (even-toed ungulates), Perissodactyla (odd-toed ungulates), and Carnivora.

It's also important to remember that these classifications are based on current understanding of evolutionary relationships, and they may be subject to change as new research emerges.