Question

Outline the evolution of Monotreme, marsupial, and placental mammals.

Answer 1

idk

Answer 2

@mrdoldum

Answer 3

what level of depth? Monotremes are mammals that lay eggs. There are only two. marsupials give live birth to an extremely immature fetus. To the best of my knowledge, neither monotremes nor marsupials have true breasts/teats, they secrete milk from skin using modified sweat glands (which is what all mammals use, more further down).

Now, at some point in mammal evolution, a virus did something that allowed for placenta mammals to evolve. I haven't kept up with the literature on that, but it is generally accepted that key genes involved in placenta formation have viral DNA in them.

Having a placenta with live birth is a huge advantage. It allows development to occur and gives the young a huge head start. Many deer/antelope young can walk and run within an hour of being born. Elephants take 2 years in the womb before being born, evidence suggests this is so the brain can develop enough to control the hugely complex mass of muscle and nerves that is their trunk.

Placental mammals have specialized structures that house the milk glands (remember those specialized sweat glands), and are dominant all over the world. Marsupials used to dominate the southern hemisphere, but when North and South America reconnected ~3 million years ago the placental mammals took over and only one marsupial survived.

Answer 4

Thank you SOO Much !!!!! @mrdoldum