<Ul> <Li> The most common view is that placentals originated in the Southern Hemisphere, but some paleontologists argue that they first appeared in Laurasia (old supercontinent containing modern Asia, N. America and Europe). </Li> <Li> Paleontologists differ as to when the first placentals appeared, with estimates ranging from 20M years before the end of the Cretaceous to just after the end of the Cretaceous . Molecular biologists argue for a much earlier origin, even suggesting appearance in the Middle Jurassic . </Li> <Li> Molecular data suggest that either Xenarthra, Afrotheria, or Atlantogenata (Xenarthra + Afrotheria), was the earliest - diverging group from the rest of the placental mammals . </Li> </Ul> <Li> The most common view is that placentals originated in the Southern Hemisphere, but some paleontologists argue that they first appeared in Laurasia (old supercontinent containing modern Asia, N. America and Europe). </Li> <Li> Paleontologists differ as to when the first placentals appeared, with estimates ranging from 20M years before the end of the Cretaceous to just after the end of the Cretaceous . Molecular biologists argue for a much earlier origin, even suggesting appearance in the Middle Jurassic . </Li> <Li> Molecular data suggest that either Xenarthra, Afrotheria, or Atlantogenata (Xenarthra + Afrotheria), was the earliest - diverging group from the rest of the placental mammals . </Li>

In the evolution of the mammalian y chromosome which of the following is assumed to have occured