Final answer:
The similarities between Australian marsupials and placental mammals are attributed to convergent evolution, where different lineages independently evolve similar traits to adapt to similar environments and ecological roles. This is most evident in the adaptation of Australian marsupials to various ecological niches in the absence of competing placental mammals.
Step-by-step explanation:
The similarities between Australian marsupials and placental mammals (eutherians) elsewhere in the world can be best explained by convergent evolution. Convergent evolution occurs when different species evolve similar traits independently, often as a result of adapting to similar environments or ecological niches.
As therian mammals, both marsupials and eutherians share a common ancestor. However, their evolutionary paths diverged, with marsupials giving birth to undeveloped young that mature in a pouch, while eutherians have developed a complex placenta to support the young's development in utero. Despite these reproductive differences, the two groups display similarities in form and function due to adapting to comparable habitats independently - a classic case of convergent evolution.
Marsupials remained widespread in Australia due to the continent's long isolation, leading to an environment with fewer competing placental species. This allowed them to radiate into a variety of ecological roles now filled by placental species elsewhere. The result is a remarkable resemblance in the ecological roles and sometimes in the physical attributes of distantly related species living in similar environments across the globe. For example, the Tasmanian tiger resembled placental canids despite being a marsupial.
In summary, the final answer to why Australian marsupials and placental mammals are similar despite their geographical and reproductive differences is that convergent evolution has led to the independent development of similar traits to adapt to similar environmental pressures.
The similarities between Australian marsupials and placental mammals in other parts of the world can be attributed to convergent evolution, which is the independent evolution of similar traits in species of different lineages due to similar environmental pressures and ecological roles.