They didn't exist. In Earths normal climate there should be a Jungle in the Antarctic. If there are any standing bodies of Ice on the Earth like there are now it's classed as an Ice Age. We are still technically in one. The melting is inevitable. If Mankind never existed all that would probably happen is the melt would slow down.
There is a bigger issue. Because we have greatly accelerated the warming process, if there is one, nature cannot catch up through evolution to evolve us, and all the current animals on earth to adapt to the heat. Thus, animals that would of adapted to a more "normal" temperature if the process was natural there would not be mass extinctions as there are now. We as humans are not immune either. Cancers, heat stroke, head aches, etc have all increased because of our activities. We have not only increased our carbon emissions that heat up the planet but we have also chemically changed our atmosphere to contain toxins that are not found in nature, such as PCB's, and toxins that are not normally air borne... such as asbestos. All of this contributes to the degredation of ecosystems, resulting in more carbon releases than normal.
For example, Acid Rain was a major problem in the 1960's-1980's and still is to an extent. It is toxic to plant and animal life. In Sudbury Ontario, Acid Rain destroyed 1000's of Hectares of forest. Since forests are 'Heat Sinks', these forests were not absorbing carbon but releasing it exponentially. Additionally, Oceans are major heat sinks, and with deforestation, and warming of the atmosphere, these two heat sinks cannot offset each other as they used to, resulting in even more accelerated atmospheric carbon increases.