Why are we the only human species left on the planet?

The most likely reason is that we are the only human species that has been able to adapt to our changing environment. We have developed tools and technologies that have allowed us to survive and thrive in a wide range of environments.

Other human species, such as the Neanderthals, may have been better adapted to their environments at the time, but they were not able to adapt to the changing climate and landscape as the ice age ended. There is also the possibility that other human species were simply out-competed by our ancestors. We may have been better at hunting and gathering, or we may have had better social skills that allowed us to form larger and more effective groups.