Human beings are on Earth, as opposed to any of the other planets in the solar system, because it is uniquely able to support the development of life. Some scientists speculate that Earth hosts the only intelligent life in the universe. Others believe there are many other habitable planets.
There are several factors which must be in place for a planet to be able to support the development of any life, especially intelligent life such as humans.
The planet must be an appropriate distance from the sun. If it is too close, then temperatures are too high. If it is too far away, then it is too cold. A habitable planet must also receive the right amount of sunlight. There must be enough for photosynthesis to occur, but not so much that potential life forms are fatally irradiated.
A habitable planet also must be large enough to hold a breathable atmosphere. Scientists don't anticipate gaseous planets to be able to support life, so a habitable planet must be solid. A solid planet also must have a molten core. That core is what allows geothermal energy to exist, cycles raw materials, and also emits a magnetic field which helps protect life from radiation.
Another important factor is water. Scientists do not believe that life can evolve or exist without water, and Earth is mostly covered in oceans. Out of all the planets in the solar system, only Earth has all of the correct factors to permit the existence of life, which is why humans are here rather than elsewhere.