The tools of modern astronomy indicate how rare any form of life, let alone intelligent life, may be in the universe. And we have just begun to appreciate how astronomically rare it is that humans even exist on earth. Through evolutionary biology we have learned that the likelihood that life would have evolved into complex intelligent creatures capable of drastically manipulating and altering the world was incalculably low.
For life to have developed as it did a series of fortuitous accidental events had to occur including the origin of life itself, photosynthesis, complex cells, sex, complex animals, and skeletons, and each of these events required an equally rare set of preceding events as well. The odds of all events occurring as they did, resulting in the emergence of humankind are so low that some scientists believe we might be the only intelligent life in the universe.
If other intelligent life did develop elsewhere it may not have successfully passed through the great filter which we are now encountering. The idea of a great filter is that, where ever it develops, life must first be able to withstand natural disasters like massive asteroid impacts and volcanic eruptions. If life managed to survive those natural great filters as we did, and eventually become capable of building a highly technological civilization, they would eventually encounter great filters of their own making resulting in numerous existential challenges.
Because of accelerating technological change with lagging social progress humankind faces such self-created great filters presently. The likelihood that intelligent life is extremely rare, in combination with the numerous existential threats it would face, all contribute to our recognition that humankind could be the only intelligent species in the universe.