What does it mean to be Human?

Many thinkers and philosophers, both of the natural and the political kind, have tried to answer this question. In order to clarify what it means to be human, I think it is important to first find out what it means to be a living being in general. Biologists define life as anything that is made up of cells and has the ability to reproduce, either sexually or asexually. However, this definition is not perfect. For instance, viruses such as HIV are not considered living things, since they are not made of cells, being in fact much smaller than body cells. But if viruses are not living beings, then what are they? they are not machines (as far as we know), and they are not naturally occurring abiotic compounds that just happen to act exactly like living parasites. The answer to this question is that viruses are probably living things, only that under the current "standard" definition they are excluded.
The same is true for the general definition of a human being. Not only does this definition change from person to person, and from one group of people to the next, but those included in this definition change as well. For instance, the Neanderthal is considered to be a different species from us, the homo sapiens, yet many say that Neanderthals are humans as well. But what about dogs, cats, or hamsters? what exactly is the difference between a human, a Neanderthal, and a common "lowly" animal? The only thing that all definitions of a human being agree on is that we are somehow different from the rest of the fauna that stalks planet Earth. But what is that difference? Are we more intelligent that other species? How can one compare the intelligence of a gerbil to that of a human? You don't have to be a gerbil expert to know that any individual of that cute species will utterly fail the SAT, ACT, or the common IQ test. It's impossible for us to accurately test the "intelligence level" of another species using our biased human testing methods.
 Then what makes us different? is it because we have souls and animals do not? Are we the chosen species that gets to relax in Heaven while our beloved pets (the soulless beasts) get to burn in hell through no fault of their own? We may never know.