I believe that the differences between humans and nonhumans are quantitative, not qualitative.
Why is this idea dangerous? It is hardly surprising, coming from someone who has spent her scientific career studying the abilities of (supposedly) small-brained nonhumans; moreover, the idea is not exactly new. It may be a bit controversial, given that many of my colleagues spend much of their time searching for the defining difference that separates humans and nonhumans (and they may be correct), and also given a current social and political climate that challenges evolution on what seems to be a daily basis. But why dangerous? Because, if we take this idea to its logical conclusion, it challenges almost every aspect of our lives — scientific and nonscientific alike.
Scientifically, the idea challenges the views of many researchers who continue to hypothesize about the next human-nonhuman 'great divide'…Interestingly, however, detailed observation and careful experimentation have repeatedly demonstrated that nonhumans often possess capacities once thought to separate them from humans. Humans, for example, are not the only tool-using species, nor the only tool-making species, nor the only species to act cooperatively.
So one has to wonder to what degree nonhumans share other capacities still thought to be exclusively human. And, of course, the critical words here are "to what degree" — do we count lack of a particular behavior a defining criterion, or do we accept the existence of less complex versions of that behavior as evidence for a continuum? If one wishes to argue that I'm just blurring the difference between "qualitative" and "quantitative", so be it…such blurring will not affect the dangerousness of my idea.
My idea is dangerous because it challenges scientists at a more basic level, that of how we perform research. Now, let me state clearly that I'm not against animal research — I wouldn't be alive today without it, and I work daily with captive animals that, although domestically bred (and that, by any standard, are provided with a fairly cushy existence), are still essentially wild creatures denied their freedom.
But if we believe in a continuum, then we must at least question our right to perform experiments on our fellow creatures; we need to think about how to limit animal experiments and testing to what is essential, and to insist on humane (note the term!) housing and treatment. And, importantly, we must accept the significant cost in time, effort, and money thereby incurred — increases that must come at the expense of something else in our society.
The idea, taken to its logical conclusion, is dangerous because it should also affect our choices as to the origins of the clothes we wear and the foods we eat. Again, I'm not campaigning against leather shoes and T-bone steaks; I find that I personally cannot remain healthy on a totally vegetarian diet and sheepskin boots definitely ease the rigors of a Massachusetts winter.
But if we believe in a continuum, we must at least question our right to use fellow creatures for our sustenance: We need to become aware of, for example, the conditions under which creatures destined for the slaughterhouse live their lives, and learn about and ameliorate the conditions in which their lives are ended. And, again, we must accept the costs involved in such decisions.
If we do not believe in a clear boundary between humans and nonhumans, if we do not accept a clear "them" versus "us", we need to rethink other aspects of our lives. Do we have the right to clear-cut forests in which our fellow creatures live? To pollute the air, soil and water that we share with them, solely for our own benefit? Where do we draw the line? Life may be much simpler if we do firmly draw a line, but is simplicity a valid rationale?
And, in case anyone wonders at my own personal view: I believe that humans are the ultimate generalists, creatures that may lack specific talents or physical adaptations that have been finely honed in other species, but whose additional brain power enables them — in an exquisite manner — to, for example, integrate information, improvise with what is present, and alter or adapt to a wide range of environments…but that this additional brain power is (and provides) a quantitative, not qualitative difference.