The Left could be "anti-evolutionists and anti-stem cell advocates", but the American Left currently isn't.
For instance if the Left for some reason thought Humans were of equal value or as much of a concern as "Nature" [as in Nature vs Human- as though humans aren't part of nature] then a similar "argument" or hysteria over GM crops could be applied to the human species. Or perhaps if they were to understand the idea that the human is sacred, is not something exclusive to Christian theology, but rather is an ancient idea which began thousands to tens thousands of years before the Jews or Christ existed.
Say, the Hindu or some other endearing minority for whatever reason started being vocal about their beliefs, the Left might then agree with it.

As for my attempt to briefly define the left; I see now that was rather futile because the proper thing to do would be to refer to some authority- say something said by Al Gore, a well liked movie star, Ms Clinton, or something like that.

But I hope Wiki is good enough for the moment:
"The meaning of the terms "left" and "right" in a political context has changed radically over time. The Right is generally against intentional political, economic and social change, the Left is in favour of it.[1]

Some commentators, such as Norberto Bobbio, have argued that the central difference between left and right is that the left prioritises social equality, while the right prioritises individual responsibility and the maintenance of natural and inherent inequalities between people. Bobbio also makes clear, however, that "left" and "right" are not absolute terms, but vary between different countries and different periods."
http://en.wikipedia.org/wiki/Left-right_politics

Now, I would add the European Left has traditionally [since around the time of blood bath of French Revolution] been focused on equality as highest value, and within last hundred years or so the America Left has been influenced [infected/impaired] with the ideas concerning this. The traditional American Left highest value has been liberty. "Give me liberty or death" kinda summarizes that.

The importance placed upon social equality led directly to authoritative regime like the Soviet Union and various fascist States- this is fairly predictable if you have any understanding of such matters.