You won't get any dissent from me, Jim. Peter Singer's arguments against our current treatment of animals are as sound as moral arguments can be: They are unanswerable. Speciesism is indeed one of the most preposterous prejudices ever generated by the human talent for self-serving sophistry. As you perceptively remark, if Neanderthals were still around, a species distinct from us but with comparable mental status, would we think it legitimate to experiment on them, hunt them, eat them, factory farm their kids, cage them, and otherwise ruin their lives? My answer is that we probably would--in the sense that all this would actually happen. Unless they had the power to fight back they would suffer the same abuse as we visit on other species now. But, of course, this would be morally monstrous, since their suffering and death is of equal moral importance to our own. Only the arbitrary stipulation that they belong to a different species from us could lead anyone to overlook their equal claim to decent treatment.
It is an odd fact of evolution on planet Earth that there is no other species around with the kind of intelligence and power that we possess. This allows us to get away with murder. But it could have been otherwise. Suppose that Mars had its own life-forms, possibly much more "advanced" than ours (as Martin Amis imagines in his story "The Janitor from Mars" in this week's New Yorker). And suppose they decide to pay us a predatory visit. Quickly we are overwhelmed, our defenses neutralized. They then embark on centuries of ruthless human exploitation, reaching ever higher levels of technologically driven barbarity: We are selectively bred for Martian culinary delight, reared in dismal factories alone and deprived, slaughtered at the age of 20 or 40 or 2 (think back to the infanticide issue for a moment); we are caged in laboratories, where we are used as subjects of tests for Martian weapons, being shot, electrocuted, vivisected and burned; and on Sundays the Martians enjoy letting us loose and tracking us down with bows and arrows. We plead with them not to treat us this way, giving them the obvious moral arguments, but with the exception of a few "sentimentalists" and "fanatics" we are dismissed as an inferior species with no inherent rights or interests. Of course, we are a distinct species from Martians, despite our psychological similarity to them, which enables them to cite this biological fact as an excuse for their immoral treatment of us. But I take it that the hollowness of this "reasoning" does not need laboring. Speciesism here is just a cloak for self-interested exploitation and systematic callousness.
Or suppose the following lies in our evolutionary future: We decline as a species, becoming sickly and complacent; all we do is watch low-grade TV and eat junk food. Meanwhile the apes are flourishing, having evolved a more powerful brain and a technology of their own. Mainly they leave us to our own wretched devices, tut-tutting over our laziness and stupidity. But then an ape leader arises with an interest in ape history. She reminds the apes of what those feeble humans used to do to them. She stirs up feeling against human decadence and points to the disgusting hairlessness that characterizes our species. Soon the apes are waging war on us, forcing us to do as they please. Before you know it we are being herded into factory farms, used as subjects for ape scientific research, hunted, etc. When we protest they remind us of our past abuse of them, citing the speciesist defense we so mechanically offered back in the old days of ape subjection. The centuries of human exploitation begin, with no end in sight.
Singer's Animal Liberation is especially powerful in simply reporting what humans do to animals every day, in the name of nutrition and science. As you rightly say, this is sickening stuff, seldom brought to the public's attention. I recommend anyone interested in these issues to read these chapters of the book, even if they read no others. The facts alone speak volumes, even without the kind of rigorous moral argument Singer appends to them.
You ask whether Singer's case has moved me to modify my diet. Well, I became a vegetarian back in 1972, chiefly as a result of reading Animals, Men and Morals. I have not always in practice cleaved to a rigidly vegetarian diet, but I have never wavered in my belief that killing animals for food is wrong in principle and that the meat industry is a vile blot on the moral record of mankind (among many such blots). What is amazing to me is that even today intelligent people can be so willfully blind on the subject. I sometimes think that the reason for this is that power over animals is psychologically important to human beings. We need to feel that there is someone we can push around, someone weaker and more vulnerable than ourselves. Slavery once served this psychological need, but we still have animals to coerce and abuse. You speak of the "insipidity" of evil, but there is nothing insipid about our attraction to it. We like the way evil tastes.