Obviously I cannot cite a reference. This is an opinion. I take it you think less than half of the sum total of what has been discovered or learned was learned in the past 400 years? Your priors suggest you assume linear advance in thinking, but hominid cranial enlargement began only 1-2 million years ago. So you must also expect, as a prior, that the difference between humans and chimps is 1/90th - 1/45th of the difference between chimps and dogs. In that case, why exclude chimps from our society?
The maximum travel speed of humans today have travelled is about 7 miles per second. Assuming a travel rate of 0 miles per second 4 billion years ago, we do not conclude that bacteria were able to propel themselves 3.5 miles per second 2 billion years ago.
I don't really think there's been a change in humans. I think there are new tools available that help us think better, much like the new machines available that let us move fast.
You don't believe that homind cranial enlargement is responsible for more than half of the difference between modern humans and dogs, so why does it matter when it happened?
Suppose that dogs are 50-100 times further away from humans than chimps are. Further suppose that bacteria are more than 100 times further away from humans than dogs are. Why is one of those a reason to include chimps, and the other not a reason to include dogs. (Rocks are more than 100 times as different from humans than fungi are, right?) Rather than use relative closeness, I'm going ...
When someone complains that utilitarianism1 leads to the dust speck paradox or the trolley-car problem, I tell them that's a feature, not a bug. I'm not ready to say that respecting the utility monster is also a feature of utilitarianism, but it is what most people everywhere have always done. A model that doesn't allow for utility monsters can't model human behavior, and certainly shouldn't provoke indignant responses from philosophers who keep right on respecting their own utility monsters.
The utility monster is a creature that is somehow more capable of experiencing pleasure (or positive utility) than all others combined. Most people consider sacrificing everyone else's small utilities for the benefits of this monster to be repugnant.
Let's suppose the utility monster is a utility monster because it has a more highly-developed brain capable of making finer discriminations, higher-level abstractions, and more associations than all the lesser minds around it. Does that make it less repugnant? (If so, I lose you here. I invite you to post a comment explaining why utility-monster-by-smartness is an exception.) Suppose we have one utility monster and one million others. Everything we do, we do for the one utility monster. Repugnant?
Multiply by nine billion. We now have nine billion utility monsters and 9x1015 others. Still repugnant?
Yet these same enlightened, democratic societies whose philosophers decry the utility monster give approximately zero weight to the well-being of non-humans. We might try not to drive a species extinct, but when contemplating a new hydroelectric dam, nobody adds up the disutility to all the squirrels in the valley to be flooded.
If you believe the utility monster is a problem with utilitarianism, how do you take into account the well-being of squirrels? How about ants? Worms? Bacteria? You've gone to 1015 others just with ants.2 Maybe 1020 with nematodes.
"But humans are different!" our anti-utilitarian complains. "They're so much more intelligent and emotionally complex than nematodes that it would be repugnant to wipe out all humans to save any number of nematodes."
Well, that's what a real utility monster looks like.
The same people who believe this then turn around and say there's a problem with utilitarianism because (when unpacked into a plausible real-life example) it might kill all the nematodes to save one human. Given their beliefs, they should complain about the opposite "problem": For a sufficient number of nematodes, an instantiation of utilitarianism might say not to kill all the nematodes to save one human.
1. I use the term in a very general way, meaning any action selection system that uses a utility function—which in practice means any rational, deterministic action selection system in which action preferences are well-ordered.
2. This recent attempt to estimate the number of different living beings of different kinds gives some numbers. The web has many pages claiming there are 1015 ants, but I haven't found a citation of any original source.