Here's another interesting set of quotes; are we even correct in assuming the most recent percent of DNA matters much? After all, chimps outperform humans in some areas like monkey ladder. From "If a Lion Could Talk":

"Giving a blind person a written IQ test is obviously not a very mean meaningful evaluation of his mental abilities. Yet that is exactly what many cross-species intelligence tests have done. Monkeys, for example, were found not only to learn visual discrimination tasks but to improve over a series of such tasks -- they formed a learning set, a general concept of the problem that betokened a higher cognitive process than a simple association. Rats given the same tasks showed difficulty in mastering the problems and no ability to form a learning set. The obvious conclusion was that monkeys are smarter than rats, a conclusion that was comfortably accepted, as it fit well with our preexisting prejudices about the distribution of general intelligence in nature. But when the rat experiments were repeated, only this time the rats were given the task of discriminating different smells, they learned quickly and showed rapid improvement on subsequent problems, just as the monkeys did.

The problem of motivation is another major confounding variable. Sometimes we may think we are testing an animal's brain when we are only testing its stomach. For example, in a series of studies goldfish never learned to improve their performance when challenged with "reversal" tasks. These are experiments in which an animal is trained to pick one of two alternative stimuli (a black panel versus a white panel, say) in order to obtain a food reward; the correct answer is then switched and the subject has to relearn which one to pick. Rats quickly learned to switch their response when the previously rewarded answer no longer worked. Fish didn't. This certainly fit comfortably with everyone's sense that fish are dumber than rats. But when the experiment was repeated with a different food reward (a paste squirted into the tank right where the fish made its correct choice, as opposed to pellets dropped into the back of the tank), lo and behold the goldfish suddenly did start improving on reversal tasks. Other seemingly fundamental learning differences between fish and rodents likewise vanished when the experiments were redesigned to take into account differences in motivation.

Equalizing motivation is an almost insoluble problem for designers of experiments. Are three goldfish pellets the equivalent of one banana or fifteen bird seeds? How could we even know? We would somehow have to enter into the internal being of different animals to know for sure, and if we could do that we would not need to be devising roundabout experiments to probe their mental processes in the first place.

When we do control for all of the confounding variables that we possibly can, the striking thing about the "pure" cognitive differences that remain is how the similarities in performance between different animals given similar problems vastly outweigh the differences. To be sure, there seems to be little doubt that chimpanzees can learn new associations with a single reinforced trial, and that that is genuinely faster than other mammals or pigeons do it. Monkeys and apes also learn lists faster than pigeons do. Apes and monkeys seem to have a faster and more accurate grasp of numerosity judgments than birds do. The ability to manipulate spatial information appears to be greater in apes than in monkeys.

But again and again experiments have shown that many abilities thought the sole province of "higher" primates can be taught, with patience, to pigeons or other animals. Supposedly superior rhesus monkeys did better than the less advanced cebus monkeys in a visual learning-set problem using colored objects. Then it turned out that the cebus monkeys did better than the rhesus monkeys when gray objects were used. Rats were believed to have superior abilities to pigeons in remembering locations in a radial maze. But after relatively small changes in the procedure and the apparatus, pigeons did just as well.

If such experiments had shown, say, that monkeys can learn lists of forty-five items but pigeons can only learn two, we would probably be convinced that there are some absolute differences in mental machinery between the two species. But the absolute differences are far narrower. Pigeons appear to differ from baboons and people in the way they go about solving problems that involve matching up two images that have been rotated one from the other, but they still get the right answers. They essentially do just as well as monkeys in categorizing slides of birds or fish or other things. Euan Macphail's review of the literature led him to conclude that when it comes to the things that can be honestly called general intelligence, no convincing differences, either qualitative or quantitative, have yet been demonstrated between vertebrate species. While few cognitive researchers would go quite so far -- and in deed we will encounter a number of examples of differences in mental abilities between species that are hard to explain as anything but a fundamental difference in cognitive function -- it is striking how small those differences are, far smaller than "common sense" generally has it. Macphail has suggested that the "no-difference" stance should be taken as a "null hypothesis" in all studies of comparative intelligence; that is, it is an alternative that always has to be considered and ought to be assumed to be the case unless proven otherwise."

EDIT: I've added this and some other points to my Evolutionary drug heuristics article.