Although it's hardly the most obvious example of bad science in the movie, one point sticks out in my mind. Toward the end, Ira and his "team" realize that selenium might be an effective weapon against the creatures, at which point Ira starts talking about how many gallons they'll need to defeat them. The problem? Selenium is a solid at room temperature, and he hadn't been made aware that selenium is the active ingredient in Head & Shoulders shampoo at that point. Shouldn't a scientist of his caliber know better?
Gallons are just as valid a measure of volume as anything (I've heard people talk about gallons of gasses, for instance), and easier/faster to say than cubic feet/meters, etc. Plus, people have a better idea of what a gallon is than other measures of volume.
Plus one assumes that he figured he would have to make some sort of solution out of it anyway, as the other option would be to make a large club out of the stuff and start clonking the monsters on the head.
Now that Pacific Rim has come out the club idea doesn't sound too rediculous...
Humanity losing sapience After the End makes no sense whatsoever. The idea that high intelligence wouldn't be useful after a catastrophic collapse of civilization is ridiculous. After a catastrophic collapse of civilization the people who would logically be best able to survive would be the ones who could do things like make fishing equipment and traps for animals, warm clothing, sophisticated weapons like atlatls and bows and arrows, stone and bone tools ... all of which require intelligence! Yes, the human brain uses up a lot of calories, but it's also our greatest survival asset. In harsher parts of the world people simply wouldn't be able to survive period if it weren't for the fact we're sophisticated tool users (good luck surviving above the Arctic Circle if you're too stupid to know how to make warm clothing). Actually if you think about the implication is sort of insulting to all our ancestors who lived before the development of agriculture, not to mention groups like the San and Australian Aboriginals (who, by the way, provide a pretty dramatic demonstration that that sort of lifestyle does not logically lead to a loss of intelligence). The idea that language would be lost is similarly absurd, as the ability to communicate complex concepts has plenty of survival utility, not to mention that language is pretty fundamental to human social existence. What exactly caused people to give up talking? A character in one of the short stories speculates that they may have had reasons for silence (though it's not elaborated on what these would be), but if that's true you would expect people to develop some alternate means of communication. It's especially ridiculous as for the scenario to play out as it does this would probably have to happen almost everywhere. If even a small minority of humanity retained high intelligence they would probably quickly begin to spread out and outcompete the morons. The whole thing comes off as nonsensical gratuitous Grimdark.
Baxter says somewhere along the course of the book that the ape-like posthumans evolved from feral children, so they wouldn't have a heck of a lot of social knowledge anyways. After that, it was just a gradual slide downwards again. Besides, it's not always energy-efficient to have a big brain.
Evolution doesn't work according to plans or logic, it is possible that humanity may have devolved, predicting the future is notoriously difficult.
The fact that you would even use a word like "devolved" shows you have no understanding of evolution whatsoever.
The point of that part of the Stephen Baxter novel is that there are no free lunches in nature. Everything has a cost. Sure, intelligence and language will always have survival utility, but they also come at a steep cost. And that cost is in energy. The human brain uses up over 20% of all the calories we consume. The size of the brain results in our children being born more helpless and vulnerable than those of any other animal. The time required to fully develop our intelligence means that we have the longest relative childhood of all animals - a period of time during which we are vulnerable to predation, before we are mature enough to reproduce. Many of the most potent aspects of social intelligence, such as language, require a minimum population density to perpetuate, and are self-reinforcing - the denser the population, the greater the advantage these traits provide. But the smaller the population, the less advantageous they become, and below a critical threshold of population density, maintaining the trait in its full complexity becomes a net resouce loss. (For example, the more people you have to interact with, the more advantageous complex language skills are. The fewer people you have the interact with, the less sophisticated your language needs to be. And if you end up living most of your life entirely alone, then complex language skills become almost useless.) Humans evolved greater intelligence in an environment where the huge advantages of intelligence outweighed the huge cost. But such environmental conditions are not guaranteed to remain so in perpetuity in the future. In an environment where the cost of maintaining high intelligence is greater than the benefit provided by that intelligence, natural selection reduces the average intelligence of the population over the generations. This has been observed to happen in many instances in many different lineages of animals. There is no reason to assume that humanity is guaranteed to be immune to this possibility. The loss of language in fact is explicitly explained in the novel. The genetic trait is a capacity for language, not language itself. Humans are born with the innate ability to learn a language, but without a social environment that provides an opportunity to learn a language, they won't. And there is a critical window in development after which they can't learn language anymore. This has been demonstrated with actual known cases of feral children. In the post-modern apocalypse envisioned in the novel, the isolated surviving populations of normal humans all succumb to starvation. The surviving lineage comes from abandoned feral children who grew to adulthood without any social contact, thus missing their window for learning complex language skills. The social continuity for the transmission of language was broken. Subsequent generations could not acquire language because their parents never acquired it. As a result the genetic capability for language, though still present, is no longer used, and therefore no longer provides any survival benefit. After several more generations of this situation, the genetic capability itself is lost and no longer penalized by natural selection. Indeed, it is favored, since precious resources are no longer spent in building and maintaining the expensive neural structures.
The original comment above with respect to the San and Australian aborigines is true in that the hunter-gatherer lifestyle is one where intelligence is really important and therefore promoted by selection, and humanity's long ancestry as hunter-gatherers is a big part of why we evolved and maintained high intelligence. (Indeed, individual intelligence is arguably more important for hunter gatherers than it is for agriculturalists of modern city dwellers, where sociability is of greater value, and an argument can easily be made that the average hunter gatherer is in fact more intelligent as an individual than your average modern city dweller). However, in the context of the Baxter novel, this is not a valid criticism for the presented loss of sapience. It is specifically shown that in the post-apocalyptic environment after the collapse of modern civilization, the hunter-gatherer lifestyle was no longer viable. There were no longer enough animals left to hunt, no longer enough edible plants left to gather. (A similar situation occurred in real life after the mass extinctions at the end of the last ice age, and it is thought that this environmental pressure promoted the rise of agriculture as a lifestyle to displace hunter gathering, which had become less viable.) The novel describes scattered populations of human survivors trying to survive in a variety of ways, but in the end, all the ones that tried lifestyles that relied on intelligence, such as hunter-gathering, all failed, and they took the last remaining large edible animals with them into extinction. (The time period over which this occurred was also quite drawn out - it could easily have been several thousand years, which arguable doesn't really constitute that great a failure, when you think about it). The ecosystem had become so depleted that essentially all the large, energy-hungry animals died out. The only way that some human populations survived was by becoming more energy-efficient, shrinking down their energy-hungry large bodies and brains, and giving up the high intelligence that came with it (and they really didn't lose that much intelligence - they still remained the smartest group of animals on the planet). The book also states the random chance played a role in this as well. The incredulity of the OP on this point in fact speaks to one of the overall themes of the novel, which is that while we humans might like to think that the evolution and future maintenance of our high intelligence was someone special, pre-ordained, or inevitable, in reality it is not. Our existence in our current state is a happy accident (which we should recognize and cherish and protect), which can very easily be lost or reversed by another series of less happy accidents.