The human brain uses up 20% of our energy. There may be easy ways to increase intelligence, that evolution avoided because it costs more energy. But in the modern world constraints like that don't matter.

...ways in which evolution produced local maxima...

There is a compelling argument against any simple way to enhance people. It's that if a simple change even the addition of a single molecule could make people significantly better, evolution would have found that change already. With nature we are stuck with much more incremental change. So I agree that science may eventually find some nice enhancements but they will require more than a single molecule or gene change.

This isn't exactly true, since the human meaning of "enhance" is quite different from the evolutionary meaning. For example, iodine (an element, not even as big as a molecule) supplementation at the fetus and early childhood stages can, among other benefits, result in as much as a 13 point IQ increase from a previously deficient region's average. This is a huge enhancement by human standards. Why hasn't evolution found a way to have our bodies produce iodine automatically (perhaps by producing iodomethane like some algae?) at that stage of life, or maybe only by the mother when the child is in the womb or perhaps produced and distributed only with breast milk? Then even if you lack natural sources of iodine you'll still benefit. It should be easy, it's just a single element!

Well even for "simple" things like the addition of a single new element, or a single new molecule, or a single new protein, the details are complicated, and any mutation that depends on a prior mutation to be beneficial has to wait until the prior mutation is close to fixation and is common among the species. Evolution is slow and only cares about reproductive fitness, and can easily get trapped in local maxima and a species can even evolve to extinction. Because of that, it can miss obvious (to human intelligence) low hanging fruit. So I don't think that "evolution would have found it already" argument is very compelling. Evolution is dumb, it's amazing it works at all.

>> For example, iodine (an element, not even as big as a molecule) supplementation at the fetus and early childhood stages can, among other benefits, result in as much as a 13 point IQ increase from a previously deficient region's average. This is a huge enhancement by human standards. Why hasn't evolution found a way to have our bodies produce iodine automatically (perhaps by producing iodomethane like some algae?) at that stage of life, or maybe only by the mother when the child is in the womb or perhaps produced and distributed only with breast milk? Then even if you lack natural sources of iodine you'll still benefit. It should be easy, it's just a single element!

FYI Iodine is concentrated by the breasts so as to provide it to babies via milk for exactly this reason. Iodine is critical to a lot of things. What you overlook is that as an element (atom) there is no way for the body to "make" iodine. The only way to get it is through what you eat and drink. In my opinion, the thyroid hormones are mostly a way for the body to store iodine - they are numbered T1, T2, T3, and T4 based on the number of iodine atoms in the molecule. The thyroid gland also concentrates iodine for use in making these hormones, but ultimately the only way to get iodine is to consume it - which I highly recommend.