On the last day, the aliens took the iron. They settled in a polar orbit, and slowly stripped the world of its mettle.
The satellites were the first to go, delicately engineered, in a hostile environment, and closest to the epicenter - a spreading zone of ignorance which satellites entered and then did not return from. The operators were mystified, then panicked; some turned their satellites around to scan the galactic skies, as did those “intelligence assets” which had not used up too much fuel in station-keeping in their endless gaze of the earth. The slowly-expanding silver lozenges were spotted quickly (to little avail but later fostering a thousand debates among academics in the ruins).
The extraction was peculiarly limited. That any human or living creature survived indicated that the removal of iron was limited to non-organic creatures: had iron been removed from hemoglobin and the oxygen content of blood reduced to zero, every mammal would have asphyxiated in seconds. Miners reported that no iron seams were accessible in existing iron mines; yet the iron core of the Earth could not have been removed as the crust had not collapsed, the Earth’s orbit seemed to be unchanged, and the magnetic field retained its usual strength. Naked-eye observations ruled out the possibility that the core had been removed from Venus or Mars, and Mars retained its red color, but the margins of error were too large to rule out orbital aberrations due to hypothetical removal of gigatons of iron from the subsurface of either.
The aliens’ motivations remained unclear. Unknown physics was involved, as no mechanism could even be hypothesized. A flotilla of vessels spoke of a rich interstellar civilization, which was impossible without millennia of peace and cooperation, but their actions supported the “intelligence implies belligerence” thesis: humanity had stored up so much data in digital libraries (often routed through communication satellites) that it was impossible that the aliens could not communicate with us; and yet they did not, communicating neither demands nor warning nor explanation. Perhaps they were doing precisely what it looked like - mining for allotropic iron? But why mine in such a destructive method, and why choose the Earth rather than any of the other rocky iron-rich planets, or better yet, the asteroids? (One astronomer noted that with the elimination of certain astronomical programs, it was possible that the asteroids had been mined out previously but we had not seen it.)
Iron in any of its derivatives such as steel, humanity was forcibly reminded, was crucial to almost all enterprises. A proud column of steel, deprived of iron, is but a mist of chromium and other adulterants which supports nothing. As the absence crept along the globe, it was followed by darkness and large red pinpricks of light.
In the former Tokyo, the most elderly looked at the rubble piles and fires with equanimity: the world had already ended for them once. In China, the national dream of the restored Middle Kingdom collapsed with Beijing and Shanghai. Ankara experienced the earthquake long prophesied by unbeliever scientists; as everywhere, they cursed God in their own way. In New York, fear of terrorism gripped the subconscious, and tens of thousands died as the bridges disappeared, and more with the towers; the waters and rubble served well to bury their millions. In San Francisco, the buildings - reinforced against earthquake and kept close to the ground by rent-seeking regulation and NIMBYism - killed fewer hundreds of thousands than one would have expected; the irony was not noted.
Crops withered and rotted in the fields of developed nations as the built-up capital of industrialization was set back at a stroke to the 1500s, with all mechanized equipment, down to the simplest iron plow, now useless. The 1% of the population could no longer feed the 99%; the masses fled the dying cities to the fields. One way or another, the problem would be solved. People in the poorest countries began killing each other with sticks and stones; the situation is indistinguishable from before.
Microchips, it turned out, were often exempt from the effect; some manufacturers used iron in key roles, but others had moved away from metals as much as possible. Most of the unaffected computing infrastructure was then destroyed by collapsing buildings, firestorms, or simply disabled by lack of power from generators that became little more than loops of copper wire sitting in puddles of gasoline. But many places continued to work. The world split between dark and light.
Remaining centers of industrialization reached a consensus that civilization could (had to) be rebuilt on the remaining non-ferrous metals like aluminum. Bauxite purification requires megawatts of electricity and ore from Africa. It was realized that smelting plants in Iceland depended on dams broken by the event, and that with the sinking of the global fleet, there was no way to transport the ore there or to other smelting plants in countries like Australia. Australia was pleased to boast bauxite deposits and endless coal for smelting; unfortunately, no substantial shipping could benefit from this - the remaining vessels were backlogged for years on humanitarian grounds. Hobbyists were consulted on the construction of wooden sailing ships.
Menstrual blood contains ~0.7mg of iron per day. A human body contains 2-4 grams of iron.
Iron mining resumed.
The existing crops were harvested without more issues than might be expected at the end of the world. The specter of iron chlorosis stalked the land. The next crop was small and sick, with pale yellow leaves. Yields were a tenth of projected. Food insecurity, already high at the time of the incident, became higher. Entire countries were denuded of any chlorophyllic or mammalian life. Rich people lamented the extinction of tomatoes, apricots, olives, pears, and nuts.
The crop yields were the first blow. With foodstuffs deficient in iron, people began to lose their own iron. The next generation of children grew up stunted, apathetic, and less intelligent than their parents. Iron supplements and iodized salt was a luxury of the past. Females were overrepresented among the deficient and the goiterous cretins, and menstruating women were most hard struck by anemia. Sexism swelled based on simple observation. With biological iron a fixed resource and further losses inevitable, the long-term trends pointed down.
Old NASA textbooks were consulted. The loss of biologic iron would be stemmed by use of self-contained agricultural ecosystems. Humanity did not go to space; space came to humanity.
As the catastrophe continued to ramify, additional speculation was added to the journals: perhaps the removal of iron was simply a way of neutering a planet permanently. Without iron ore and with most hydrocarbon reserves depleted, it would be impossible for a space-faring civilization to ever develop again.
This explanation seemed as plausible as any.