The Missing Summer Hypothesis: Volcanic Summerless Years, the Vitamin D Panel–Battery, and the Neolithic Selection of Light-Toned Skin

Statement of the theory Light-toned skin in northern human populations was selected in the rare years in which summer failed. Several times per millennium, and in the clearest cases after large volcanic eruptions, a year arrives in which the summer ultraviolet-B needed to recharge the body's stored vitamin D does not arrive. In every ordinary year, dark-toned skin in an unclothed outdoor life banked more vitamin D than the body required, at every latitude humans then occupied, so no advantage for lighter skin existed and none was selected. In a summerless year the store did not fill; the dietary reserve failed at the same moment, because dietary vitamin D is itself made by ultraviolet-B; and the following winter killed through acute immune failure rather than through the bone diseases with which vitamin D deficiency is identified. Settled farmers could not escape such a year as mobile foragers could, so the Neolithic is a necessary condition of the selection. What lighter skin supplied in those years was not speed of synthesis but a lower threshold: the capacity to make vitamin D at irradiance levels at which darker skin makes none. The theory keeps vitamin D as the selected function and replaces the occasion of selection, and in doing so reconciles two positions held to be in conflict — Robins's demonstration that pigmented skin met the vitamin D requirement at high latitude, which describes the typical year, and the vitamin D–folate account of depigmentation, which describes a selection whose occasion it does not identify. Claims Light-toned skin in northern human populations was selected in the rare years in which summer ultraviolet-B failed — several per millennium, volcanic in the clearest cases — and not by the ordinary annual ultraviolet-B regime of high latitudes, which imposed no vitamin D penalty on dark-toned skin. [first stated: v1.0, 2026-09-19] This selection was episodic and catastrophic, concentrated into the single winters that followed failed summers, and was not a gradual response accumulating across generations along a latitudinal ultraviolet gradient. [first stated: v1.0, 2026-09-19] Robins's demonstration that dark-toned skin met the vitamin D requirement at high latitude and the vitamin D–folate account of depigmentation are both sound and describe different years — the typical year and the outlier year — so the conflict between them is apparent only, and neither account needs to be withdrawn once the outlier year is supplied. [first stated: v1.0, 2026-09-19] The measured correlation between skin reflectance and ultraviolet dose rests on satellite irradiance figures averaged across fifteen years, and an averaged series cannot register a selective event confined to outlier years, so that correlation is silent on the occasion of depigmentation rather than evidence that selection was gradual. [first stated: v1.0, 2026-09-19] The lethal consequence of a failed vitamin D year is acute immune failure in the following winter, not the chronic bone diseases — rickets and osteomalacia — with which vitamin D deficiency is identified; bone pathology deforms and pains, but it does not kill at the rate selection requires. [first stated: v1.0, 2026-09-19] A summerless year struck through four coupled failures — loss of ultraviolet-B, loss of the dietary vitamin D that ultraviolet-B makes, crop failure and famine, and epidemic disease amplified by the loss of ultraviolet-B's germicidal and antiparasitic action on surfaces, in air, and on ectoparasites — and these were consequences of one missing variable, not four independent misfortunes. [first stated: v1.0, 2026-09-19] Dietary vitamin D is itself a product of ultraviolet-B, made either in the organism eaten or in what that organism ate, so a year without ultraviolet-B removes the dietary reserve at the same moment it removes cutaneous synthesis; the two supplies fail together, and diet offered no compensation in the years that did the selecting. [first stated: v1.0, 2026-09-19] A summerless year was survivable for mobile foragers, who could move, switch foods, and draw on vitamin D banked in the food chain from earlier seasons, and not for settled farmers holding a single failed store, so the Neolithic is a necessary condition of the selection and depigmentation is Neolithic rather than Paleolithic for that reason. [first stated: v1.0, 2026-09-19] The selectively relevant difference between dark- and light-toned skin is the irradiance threshold below which synthesis fails altogether, not the rate of synthesis above it, since both tones reach the same synthetic ceiling; experiments dosing each tone at the same fraction of its own sunburn threshold conceal this difference, because that fraction delivers more ultraviolet-B to darker skin. [first stated: v1.0, 2026-09-19] The quantity on which selection acted is the size of the vitamin D store held in fat and muscle at the end of summer, not the rate at which skin synthesizes on a given day; the system is a seasonal charge-and-drawdown device — a panel and a battery — and comparisons of skin tones by synthesis rate measure the wrong variable. [first stated: v1.0, 2026-09-19] The body-exposure figure belonging in a whole-body vitamin D calculation is about 51 percent of the ground-level dose — the measured annual average across body sites, on an erythemally weighted proxy, once diffuse sky radiation is counted — and neither the 100 percent assumed in the original calculation nor the 7 percent sometimes substituted for it, that 7 percent being the direct-beam share for a standing body under a tropical noon sun and excluding roughly four-fifths of the annual dose. [first stated: v1.0, 2026-09-19] Applying that 51 percent to the original synthesis estimates leaves deeply pigmented skin producing on the order of 20,000 to 41,000 IU a day under an unclothed outdoor regime, twenty-six to fifty-one times the daily requirement, so the pre-Neolithic vitamin D margin for dark-toned skin at high latitude was large rather than marginal. [first stated: v1.0, 2026-09-19] Meeting a daily requirement of 600 to 800 IU asks the epidermis for roughly 3 to 12 molecules of vitamin D per billion water molecules, so vitamin D sufficiency is a trace-quantity problem, and any argument resting on a substantial synthetic burden on the skin misstates the physiology. [first stated: v1.0, 2026-09-19] The published ratios for how much longer dark-toned skin needs under the same sun, running from about 1.3-fold to sixfold, are in part artifacts of model construction rather than measurements — where a model applies a fixed multiplier to a base exposure time, the ratio is an input and not a result — so no settled figure for the pigment penalty exists. [first stated: v1.0, 2026-09-19] Human skin in spring, summer, and autumn was habitually uncovered until the recent past, and daily body covering is an artifact of cheap textiles, mechanical laundering, and heated interiors rather than an ancient human condition; the pictorial and burial evidence for prehistoric clothing records ritual, status, and winter wear, not ordinary working dress. [first stated: v1.0, 2026-09-19] Three independent pressures kept warm-season clothing off pre-industrial bodies — ectoparasite burden, the labor cost and abrasiveness of hand-made cloth, and a cold tolerance that modern rearing has removed from us — so habitual ancient nakedness follows from costs those people faced and is not an assumption introduced to serve the vitamin D argument. [first stated: v1.0, 2026-09-19] The Neolithic raised the parasite cost of habitual clothing rather than lowering it, because permanent dwellings let the flea complete a life cycle that frequent camp moves had broken and stored garments gave ectoparasites a reservoir, so a year-round covering norm would have been out-competed by the practice of stripping in warm seasons rather than sustained by custom. [first stated: v1.0, 2026-09-19] Hand-sewn seams, thinning, tearing, and wetting give pre-industrial garments a substantially higher ultraviolet-B transmission than modern fabric, and that quantity has never been measured; it is the missing number in every reconstruction of prehistoric ultraviolet exposure, not a term that can be set at zero. [first stated: v1.0, 2026-09-19] The Neolithic eroded the vitamin D margin without breaking it — less bare skin, more hours under roofs, a food shift away from vitamin D-bearing foods — so in typical Neolithic years dark-toned skin at high latitude still banked enough for winter, and the Neolithic transition by itself does not account for depigmentation. [first stated: v1.0, 2026-09-19] The Neolithic cereal shift lowered vitamin D status by a route involving no ultraviolet-B at all: phytate binds dietary calcium, the parathyroid response raises the active hormone, and the raised hormone drives hepatic destruction of the stored 25D, so tissue functions depending on cellular 25D fail while circulating active hormone remains normal and blood calcium is little disturbed. [first stated: v1.0, 2026-09-19] The pre-modern hungry gap ran from February to June, the same months in which the skin had made nothing since autumn and the store stood lowest, and burning body fat in those months released stored vitamin D at the one time of year the skin could replace none; seasonal hunger and seasonal vitamin D failure are one coupled event, not two that happen to coincide. [first stated: v1.0, 2026-09-19] At high latitude in winter the vitamin D store leaks outward through the same skin that cannot charge it, because the precursor absorbs only below 315 nm while vitamin D itself goes on absorbing out to 330 nm and winter light carries the longer photons and not the shorter; winter sun therefore unmakes vitamin D without making any. [first stated: v1.0, 2026-09-19] Neolithic winter vitamin D came from an enumerable set of ultraviolet-B-derived charge routes — the marine and anadrom