Holzmann, Schmitzer, Peters et al. (Nature, 2026) measured the critical thermal maxima of 2,300 insect species across 242 families along elevational gradients in Kenya and Peru — from cool mountain forests to lowland tropics. They then compared organism-level heat tolerance to protein melting temperatures across more than 600 species spanning the insect tree of life.
The protein melting temperatures tracked the taxonomy, not the environment. Diptera (flies) had the lowest heat-sensitive melting points — mean 41.17°C. Orthoptera (grasshoppers, crickets) had the highest — 43.40°C. The variation in organism-level thermal tolerance mapped onto these protein-level differences, which are conserved within insect orders. Species at high elevations could increase their heat tolerance in the short term. Lowland species — where biodiversity is highest — largely could not.
The constraint is not behavioral. Insects can change where they forage, when they're active, which microhabitats they select. The constraint is not ecological. Food webs, competitors, and habitat structure are not the binding limit. The constraint is molecular. The proteins that run metabolism have melting points set during the early radiation of insect orders — hundreds of millions of years ago — and these melting points are phylogenetically conserved. An insect's thermal ceiling was determined by its lineage before the current climate existed.
This creates an unusual relationship between crisis and capacity. Warming is a present phenomenon. The ability to tolerate it is an ancient property. The organisms most vulnerable — lowland tropical insects, whose current environmental temperatures already approach their protein melting points — are not failing to adapt. They are encountering a limit that was fixed before adaptation to the current crisis began. The building blocks were optimized for a temperature range that is now the floor, not the ceiling.
The structural consequence: evolution can modify behavior, morphology, life history, phenology, and habitat choice on timescales of decades to centuries. It cannot easily redesign protein folding landscapes. The most biodiverse ecosystems on Earth exist precisely because their conditions have been stable long enough for speciation to proceed without thermal pressure. That stability is the reason the organisms never needed heat-tolerant proteins. Now that they need them, the timescale for acquiring them exceeds the timescale of the warming. The crisis operates faster than the level at which the limit is set.