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You may not know that our ancestors hibernated during the winter months. Why did we evolve out of this biologically prolific survival mechanism?<\/p>\n
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Deep inside a limestone cave in northern Spain, researchers made a discovery that upended what we thought we knew about our own ancestors. The bones \u2014 430,000 years old, pulled from a shaft called Sima de los Huesos, or \u201cthe pit of bones\u201d \u2014 were riddled with a very specific kind of damage. Tunneling through the trabecular structure. Lesions consistent with seasonal metabolic shutdown. Bone signatures, in other words, that look strikingly like those left on animals that hibernate.<\/p>\n
The implication was difficult to sit with. Early hominins, our own relatives, may have once curled up and gone dormant for the winter. A 2020 paper<\/a> published in L\u2019Anthropologie by paleoanthropologists Antonis Bartsiokas and Juan-Luis Arsuaga made the case: lesions on the fossils, including subperiosteal resorption and rickets-like deformities concentrated in adolescents, mirrored the bone pathology of hibernating cave bears found in the same deposit. These were the physiological signatures of a body that cycled through months of metabolic near-silence.<\/p>\n And yet, you know today that no matter how harsh winter is, you won\u2019t be shutting down for the season. So, somewhere between then and now, evolution ruled hibernation out for us. To understand why, we need to understand the process of hibernation first.<\/p>\n What\u2019s Happening In Bodies That Hibernate?<\/p>\n The science of hibernation is more sophisticated than its reputation. It is not sleep, not even close. True hibernators, such as the Arctic ground squirrel, little brown bats and thirteen-lined ground squirrels, reduce their basal metabolic rate to as little as 1 to 4% of normal waking levels. <\/p>\n Body temperature in the Arctic ground squirrel<\/a> (Urocitellus parryii) can plunge to -2.9\u00b0C (26.78\u00b0F), and a single torpor bout (hibernation cycle) can last more than three weeks. The animal is, by most physiological definitions, barely alive.<\/p>\n What keeps it from dying is their molecular machinery of extraordinary complexity. The cascade of changes required to enter a state of hibernation, depending on the animal, can include anything from cold-inducible RNA-binding proteins, seasonal shifts in gene expression across 14,000+ genes, alterations in membrane fluidity and cell-cycle arrest. <\/p>\n A 2020 transcriptome study<\/a> of the meadow jumping mouse found that organs modify their own gene expression profiles independently during torpor \u2014 the kidney adjusting filtration, the brain dialing back synaptic activity \u2014 in a coordinated shutdown that nothing in the human genome is equipped to replicate.<\/p>\n If So Many Mammals Hibernate, Why Don\u2019t Humans?<\/p>\n The discovery that echidnas hibernate \u2014 monotremes<\/a> that diverged from other mammals more than 120 million years ago \u2014 pushed researchers to reconsider the timeline entirely. If the earliest diverging lineage of mammals hibernates, it raises the possibility that the common ancestor of all mammals was itself a hibernator.<\/p>\n Today, hibernation appears in at least seven distinct mammalian orders, from placental mammals to marsupials to monotremes. A 2025 genomic study<\/a> published in Science used comparative analysis across hibernating and non-hibernating mammals to identify conserved regulatory elements in the hypothalamus, revealing convergent genetic signatures that point to similar molecular mechanisms underlying hibernation. <\/p>\n Closer to home on the evolutionary tree, something remarkable was documented in Madagascar: lemurs hibernate. Specifically, several species of the family Cheirogaleidae enter seasonal torpor during the dry winter months. This matters because lemurs are primates \u2014 our relatives. Research<\/a> published in Genomics, Proteomics & Bioinformatics identifies the gray mouse lemur as a key primate model for studying metabolic rate depression, highlighting its unique ability among primates to enter torpor and suppress energy-intensive cellular processes. <\/p>\n The genetic scaffolding for torpor, it turns out, exists somewhere in our primate lineage. We just don\u2019t express it. The question isn\u2019t why animals hibernate. It\u2019s why we stopped. The answer has three layers, and they compound each other with almost elegant finality.<\/p>\n The Three Main Reasons Why Humans Don\u2019t Hibernate<\/p>\n The first is geography. Homo sapiens and our immediate ancestors evolved in equatorial Africa, where temperatures are relatively stable year-round and food, while sometimes scarce, is available across seasons. There was no winter famine pressing our lineage toward dormancy. Hibernation is metabolically expensive to maintain as a genetic toolkit, and without selection pressure to keep it, those molecular systems degraded over millions of years.<\/p>\n