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Reference topic 1: The 200-Million-Year Warm-Up

How mammal evolution still shapes the human body

ProbableEvidence grade: Probable. Good observational evidence with a plausible mechanism, or small or few trials.68 sources0 graded recommendationsTopic hub: Sleep and light

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The study began with an extracted tooth. Pam Gill, of the University of Bristol, recalled that a colleague had a tooth removed and wanted it X-rayed, "because it can tell all sorts of things about your life history. That got me wondering whether we could do the same to learn more about ancient mammals."1

Her team turned to Morganucodon and Kuehneotherium, shrew-sized insect-eaters near the base of the mammal family tree.2 Nearly 200 million years ago, in what is now South Wales, they fell into caves and holes in the rock. Their teeth preserve the cementum, the tissue that anchors a tooth's roots and, like a tree, lays down a ring each year. When Ian Corfe of the University of Helsinki scanned one with synchrotron X-rays, the intense beam of a particle accelerator, "the image from the scan was so clear the rings could literally be counted."1

Mismatch card

The system

The mammalian operating system: warm blood, night-built senses, milk, late-added color vision.

What it expects

High energy costs paid for by effortful foraging; real darkness at night; changing temperatures; food judged by color and smell.

What it gets

Abundant calories, climate-controlled rooms, bright evenings, engineered food colors.

What happens in the gap

Sleep and light (Topic 23), temperature (Topic 25), food reward (Topic 11); even the body's thermostat is drifting, for unknown reasons.

The handrail

Dim the evenings, and let your body meet some everyday temperature variation within the safety limits in Topic 25; Topic 41 gives doses. (●●○ for dim evenings; ●○○ for temperature)

Strength of evidence

●●○ Several independent lines of evidence support the deep-time outline; its dates and causes are contested.

What this topic covers

  • ·A Furnace That Must Be Fed
  • ·Everything Changes Together
  • ·Life After Dark
  • ·The Morning After
  • ·Into the Trees
  • ·Seeing Red
  • ·The Thermostat and the Calendar

The practical version is in the book: Chapter 2, What Your Body Expects, and How to Trust This Book.

The skeptic's corner

The case against this topic, from the research itself.

Every date in this chapter is an inference, and the methods disagree. Oxygen isotopes in fossil bones and teeth point to raised metabolism in the middle to late Permian.13 Ridges in fossil noses once suggested a Late Permian start too; they probably anchored turbinates, thin scrolls of bone that cut water loss from breathing.12 But turbinate size in living mammals does not track metabolic rate or body temperature,59 and five scanned Triassic skulls of mammal relatives show no bony turbinates.60 The inner-ear method rests on fluid viscosity measured in only eight species.4 Critics of the tooth-ring argument note that birds and bats live long despite high metabolic rates;61 the tooth team replied that fliers should be analyzed separately.62 A minority argues that warm blood is ancient, shared by the ancestors of mammals, birds and crocodilians.9 The fairest reading is that warm blood was assembled in stages, and "when" depends on which part you measure.

The nocturnal bottleneck risks becoming a just-so story, a tidy tale that is hard to test, as Walls himself shows. While "the twenty-foot tyrannosauri were mangling the ninety-foot diplodoci," he wrote, "the first of the mammals were furtively sneaking about looking for dinosaur eggs to suck."23 (The two dinosaurs never met: they lived tens of millions of years apart.) The nocturnal phase itself rests on several independent lines of evidence, though reading fur color from fossil pigment calls for caution.63 Its supposed cause, hiding from dinosaurs, is less secure. The late arrival of daytime living fits it,29 but night activity in the mammal line goes back about 300 million years,64 and the first dinosaurs and first mammals both arose in the Late Triassic.31 The dinosaurs' own rise owed more to historical accident than to superiority.65

Color vision carries the same risk. Fruit, young leaves and reading the flush of bare faces all have advocates,495166 and two-color vision has real advantages. In one small study, seven red-green color-blind people beat people with normal vision at spotting targets hidden by red-green camouflage.67 In another, marmosets with two-color vision foraged better in shade than those with three.68

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Sources cited above

  1. 1.University of Bristol (2020). Ancient tiny teeth reveal first mammals lived more like reptiles. https://www.bristol.ac.uk/news/2020/october/ancient-tiny-teeth.html
  2. 2.Newham, E., Gill, P.G., Brewer, P., Benton, M.J., et al. (2020). "Reptile-like physiology in Early Jurassic stem-mammals." Nature Communications, 11(1), 5121. https://doi.org/10.1038/s41467-020-18898-4
  3. 13.Rey, K., Amiot, R., Fourel, F., Abdala, F., et al. (2017). "Oxygen isotopes suggest elevated thermometabolism within multiple Permo-Triassic therapsid clades." eLife, 6, e28589. https://doi.org/10.7554/eLife.28589
  4. 12.Hillenius, W.J. (1994). "Turbinates in therapsids: evidence for Late Permian origins of mammalian endothermy." Evolution, 48(2), 207-229. https://doi.org/10.1111/j.1558-5646.1994.tb01308.x
  5. 59.Martinez, Q., Okrouhlík, J., Šumbera, R., Wright, M., et al. (2023). "Mammalian maxilloturbinal evolution does not reflect thermal biology." Nature Communications, 14(1), 4425. https://doi.org/10.1038/s41467-023-39994-1
  6. 60.Fonseca, P.H.M., Martinelli, A.G., Gill, P.G., Rayfield, E.J., et al. (2024). "New evidence from high-resolution computed microtomography of Triassic stem-mammal skulls from South America enhances discussions on turbinates before the origin of Mammaliaformes." Scientific Reports, 14(1), 13817. https://doi.org/10.1038/s41598-024-64434-5
  7. 4.Araújo, R., David, R., Benoit, J., Lungmus, J.K., et al. (2022). "Inner ear biomechanics reveals a Late Triassic origin for mammalian endothermy." Nature, 607(7920), 726-731. https://doi.org/10.1038/s41586-022-04963-z (341 living and extinct species measured, including 56 extinct relatives of mammals; the proposed muscle heat comes through a protein called sarcolipin)
  8. 61.Meiri, S., and Levin, E. (2022). "Revisiting life history and morphological proxies for early mammaliaform metabolic rates." Nature Communications, 13(1), 5562. https://doi.org/10.1038/s41467-022-32715-0
  9. 62.Newham, E., Gill, P.G., Benton, M.J., Brewer, P., et al. (2022). "Reply to: Revisiting life history and morphological proxies for early mammaliaform metabolic rates." Nature Communications, 13(1), 5564. https://doi.org/10.1038/s41467-022-32716-z
  10. 9.Grigg, G., Nowack, J., Bicudo, J.E.P.W., Bal, N.C., et al. (2022). "Whole-body endothermy: ancient, homologous and widespread among the ancestors of mammals, birds and crocodylians." Biological Reviews, 97(2), 766-801. https://doi.org/10.1111/brv.12822
  11. 23.Walls, G.L. (1942). The Vertebrate Eye and Its Adaptive Radiation. Cranbrook Institute of Science Bulletin No. 19. Cranbrook Institute of Science, Bloomfield Hills, Michigan. Quotations from pp. 138 and 283-284, checked against the 1963 Hafner reprint. https://archive.org/details/vertebrateeyeits00wall
  12. 63.Li, X., Bi, S., Clarke, J.A., Li, Z., et al. (2025). "High melanosome diversity exhibits weak correlation with color and environmental variables in the early evolution of therian mammals." Science Advances, 11(43), eadw8707. https://doi.org/10.1126/sciadv.adw8707
  13. 29.Maor, R., Dayan, T., Ferguson-Gow, H., and Jones, K.E. (2017). "Temporal niche expansion in mammals from a nocturnal ancestor after dinosaur extinction." Nature Ecology & Evolution, 1(12), 1889-1895. https://doi.org/10.1038/s41559-017-0366-5
  14. 64.Angielczyk, K.D., and Schmitz, L. (2014). "Nocturnality in synapsids predates the origin of mammals by over 100 million years." Proceedings of the Royal Society B, 281(1793), 20141642. https://doi.org/10.1098/rspb.2014.1642
  15. 31.Han, G., Mallon, J.C., Lussier, A.J., Wu, X.-C., et al. (2023). "An extraordinary fossil captures the struggle for existence during the Mesozoic." Scientific Reports, 13(1), 11221. https://doi.org/10.1038/s41598-023-37545-8 (fossil found in Liaoning, China, on 16 May 2012; the describers wrote that it "convincingly demonstrates that mammals could pose a threat even to near fully-grown dinosaurs")
  16. 65.Brusatte, S.L., Benton, M.J., Ruta, M., and Lloyd, G.T. (2008). "Superiority, competition, and opportunism in the evolutionary radiation of dinosaurs." Science, 321(5895), 1485-1488. https://doi.org/10.1126/science.1161833
  17. 49.Regan, B.C., Julliot, C., Simmen, B., Viénot, F., et al. (2001). "Fruits, foliage and the evolution of primate colour vision." Philosophical Transactions of the Royal Society B, 356(1407), 229-283. https://doi.org/10.1098/rstb.2000.0773
  18. 51.Dominy, N.J., and Lucas, P.W. (2001). "Ecological importance of trichromatic vision to primates." Nature, 410(6826), 363-366. https://doi.org/10.1038/35066567
  19. 66.Changizi, M.A., Zhang, Q., and Shimojo, S. (2006). "Bare skin, blood and the evolution of primate colour vision." Biology Letters, 2(2), 217-221. https://doi.org/10.1098/rsbl.2006.0440
  20. 67.Morgan, M.J., Adam, A., and Mollon, J.D. (1992). "Dichromats detect colour-camouflaged objects that are not detected by trichromats." Proceedings of the Royal Society B, 248(1323), 291-295. https://doi.org/10.1098/rspb.1992.0074
  21. 68.Caine, N.G., Osorio, D., and Mundy, N.I. (2010). "A foraging advantage for dichromatic marmosets (Callithrix geoffroyi) at low light intensity." Biology Letters, 6(1), 36-38. https://doi.org/10.1098/rsbl.2009.0591

This topic carries 68 notes in the research manuscript; the 21 above are those behind the material on this page. Full source register.