Paleohistology (WHY TEETH EXIST) with Yara Haridy

Episode Summary

Executive Summary: This episode explores paleohistology and the origin of vertebrate teeth through Dr. Yara Haridi’s work on ancient tissues. It traces how tooth-like structures likely began as sensory, armored odontodes on early vertebrates, how teeth evolved in the mouth, why mammals mostly stopped replacing teeth, and how a Nature paper revised a key Cambrian fossil from vertebrate to arthropod using synchrotron imaging and comparative anatomy.

Main Topics: Paleohistology and ancient tissues (Priority: 5/5): The episode defines paleohistology as the study of ancient tissues and explains how microscopic structure in fossils reveals growth, age, blood vessels, and preserved tissue organization. Origin of teeth and odontodes (Priority: 5/5): Haridi explains that tooth-like odontodes first evolved outside the mouth as enamel- and dentine-bearing structures on early fish armor, likely before true oral teeth appeared. Teeth as sensory structures (Priority: 5/5): The conversation emphasizes that teeth and odontodes may have originally functioned partly as sensory organs, not just armor, supported by modern examples like sharks, catfish, elephants, and narwhals. The Nature paper and fossil reclassification (Priority: 5/5): Her 2025 Nature paper used synchrotron scans and comparative histology to show that a previously claimed earliest vertebrate fossil was actually an arthropod with sensory sensilla, pushing back the earliest confirmed mineralizing vertebrates but removing a proposed transitional fossil. Why mammals replace teeth differently (Priority: 4/5): The episode contrasts constant tooth replacement in reptiles, sharks, and dinosaurs with mammals’ limited two-set dentition, linking this to precise occlusion and dietary evolution. Tools, fieldwork, and science communication (Priority: 4/5): Haridi discusses fieldwork in Colorado, museum collections, particle accelerators, embryo imaging, and the importance of sharing science through social media and public outreach. Broader implications for fossils and evolution (Priority: 4/5): The discussion shows how tiny fossil fragments can overturn major evolutionary assumptions and how new technologies reopen old questions about skeletons, teeth, and tissue evolution.

Key Arguments: Teeth likely evolved from external odontodes on the dermal exoskeleton rather than appearing first inside the mouth. Odontodes were probably not only protective armor; they may also have been sensory structures that helped early vertebrates detect their environment. Modern evidence from sharks and catfish shows that external odontodes can be innervated, supporting the sensory hypothesis. A famous late Cambrian fragment previously thought to be the earliest vertebrate tooth-bearing fossil is better interpreted as an arthropod with sensory sensilla. Mammals reduced tooth replacement to two generations because stable occlusion and chewing efficiency may have been favored evolutionarily. Fossil teeth preserve chemistry that can reveal diet, while shape alone also provides dietary clues. Science progresses nonlinearly: reanalyzing old fossils with new tools can overturn long-held claims and open new research directions.

Data Points: Estimated age of earliest odontode-bearing vertebrates: ~455 million years ago - Haridi places the earliest confirmed mineralized vertebrate tissues in the middle Ordovician. Age of candidate earliest fossil from the late Cambrian: ~475 million years ago - The paper examined a fragmentary fossil once thought to predate known vertebrates. Duration of the Devonian: ~60 million years - Presented as the era in which many major vertebrate and plant changes occurred. Age of dinosaurs’ extinction: ~65 million years ago - Used for comparison with much older tooth and bone tissues. Proportion of fossils that are teeth: 99.5% - Haridi states that most fossils are teeth because enamel preserves so well. Number of scientists in the niche field: Fewer than 10 - Estimate for researchers doing paleohistology/related tiny-teeth work. Years since PhD award: January 2022 - Haridi says she was officially awarded the PhD then. Fieldwork budget: $2,000-$3,000 - Approximate cost for a week of fieldwork mentioned during discussion. Alternative fieldwork comparison: Less than a Camry - A joking comparison for how inexpensive some dig work can be relative to public perception. Catfish diversity share: ~5% of vertebrates - Haridi says catfish make up about one in twenty vertebrates on Earth. Two sets of human teeth: Baby teeth + adult teeth - Described as the mammalian reduction from ancestral constant replacement. Teeth lost by shark per week: About 1 tooth/week - Used to illustrate constant tooth replacement in sharks. Marsupial tooth count: 40-50 teeth - Mentioned in a listener question about marsupial dentition. Crocodilian/dinosaur tooth replacement: Constant replacement - Haridi contrasts these reptiles with mammals’ limited replacement. Population affected by menopause-related bone loss: Women - Hormone-linked bone density loss discussed in the bone-density question. Twin study duration: 25 months - Referenced in the NASA twin study example about physiological change in space.

Pivotal Quotes: "What if these external odontodes were not only for protection, but they were also for sensation?" — Dr. Yara Haridi: Core hypothesis emerging from her research on ancient tooth-like structures. "The origin of vertebrate teeth and evolution of sensory exoskeletons" — Dr. Yara Haridi: Title of the 2025 Nature paper discussed in the episode. "We finally have a little bit of proof that the external odontodes were not only for protection... but they could have also helped our ancient ancestors with sensation." — Dr. Yara Haridi: Summary of the paper’s implication for tooth evolution.

Implications: The episode suggests teeth evolved from sensory armored structures and that new imaging can rewrite fossil history. For listeners, it reframes tooth sensitivity as ancient biology; for science, it highlights the power of reexamining fossils with modern methods.