Phytoliths and Hypsodont Teeth: The Silica Arms Race
Plants deposit microscopic silica particles in their tissue — hard enough to wear down tooth enamel, and durable enough that phytoliths in sediment identify plants long after organic material has decayed. Grasses are heavy accumulators, and grazing mammals answered with high-crowned or ever-growing teeth, a spread that tracks Miocene grassland expansion. Human teeth are low-crowned and don't regrow, making abrasion a barrier independent of the enzymatic one.
Grass is abrasive. Its tissue contains **phytoliths** — microscopic particles of silica (SiO₂) that plants deposit within and between cells, essentially small pieces of glass distributed through the leaf. ## Why plants make them Silica is taken up from soil as silicic acid and deposited in a range of shapes characteristic enough that phytoliths surviving in sediment are used by archaeologists and palaeobotanists to identify plants long after everything organic has decayed. The principal function is defence. Silica is hard — comparable to or exceeding tooth enamel — so a leaf full of it wears down the mouthparts of whatever eats it, and grasses are among the heaviest accumulators. It also provides structural support at low metabolic cost, since silica is cheaper for a plant to deposit than lignin is to synthesise. ## The mammalian response: hypsodonty Grazing mammals evolved **hypsodont** teeth — high-crowned, with a large reserve of enamel and dentine above the gum line, extending as the surface wears. Some lineages went further to **ever-growing** teeth that never stop erupting. Horses, cattle, sheep and many rodents show this. The alternative is **brachydont** — low-crowned teeth like those of humans, pigs and most primates, adequate for softer food and destroyed by a diet of abrasive material. The spread of hypsodonty in the fossil record tracks the expansion of grasslands in the Miocene closely enough to be a standard example of coevolution — a change in vegetation registering in mammalian dentition. The interpretation has been refined, since grit and dust ingested along with ground-level forage contribute abrasion alongside phytoliths, but the association is robust. ## The consequence for humans Human teeth are brachydont and do not regrow. A diet of grass blades would grind them down, and this is a **separate and independent** barrier from the enzymatic one in Cellulose and the Missing Enzyme: Why Humans Cannot Digest Plant Fibre — solving cellulose digestion would not solve this. It is one reason the fungal route is attractive: growing mushrooms on grass sidesteps the enzyme problem and the abrasion problem simultaneously, since you never chew the grass at all. See Growing Mushrooms on Lignocellulose: Letting a Fungus Do the Digestion.