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Darwin, We Have a Problem: Horse Teeth Are Not Less Evolved

David Coppedge
Photo credit: David Coppedge.

It was a perfect Darwinian tale. The evidence was right there in the fossils. Teeth evolved to have higher crowns in ruminants (e.g., cattle, sheep, antelopes, deer, giraffes) over time, because the rise of grasslands caused more tooth abrasion and required more durability. Evolution met the need and provided the dental and digestive toolkits for the evolving diet. Here’s how it was told, according to Gordon D. Sanson in a PNAS commentary:

The rise and spread of grasslands on different continents during the Tertiary coincided with the appearance of dental characters assumed to be adaptations for eating grass. The dogma was, and largely still is, that grasses are particularly tough and abrasive compared to the ancestral diet of woodland plants. Grasses have long been thought to be particularly abrasive because of their high levels of endogenous silica bodies (phytoliths) although exogenous dust or grit on the surface of grass leaves can also cause tooth wear. Grass-eaters evolved very durable teeth with sacrificial high crowns that could endure high levels of wear (Fig. 1). The teeth also developed highly folded and more complex enamel ridge patterns, assumed to be necessary for chewing a tough fibrous diet. There were other adaptations associated with moving onto grasslands, including changes in locomotory morphology, herd behavior, and body size, but the linkage between tooth form and function and the changing biomechanical properties of the diet are of interest here. It is a particularly rich story because teeth, being so hard and durable, are well preserved in the fossil record. [Emphasis added.]

In addition, ruminants evolved forestomachs that wash and sort some of the grit from the grass, allowing the animals to regurgitate the food, chew the cud, and break down the bolus into finer particles. This provided an “inadvertent advantage” over mammals that didn’t evolve a forestomach, like horses.

A Difficulty with Tooth Evolution

Alas, an earlier paper in PNAS by Valerio et al. raised a difficulty with the tooth evolution story. As evolutionists, these agricultural scientists agreed with part of the tale. It appears true that cattle and other ruminants sort out the dirt and grit in the forestomach. The team proved this in a series of experiments. The sorting mechanism does appear to reduce wear on a cow’s teeth. 

Many reasons have been suggested for the evolutionary success of the highly diverse clade of ruminants. Ruminants have evolved a forestomach physiology that leads to unparalleled chewing efficacy for mammals of their size, with an extreme particle size reduction. This is due to a well-documented particle sorting mechanism in their forestomach that is based on the density of the forestomach content, which floats/sediments in a liquid medium. This mechanism should, inadvertently, also wash off a large proportion of grit and dust before the material is regurgitated for rumination. Here, we show in live animals that this suspected washing actually takes place.

Sanson thought about this finding. He put the new evidence alongside the old evolutionary story and started asking questions. Recalling Kuhn’s philosophy of science, he wondered if biologists had been defending a paradigm without questioning its assumptions. If so, they’ve been doing it for a century and a half! 

It is inevitable that we conduct research within the lens of existing paradigms, but Thomas Kuhn argued that reevaluation of assumptions encourages paradigm shifts. For over 150 y, the coevolution of grasses and large mammalian herbivores has interested biologists and has become a classic textbook paradigm of adaptation. Valerio et al.’s contribution prompts a fresh look at the assumptions underlying this paradigm. … Valerio et al.’s paper raises questions that are worth unpacking.

A simple observation should have perturbed the story long ago: horses are not ruminants. They eat grass but do not chew the cud. Why do equids appear so well adapted to grazing? Like ruminants, they can eat grass for eight hours a day and live long, healthy lives. 

When Sanson started questioning assumptions, he found many dubious claims in the story that were never well established:

  • Is rumination a greater advantage than tooth complexity in avoiding wear?
  • Is crown height (hypsodonty) more advantageous than reduction of particle size in food?
  • Are grasses more abrasive than other plant diets? There is “little evidence” for this, Sanson asserts.
  • Did hypsodonty precede the emergence of grasslands? Apparently not in South America.
  • Are geological particles (e.g., volcanic ash) more abrasive than grass phytoliths?
  • To what extent do grasses accumulate more exogenous sources of grit (e.g., wind-borne sediments) than the endogenous grit in grass phytoliths? Where is the true grit?
  • Are high crowns more adaptive than enamel ridge complexity?
  • Do all ruminants benefit the same from the forestomach sorting mechanism if they ingest different quantities of grit?
  • Most importantly, “What is the driver for the evolution of tooth enamel complexity, durability, or cutting efficiency or effectiveness, or both?”

A Simplistic Scenario

Here’s a sample of Sanson ruminating on the complexities of these issues. They weaken, if not undermine, the simplistic evolutionary scenario.

The cause of abrasion has become more contentious since several studies have questioned the hardness of plant phytoliths and consequently their capacity to wear tooth enamel. However, even if plant phytoliths do contribute to enamel wear, it has been estimated that African buffalo may consume between 10 and 28 kg of grit and 300 and 400 kg of endogenous silica per year depending on the soil type. A fifteen-year-old buffalo on granite soils might have chewed over 200 million times on a diet containing about 6,000 kg of silica, 14 times the amount of grit on the food. These are formidable quantities and attest to the durability of teeth and the necessity for high crowns. With the potential for such quantities of abrasives in the diet, does any inadvertent advantage become less important in grazers if the wear from endogenous silica swamps the wear from exogenous grit? On the other hand, if browsing ruminants consume lower grit levels but virtually no silica, they may have a relatively higher inadvertent advantage. The relative contribution of endogenous to exogenous abrasives needs to be systematically measured over diets, seasons, and soil types and integrated with studies on chewing behavior.

Sanson remarks, “Arguably, Valerio et al. inadvertently highlight just how much we do not know about chewing, which is such a vital part of food mechanical preparation that a large herbivore might invest 8 h a day in the activity.” Then he proceeds to ask more questions:

Ruminant teeth must deal with fresh abrasive food on ingestion and softened, washed, and sorted food on rumination, possibly engaging with a wider range of biomechanical properties than a nonruminant. A horse must accommodate unwashed and unsorted food particles. Why then are the teeth so similar in many ways and why does the fundamental paradigm still make sense when the assumptions may not be so robust after all?Are the biomechanical properties relevant? Diet toughness is often considered in terms of the energy required to chew the food, but that may not be a limiting factor. Rather toughness might affect how the food locally resists fracture and flows along the basins between the complex enamel ridges when chewed.

On he goes, questioning assumptions that have supported a paradigm for a century and a half. Darn Valeria et al.! They just made it tougher to chew the Darwinian story.

Valerio et al. suggest an added level of complexity. Their perspective, as agricultural and veterinary scientists familiar with the intricate workings of the ruminant’s digestive system, bears on the assumptions made by paleontologists about the coevolution of grasses and grazers. Unraveling the selective forces that have led to the patterns of dental evolution has just become more difficult.

Unnecessary Difficulty

The difficulty lies in the narrative, not the evidence. Each mammal — ruminant or not — is living well in its habitat, because it has the right tools for the job. We don’t see horses or cows keeling over in the grass from starvation, suffering from worn-out teeth in aging gums. Ranchers have more horse sense than this; they can tell a horse’s age by looking at its teeth, even if they are courteous enough to avoid looking a gift horse in the mouth. Retired horses put out to pasture continue to graze and usually die of other causes than tooth loss. The cows are not laughing at them, mooing that they should have evolved forestomachs. 

Isn’t that the motivation that causes so many evolutionary tales to fall? In their myth of progress, they envision animals emerging with more complexity over millions of years. Their “universal tree of life” icon starts at a single root and branches out in all directions, each branch “innovating” the tools necessary for whatever creature happens along. Innovations emerge because of “selective forces” that impel them toward solutions for the challenges that the environment throws at them. It’s such a blissful scenario. How can it be wrong when it feels so right?

Evolutionists have been chewing this cud for too long, assuming that Darwin’s magical “selective forces” have the true grit to deal with gritty grass. Time to change the channel and watch The Way Things Work.