In the mid-1800s, Claude Bernard – the “father of experimental physiology” – began a series of experiments to create carnivorous rabbits.
Don’t worry: Bernard wasn’t cultivating predatory beasts like the angry rabbit in Monty Python and the Holy Grail. At first he was simply starving animals until their acidic urine indicated that they’d begun to consume their own flesh as fuel. Deprived of calories, any animal will metabolize its own muscle.
But Bernard went further. As he describes in An Introduction to the Study of Experimental Medicine (translated by Henry Copley Greene), Bernard felt that:
“A carnivorous rabbit had to be experimentally produced by feeding it with meat …”
“I had rabbits fed on cold boiled beef (which they eat very nicely when they are given nothing else). My expectation was again verified …”
“To complete my experiment, I made an autopsy on my animals, to see if meat was digested in the same way in rabbits as in carnivora. I found, in fact, all the phenomena of an excellent digestion in their intestinal reactions …”
This was a strange experiment. Claude Bernard did make a novel discovery, but I don’t think the gains were commensurate with the cost.
Bernard, however, was proud of his willingness to inflict pain for the sake of science. He had a reputation for live dissections of unanesthetized dogs; his spouse called him a monster, divorced him, and created France’s first animal welfare organization.
By all outward appearances, Bernard was unperturbed. He told his friends and colleagues that he’d only married that woman for her money, anyway, which he’d needed to build his first laboratory. By the time she left, she was of no further use to him. And he was disinterested in her “fashionable” morality.
In An Introduction to the Study of Experimental Medicine, Bernard writes that,
“A physiologist is not a man of fashion, he is a man of science, absorbed by the scientific idea which he pursues: he no longer hears the cry of animals, he no longer sees the blood that flows, he sees only his ideas and perceives only organisms concealing problems which he intends to solve.”
Claude Bernard tortured animals, disdained Charles Darwin’s theory of evolution, and loathed the introduction of statistics into biological research. Still, the conclusion of Bernard’s rabbit experiment is correct.
Herbivores can digest meat.
And this transition – an herbivore switching to a meat-based diet – is far more common than its opposite. Plants are much more difficult to eat!
Most species that we think of as herbivores will occasionally eat meat. Cows consume chickens, mice, dead rabbits (especially if the cows are mineral deficient, such as the experimental herds intentionally fed a low-phosphorous diet in the 1990s). Squirrels raid nests to eat baby birds. Pandas will eat roadkill if they can’t find enough bamboo.
After all, another animal’s body provides the full compliment of nutrients that an animal needs – it’s much easier to live as a mere meat refurbisher than to create your own animal body from scratch!
In a research article published in 2019, “Evolution of Diet Across the Animal Tree of Life,” Christian Roman-Palaclos, Joshua Scholl, and John Wiens speculate that the very first animals were carnivores. They write that:
“Remarkably, our results suggest that many carnivorous animals alive today may trace this diet through a continuous series of carnivorous ancestors stretching back for >800 million years.”
Their data don’t actually support this claim. Roman-Palaclos, Scholl, and Wiens categorized the diets (herbivorous, carnivorous, or omnivorous) of a wide range of animal species and found that a statistical model in which the first multicellular animals consumed other heterotrophs would be mathematically parsimonious. Historically, it would take fewer genetic changes to produce our world if herbivory had evolved independently many times over.
But evolution tends to be quite rapid when organisms encounter an empty ecological niche, which is why we see sudden increases in diversity during periods following mass extinctions (like the “Cambrian explosion”) and when animals reach new islands. And we know that multicellular life arose multiple times – at the very least, happening independently in both plants and animals.
The earliest multicellular animals were probably simple aggregates of cells that failed to separate after dividing. Even after genes could cause intentional multicellular development, these early animals were probably blobby things that pursued the same diets as their single-celled precursors.
All told, many of the assumptions made by Roman-Palaclos, Scholl, and Wiens seem dubious at best.
And yet. It probably would have been easier for earliest animals to eat other heterotrophs than to eat autotrophs. Single-celled protists already liked to eat the autotrophs, so most autotrophs had defenses. The autotrophs might be toxic; their tasty molecules were hidden behind indigestible cell walls. If the first multicellular animal gobbled these up, it would’ve gotten such a bellyache!
Except, right. The first multicellular animal didn’t have a belly.
It would’ve gotten such a lysosome-ache!!
In more recent evolution, herbivorous mammals often developed pretty major adaptations to accompany their diet. For instance, herbivores typically have more complicated teeth than carnivores – by chewing their food, herbivores can rupture a plant’s cell walls to access the nutrients inside. And instead of stealing a full compliment of essential amino acids and vitamins from another animal, an herbivore has to synthesize these inside its body. Biosynthesis of Vitamin B12 is pretty tricky – my cells certainly can’t do it. Can yours?
A human whose body contained only human cells could barely digest anything, and certainly not plants. Indeed, most human babies begin life this way – as wholly human. Newborns seem to have very few bacteria inside their bodies, and it’s difficult for newborns to digest anything other than milk.
Soon after birth, though, humans acquire a wide range of passengers. On the surface of our skin and throughout our intestines, we harbor bacteria, many of which are essential for our health. Our passengers interact directly with our brains: certain gut microbes make exercise more pleasurable; other gut microbes are so closely linked with mood & mental wellness that researchers are exploring ways to use them therapeutically.
Without the help of the bacteria who build empires inside our bodies, we would be miserable – achy, asthmatic, bloated, and mentally unsound. And also, yes – without their help, we could not eat plants.
How fortunate that we are not alone!
Header image credit: Darryl Leja for the National Human Genome Research Institute’s photostream on flickr.