As opposed to the traditional view of evolution by natural selection, niche selection theory holds that we aren’t passive participants in our fate. Therefore, a new study about changes in our diet and our teeth millions of years ago should not have been surprising.
Sometimes, even the experts can miss the forest for the trees to use the old expression. Evolutionary biology, a complex field if ever there was one, is no exception if the surprise that accompanied the results of a recent study on the relationship between diet and teeth in early humans is any indication. As part of the study itself, scientists at Dartmouth University conducted an extensive analysis of the teeth of various human ancestors dating back to Australopithecus afarensis, made famous by the Lucy fossil that lived some 4 million years ago, to identify isotopes left behind by eating plants, known as graminoids. These isotopes, present on fossilized teeth, would reveal certain aspects of their diet, particularly changes in oxygen and carbohydrate continent. They went on to compare findings from these early hominids with the teeth of two extinct primate species, theropiths, giant ground-living monkeys that were something like modern baboons, and smaller, leaf eating monkeys known as columbines. Based on their findings, the three different species began moving away from feeding primarily on fruits, flowers, and insects towards more grasses and sedges, a grass-like flowering plant, some 4.8 to 3.4 million years ago even though their teeth hadn’t yet kept pace with the new diet. While the two species of monkey continued down that path into the present, changing relatively little in the interim, early humans radically altered their diet yet again about 2.3 million years ago. This change was reflected by marked differences in both carbon and oxygen isotypes found on their fossilized teeth during this period, suggesting that our ancestors both reduced the grasses in their diet and increased the amount of oxygen depleted water. While it’s possible the change was due to adopting a radically different lifestyle, effectively spending a lot of time submerged in water during the day like a hippopotamus while feeding at night, the most likely explanation is that we began to exploit plants that were previously hidden underground, including tubers, bulbs, and corms, all of which rely on storing large amounts of oxygen depleted water to protect them from most planet eating animals. As Dartmouth University explained it, “The transition from grasses to these high-energy plant tissues would make sense for a species growing in population and physical size…These underground caches were plentiful, less risky than hunting, and provided more nutrients for early humans’ expanding brains. Having already adopted stone tools, ancient humans could dig up tubers, bulbs, and corms while facing little competition from other animals.” “We propose that this shift to underground foods was a signal moment in our evolution,” noted Luke Fannin, a postdoctoral researcher who worked on the study. “It created a glut of carbs that were perennial—our ancestors could access them at any time of year to feed themselves and other people.”
After the change in diet, hominem dentition began to shift. Teeth overall got smaller, averaging about 5% in reduced size every thousand years while the molars grow longer, but around 2 million years ago there is another marked change over a much shorter period which was likely associated with eating cooked versions of the existing food sources. In both cases, however, the change in diet came first followed by the change in genotype and phenotype suggesting a lag time of about 700,000 years for anatomy to catch up to what we ate. “We can definitively say that hominins were quite flexible when it came to behavior, and this was their advantage,” Mr. Fannin concluded. “As anthropologists, we talk about behavioral and morphological change as evolving in lockstep. But we found that behavior could be a force of evolution in its own right, with major repercussions for the morphological and dietary trajectory of hominins.” “Anthropologists often assume behaviors on the basis of morphological traits, but these traits can take a long time—a half-million years or more––to appear in the fossil record,” added Nathaniel Dominy, Charles Hansen Professor of Anthropology and lead author of the study. “But these chemical signatures are an unmistakable remnant of grass-eating that is independent of morphology. They show a significant lag between this novel feeding behavior and the need for longer molar teeth to meet the physical challenge of chewing and digesting tough plant tissues,” he added. While the study is undoubtedly ingenious and the detail of the findings are groundbreaking, the fact that we can learn what people were eating millions of years ago astounding, the results should not be surprising. Indeed, there’s an argument to be made that they are to be expected thanks to what I refer to as the second most under-appreciated concept in evolution (I will get to the first later), niche selection theory, sometimes known as niche construction theory. As opposed to the traditional view of evolution by natural selection as Mr. Fannin described above, where changes move in lockstep and organisms respond to changes in their environment like organic robots, niche selection holds that we aren’t passive participants in our fate. Instead, animals make choices, even unconscious ones, that influence both the environment around them and how they themselves evolve in response, at times in very surprising ways because there is frequently a compounding effect. The idea is deceptively simple. The environment is filled with different ecological niches that are ripe for exploitation by enterprising creatures, whether or not they have evolved fully to take advantage of them. The enterprising organism in question moves into a niche that is either unoccupied or where there is an opportunity other organisms are not exploiting, alters the environment as a result, and are then altered themselves over time as the selection pressure causes them to further adapt. While the classic examples – beavers building a dam or earthworms changing the soil – are a bit more dramatic than humans exploiting a new food source, the concept is the same. Beavers didn’t evolve their flat tails, ability to swim with their heads above water to carry pieces of wood, or the complex behavior to build large obstructions across rivers and streams before they started building dams. Instead, some prehistoric ancestor of a beaver likely stumbled on the strategy by accident, perhaps taking advantage of the pool near a partially blocked stream, taking up residence underneath whatever was blocking it and somehow benefiting from that protection and easy access to the water source. Over the generations, those who better secured their homes by building upon them had a strategic advantage which resulted in significant genetically driven changes to their anatomy and their behavior. From the perspective of niche selection theory, the first step in this process was some precursor of the beaver settling under a partial blockage by a stream, the ecological niche, not an adaptation that allowed them to do so. Once there, the beavers began adapting to their new mode of life which further altered the niche and further altered them, causing a feedback loop that increased selection pressure in a way that random environmental changes would not.
Humans are not above this process. In the case of the Dartmouth study, I would argue that our ancestors discovered a new niche – underground food – that was as ripe to be exploited as the beaver’s partial blockage. Once we began exploiting it, the benefits of energy rich foods prompted changes in our genetics – and ultimately our behavior considering the still unsung benefits of food preparation and processing that lead to cooking – that better adapted us to exploit the new niche further. In other words, the niche was chosen first and instead of it being a surprise that we changed as a result, this should have been expected, especially when this would not be the first time humans benefited from niche selection theory. The late great linguist and anthropologist Derek Bickerton has previously used the concept to explain the evolution of language – and connect it to how communication evolved in honeybees. In his view, sometime after the shift in diet that was the subject of this study, humans began scavenging the corpses of large animals like mammoths, but unlike an animal adapted to this lifestyle such as a vulture armed with both flight and an incredible sense of smell, this presented a unique challenge. First, we had to find the animal’s decaying body being only able to walk and run across a prehistoric savannah dotted with trees. Second, we had to protect the body from other scavengers while getting help from the rest of our family group or clan. Doing so required a form of communication about objects or stimuli that aren’t present in the immediate environment that is extraordinarily rare in the animal kingdom. Generally speaking, almost all animals only communicate about what’s right in front of them, whether food, threat, or even an evolved trick to steal food. The stimulus – or in the case of monkeys who have evolved that trick, lack of a stimulus – is present at the time of the communication itself. The monkeys are an excellent, though seemingly counter intuitive, example. As a protection against predators, certain species of monkey have adopted various calls to indicate the presence of snakes and other threats in the forest nearby while they are foraging for food. Observing a potential threat, a monkey will signal its presence to others to clear the area and take cover, but some of them cheat. They’ll make the call when nothing is present in an effort to clear the area to get more food for themselves. At the same time, they only do so when the group is foraging in this fashion, under the specific circumstances when a predator might be present. They don’t use the trick to help them mate or do anything else; in other words, they do it only in the presence of the stimulus, in this case food. Early scavenger human communication was fundamentally different. After discovering the deceased animal, our ancestor would have to travel back to the main group and tell them about it somehow, convince them it was worth the effort, then lead others there. If there were competing finds, they needed some way to communicate the size and the distance for the group to make a decision. While no one knows quite how this worked without anything resembling the spoken word – presumably some combination of gestures, grunts, etc. – it occurred before complex language evolved, indeed somewhere between one and two million years earlier. From this perspective, it can perhaps best be seen as moving into a new niche, group scavenging which ultimately separated communication from a specific stimulus and over about two million years led to language. As with the earlier move to underground feeding, the change in behavior occurred first, the change in our genetic machinery and the evolution of more complex language that accompanied it, came significantly later.
Interestingly, there are only two other known animals that do the same thing, the aforementioned honeybees and a certain species of crow. Honeybees, in particular, appear to hold a lesson for humanity. Though they aren’t scavengers, they have the same communication challenge. Rather than deceased animals, they search for pollen and report back to the hive; to communicate their findings to their fellows, they have developed a complex dance that addresses the size of the flower patch, its direction, and distance. In other words, there’s a strong argument to be made that honeybees found themselves in a similar niche to early humans, group scavenging without the right communication tools (such as ants, who leave behind trails of chemicals as they travel) to do so. In this view, niche selection theory describes the niche of the individual organism in question and provides a framework to connect an individual species’ approach to solving the challenge of adapting to the niche with other wildly diverse species. While honeybees and humans solved their challenges via completely different means, the fact that both a hive insect and a primate ultimately evolved communication strategies that are almost unique in the entire animal kingdom, is undoubtedly meaningful. What are the changes otherwise? Generally speaking, when evolutionary biologists consider instances of convergent evolution, they are referring to structures that have evolved repeatedly – such as wings in birds, bats, and honeybees – rather than complex behaviors. Niche selection theory offers the ability to consider both, making it both a powerful explanatory tool and a paradigm with which to view evolution as a whole. While it remains under appreciated and there is very little literature devoted to the subject, the concept isn’t alone. For decades, the renowned evolutionary biologist Richard Dawkins has been arguing on behalf of a gene’s eye view of evolution that goes beyond the body of the organism in question such as a beaver evolving a behavior that builds dams and fundamentally changes the landscape. Most recently, he reset this idea to consider genes as a model of the environment. Astute readers will realize both are pretty fundamentally connected to niche selection theory. If only the evolutionary biologists themselves could keep up.
Confessions of a Conservative Atheist 