Plants make sense

Organisms demonstrate biological agency as the universal behavioral propensity to survive, reproduce, and flourish (biological axiom). This is a life-defining characteristic that demonstrates evolutionary continuity.
As autonomous biological agents acting on, and responding to, their environments they have the capacity for both short-term sensory, and long-term genetic information exchange with their surroundings. Three articles extend this general theme into the realm of plants and sensory systems: plant agency introduces the unpopular notion of plants as agents; plant sense explores the ways that plants sense their environments, using the human senses as a point of comparison; and plants make sense describes how, as human agency evolved out of biological agency, the evolution of human senses was influenced by plants.

Our historical human dependency on plants has left a permanent impression on our biology – incorporated into our genetic make-up as both physical and psychological adaptations. These arose in ancient environments of evolutionary adaptation, many of them emerging in our deep evolutionary history before we had become the species Homo sapiens.

Modern scientific research can help us better understand these adaptations to the world of plants and ancient environments and, in so doing, help us refine our understanding of human nature and the human senses.

Our interest in the cultural utility of plants – the way we use them for fibres, dyes, furniture, medicines and such, ignores a much deeper interdependence forged over deep evolutionary time, a dependence that has been cemented into the human genome. Plants are not just cultural adjuncts to our industrious lives, they are part of the fabric of our universal human nature.

So, what are the biological traits that evolved as ancient adaptations to plants?

Plant-human coevolution

Coevolution occurs when two or more species influence one-other’s evolution through the process of natural selection. This means that coevolved organisms have shared a particular environment of evolutionary adaptation. Sometimes an organism can influence its own evolution by modifying its environment, humans being a good example of this, living in mostly artificial cultural (rather than natural) environments.

Shared genes

One of the great revelations that followed the cracking of the genetic code was the discovery that we share many genes with other organisms. Not just with our close ancestors but with worms, plants, and bacteria. This should not surprise us since we know that all life has evolved from a common ancestor – and that includes animals and plants. Organisms share many metabolic processes. Respiration is one example but there are many others. That ‘unique group of genes necessary for a plant to determine if it is in the light or or in the dark‘ is also present in human DNA, where it regulates certain human responses to light.

There was a time, not so long ago, when it was believed that every species was created uniquely and independently by god. Today we know that, since we are made of stardust, the chemicals created in ancient supernovae we share our chemistry with the universe. Further, since the entire community of life evolved from a common ancestor, we know that there is a biological connection between all organisms.

Early plant influences

So in what way has our human genetic make-up been moulded by plants?

This is an interesting but sometimes controversial area of study and there is plenty of room for original research on the topic as it has received surprisingly little attention. In fact, understanding the biological relationship between plants and people can be regarded as part of the 21st century’s endeavour to really come to grips with our human nature. Plants have a direct influence on our moods and behaviours, through our diets. Through photosynthesis an enormous variety of organic molecules is created, from sunlight, water and carbon dioxide, which is amazing in itself. The molecules in our brain that determine our moods and behaviour, such as dopamine and serotonin, are plant compounds.

Among our possible plant-related biological traits can be included: bipedalism; dentition; our senses of taste and sight, especially of colour vision; and our general appreciation of nature and landscapes.

Bipedalism

Among the more speculative ideas about the way the broad environment has influenced the evolution of our bodies relates to the origin of bipedalism. The human ability to stand upright on two legs has been related to an ancient need to stand upright to peer over the savannah grasses on the look-out for potential predators. It is a plausible theory that needs more evidence.

Recent research suggests that dense forests of eastern Africa gave way to open woodland 10 million years before previously thought driving upright gait of apes that gave rise to humans. Fossil soils from sites in Kenya and Uganda reveal the presence and predominance of C4 grasses up to 21 million years ago probably driving the development of an upright posture.^[20]

Diet & digestion

Perhaps the most obvious evolutionary connection with plants is through our diet. We might expect to have biological adaptations in the way we process food in a complex interplay between evolution, ecology and culture. One peculiarity of the primate diet is that it needs to include vitamin C, probably because the diet of the early primates always included plants and fruit. Leaf-eaters (folivores), like gorillas, need to ferment their main food in both stomach and gut and this resulted in their increase in general bodyweight due to modifications in the digestive tract which was unnecessary for, say, frugivores (fruit-eaters). A wide variety of plants must be included if the diet is to provide all the essential amino acids. All-in-all the diet chosen can limit the available habitats and this in turn can be affected by climate and seasonality. Cooking food made plants more palatable, softer for mastication, burst the cells to release nutrients that could be digested more quickly, aided preservation, and since it often nullified toxins it extended the potential range of foods. This is all part of our selection of food preferences and the way our physiology and morphology (the coevolution of genes and culture) have created boundaries to the kinds of things we eat.

However, we should be aware that interpretation of many aspects of diet can be a minefield. Food is often at the heart of human innate feelings of ‘purity and sanctity’ so what we might think are just matters of fact quickly move into areas of ideology and taboo.

We cannot boast the multiple stomachs that ruminants have to aid their digestion of vegetation but, even so, we have inherited from our hominin ancestors the dentition and digestive system of omnivores. There is no doubt that our ancient plant environments and diets have influenced the evolution of our dentition and the processes that go on in our gut. We can study these this side of our nature through the processes we see today but we can also study ancient diets through the physical evidence of archaebotany.

Archaebotany

Archaebotany studies plant remains in the archaeological record. Microremains, tiny plant structures with distinctive morphologies, can record the exact plant foods that our ancient ancestors consumed. Plant remains like this can be recovered from recently deceased and fossil primate samples, and can also be used to supplement traditional dietary analyses in living groups. This is a developing field of study that can now give us insights into ancient diets. In very general terms our ancestors ate more plant food than meat with males probably consuming more meat than females. These diets have been analysed in part from the phytoliths (starch grains) embedded in tooth plaque (calculus) and on grinding stones. Neanderthals were plant eaters across their range, eating grains, tubers and roots.^[15]

Dentition

Dentition is a rewarding area of study for archaeologists because teeth are preserved better in the archaeological record than bone and the characters of structure and arrangement are more conservative that other skeletal characters.

Primates eat a wide range of different foods and their dentition consisting of incisors, canines, premolars, and molars reflects this dietary plasticity. Incisors and canines are used for biting and tearing, the premolars and molars for grinding. Primates have fewer teeth than their mammalian ancestors, the Old World higher primates, including humans, having the fewest: 8 incisors, 4 canine, 8 premolars, and 12 molars. Human dentition has also evolved on its own due to dietary changes. Human teeth differ from other primates in having non-honing chewing teeth, the large canines disappearing about 5.5 million years ago. Perhaps the ability to modify foods before eating led to the loss of the honing canines and their replacement with vertical, incisor-shaped canines. Humans also lack the diastema, a gap that other primates s have between their lower canines and first premolars into which the large honing canine slots. Human teeth are also relatively small compared with other primate teeth.

In short, the adoption of easily chewed cereals led to changes in the jaws and teeth, th ejaws becoming mor edelicate and the teeth smaller. In the Near East the spread of agriculture has been related to a decline in adult height until about 4000 BP and even today we remain on average about 3 cm shorter than our ancestors.^[17]

More recent changes in human dentition may have come about following the advent of sedentary lifestyles and a change from foraging to agriculture with domesticated plants and animals. Softer cooked foods put less stress on the chewing muscles with a reduction in size of the jaw bone possibly giving rise to the modern problems of overbites and underbites as smaller jaws have retained similar sized teeth. Tooth decay may be in part a result of dietary change since domesticated plants are rich in the carbohydrates that encourage lactic acid-producing bacteria that attack the tooth enamel. The type of plant may be important though as rice, unlike corn, causes very little decay.^[14]

Herbivory

Though generally described as omnivores, meat-eating is not a highly evolved characteristic in us: we lack the well-developed physical characteristics of carnivores. We have poorly-developed ‘claws’ (nails) and impotent canine teeth. Carnivores lack the side-to-side molar-grinding ability of herbivors and humans. The molars themselves, unlike those of carnivores, are flat for grinding fibre.

Highly acidic digestive stomach juices allow carnivores to swallow meat in large pieces, the acid killing potentially dangerous bacteria, while human stomach juices are much more dilute. Carnivory is also associated with short intestinal tracts and colons that process meat rapidly before it rots, while human intestines are much longer permitting more extended process of fibre digestion and nutrient uptake.

Plants & human senses

Our senses – the way we perceive the world of smells, sights, sounds, and tastes – have, like everything else to do with our bodies and brains, evolved by the process of natural selection acting over countless generations in our environment of evolutionary adaptation, the EEA. It is likely that our sensory limits: the limit to our field of view, angular resolution (detail), visible spectrum (wavelengths between 390 and 750 nm)and hearing range, reckoned as lying between about 20 — 20,000 Hz are all the result of evolutionary selection: they approximate the limits most useful for our survival. We must also bear in mind that the way we percieve the external world is the way that our senses construct it.

One of the most obvious way our brains have been ‘moulded’ by natural selection is in the area of sensory input, an area of active research. And plants have a key role in this story.

Sight

Our visual apparatus discriminates discrete objects within our field of view, gives us a sense of distance, and recognises colours (we now know this runs into the billions) and shades. From highly complex visual sensory input the retina converts patterns of light into neuronal signals that become meaningful images in our brains. Remarkably we do not see a meaningless visual jumble light and shade. Needless to say the physiology and psychology of vision is still an active area of research.

So where do plants fit in?

Binocular or stereoscopic vision

Animals, like the hooved grazers of the open plains, have eyes on the sides of their heads giving them almost a 360° arc of view that can pick out predators approaching from almost any direction. In contrast predators and animals living in trees tend to have forward-directed eyes giving improved depth-perception that has obvious benefits when swinging from perch to perch or attacking prey. This suggestion of function is probably an oversimplification as it can be countered by examples like birds and squirrels. However, our ancestry as tree-living primates might well account for our binocular vision.

Colour vision

Colour vision first evolved in the form we know within the primates, its utility being sometimes attributed to the colouring of the fruits that were a major part of the diet at this time in evolution.

Humans and Old World primates are the only mammals with trichromatic color vision based on three kinds of photoreceptors (cones) within the retina containing different photopigments red, green and blue. The spectral peaks of the three pigments are in the short-wave violet, middle-wave green, and long-wave yellow-green regions of the spectrum.

Human vision operates over the wavelengths 390 to 700 nm. The mid-point of this visible spectrum, the point around which our colour vision is centred is 545 nm. The dominant colour at this wavelength is green. Anticipating that natural selection would favour colour discrimination that is most likely to affect our survival it is perhaps no surprise that such a large proportion of our colour vision is devoted to green, the colour reflected by vegetation. Ability to perceive red and orange hues allows tree-dwelling primates to distinguish them from green. This is particularly important for primates in the detection of red and orange fruit, as well as nutrient-rich new foliage with more carotenoid pigments. Another theory suggests that skin-flushing is a measure of mood, a useful trigger to primate trichromate vision.^[9]

Spectral absorption curves of the short (S), medium (M) & long (L) wavelength pigments in human cone and rod (R) cells
Courtesy Wikimedia Commons

Taste

Our sense of taste operates as a quality-control for the foods we are about to ingest. Our early ancestors were omnivorous foragers who would have benefitted from eating high-energy nutritious and digestible foods that did not contain toxins or other unsuitable substances.

Our sense of taste today was probably shaped by our EEA which was, at first, the closed tropical forests of our early hominin ape-like ancestors who ate mostly fruit (rich in sugars and vitamin C) and young leaves before the move to the African savannah about 4.5-2.3 million years ago resulted in a much more diverse diet.

We perceive taste through sensory taste buds in our mouths, science currently recognising five basic tastes of sweet, sour, salt, bitter and umami. Mixtures of these basic tastes combined with our sense of smell gives rise to food flavours. They also surely regulate to a degree our intake of the salt, protein and energy that are vital to our survival and which, together with our sense of smell, set off the automatic revulsion we have for many poisonous, rotting, or other foetid substances.^[1]

‘Our unique human evolutionary history has given us taste preferences for sugars and acids that provide energy and vitamin C, as well as newly developed preferences for higher intakes of salt and starch. In addition we have developed a taste for umami-tasting fermented foods, which have the benefit of introducing more digestible nutrients and probiotic bacteria to our diet’.^[16]

Expressed very simply we have nutritional rewards and punishments. Foods like sugar and other simple carbohydrates that are energy-rich taste good while food that may give us problems tastes bitter: we tolerate some sourness. Making a mistake is ‘corrected’ by responses of pain, nausea and vomiting. Pregnancy alters taste responses and feeding patterns in women especially sensitivity to bitterness which can cause nausea and presumably protects the foetus from low-level toxins. Babies fed on mothers milk rich in glutamates and sugars notoriously avoid some vegetables.

In very simple terms:

• Sweet – carbohydrate.
• Salty – electrolytes.
• Sour – acids.
• Bitter – toxins.
• Umami – glutamate and nucleotides.

Sweet

Sweetness, a pleasurable sensation, is produced mostly by sugars and this helps us identify energy-rich foods. An ecological ‘optimal foraging theory‘ suggests that many organisms forage in a manner that will maximize their energy intake per unit time.

Salt

Like other mammals we like the taste of diluted salt in liquids and the quantity of salt added to meals tends to be uniform across cultures. Salty foods contain electrolytes necessary for internal regulation especially for the nervous system. Sweating humans need to regularly replace these vital chemicals, especially as sodium and potassium, and will seek out salty foods. Herbivores can quickly become salt-depleted and seek out salt licks.

Bitter

Bitterness, which serves as a warning sign of poisons at low thresholds, can be especially aversive but is tolerated in some foods like coffee, citrus peel, brassicas, and the quinine of tonic water. Plant leaves often contain toxic compounds and primates prefer tender young shoots that are often higher in protein while being lower in fibre and poisons than mature leaves. Cooking techniques that remove poisons, such as the cooking of cycad seeds by Australian Aboriginals, are well documented.

Sour

Sourness occurs not only in citrus but many other fruits. Acids are a feature of foods that have begun to rot and a reason why they signal caution. We tolerate low levels of sourness as it is often associated with high levels of nutrients in plants and some have medicinal or other properties as is the case with Beer and wine.^[7]

Umami

Umami, a savoury taste, is a relatively recently added taste and is associated with meats, mostly those that are aged or cooked and associated with hydrolysed protein and fermentation associated with improved nutrition and probiotic bacteria making the meat more digestible. These tastes are present in both babies and adults. Glutamate is the most abundant amino acid in nature and like other amino acids, is integral to growth and presumably serves as an indicator of the presence of substances needed for body protein synthesis.

Evidence today

Today we have ready access to foods high in salt, glutamate, sugar and fat. Though this was useful to our ancestors, when eaten to excess it is an energy-rich combination of foods resulting in obesity and diabetes, together with a deficiency of micronutrients.

Such an argument might also be applied to some of our evolutionarily acquired behaviour.

Taste preference

Taste preferences are not all under genetic control; they may be both culturally and genetically influenced. For example, some people cannot bring themselves to eat snails, shark, or raw meat. However, the presence of taste receptors that enable us to avoid toxins in foods has now been demonstrated and manifest largely as a genetic variation in sensitivity to bitterness that has given rise to a number of hypotheses: in Africa where sensitivity is low it might assist in resistance to malaria. It also appears to be linked to the use of spices (used to make bland food tasty, obscure the taste of bad meat, or simply cultural tradition) which might have the evolutionary advantages of being anti-microbial (more spices are used in hot climates), providing micronutrients (they tend to have antioxidants), or allowing users to display their wealth.^[2]

Lactose intolerance

Most mammals are lactose intolerant: after weaning they cease to produce the enzyme lactase which digests the milk sugar lactose found in dairy products– they no longer drink milk. In humans there is lactase persistence. This is believed to have arisen at the dawn of agriculture when milk and dairy products became a routine part of the human diet in parts of Europe about 10,000 years ago. It is a distinct adaptation, a genetic change produced in response to cultural change. Caucasians rarely develop the lactose intolerance that is more common among Australian Aborigines and people from Africa, the Middle East, some Mediterranean countries and Asia. In a similar way many people, mostly from Africa, the Mediterranean, and Middle East, have a mutation affecting the activity of the red blood cell enzyme G6DP which deprives the malaria parasite of oxygen from the red blood cells. The mutation is sex-linked to males who become unable to digest broad beans, a condition called favism, which is correlated with the geographical distribution of malaria in the Middle East and Mediterranean indicating a tension between the relative benefits of disease resistance and food consumption.^[3]

Dietary irregularities

Carnivores do not appear to suffer from the modern human complaints of heart disease, diabetes, cancer and obesity, conditions related in humans in large part to the consumption of saturated fats and cholesterol. Carnivores do not develop hardening of the arteries, even with the consumption of large quantities of fats and cholesterol. Perhaps the illness experienced by humans indicates a low meat intake evolutionary diet, this being a good argument for a vegetarian diet?

Then there is the conflict between our evolved preferences and present-day ‘Western’ environments which might be connected to lifestyle illnesses like obesity and type 2 diabetes. It appears that one component of multifactorial causation is that the body regulates protein intake (more than it does carbohydrate or fat) so when our diet contains insufficient protein we compensate by eating more of the other food types. One obvious remedy is to increase protein content of processed foods and to manage our predispositions through sensible eating habits although any form of government regulation is likely to be perceived as an infringement of free choice.^[4] Type 2 diabetes appears to affect different groups of people in different ways prompting the question as to whether at base this is an environmental or genetic effect. Environmentally we know that obesity, lack of exercise and a high calorie diet, stress, low self-esteem, all contribute but there is also the possibility of a genetic component related to our past when we would have feasted and fasted in turn and when rapid energy uptake would have been a distinct advantage. Nowadays this adaptation would be a distinct disadvantage because we are exposed to a continuous supply of food to excess. It seems possible that present-day type 2 diabetes sensitivities might depend to a degree on past evolutionary exposure to feast and famine.^[5]

This is a rich field for further research although caution is needed in allocating relative influences of genes and environment. It would seem highly likely that aspects of our other senses, have a component that has evolved as a response to plants.

With agriculture came an increased use of starch, a complex glucose polymer digested ultimately into glucose itself which passes into the bloodstream. Humans are unique among the mammals in having a large amount of salivary amylase, an enzyme that quickly breaks down cooked starch.

Smell

Food flavours are the combined response of our chemical senses of taste and smell (odour) – with the sense of smell more sensitive than the sense of taste and the greater contributor to our perception of flavour.

Our sense of smell (olfaction) comes from special nasal sensory cells which interpret certain molecules in the nasal cavity as having a particular smell, the sensory signals passing to the olfactory bulb below the frontal brain lobe). Like the sense of taste, smell is detecting chemicals with chemoreceptors, the chemicals being mostly volatile small molecules, non-volatile proteins, and non-volatile hydrocarbons.

The olfactory gene family makes up about 1% of all the genes and is the largest in the mammalian. Each species has an olfactory repertoire unique to the genetic makeup and humans can detect millions of airborne odorants and there is great genetic variation in olfactory genes reflecting cross-cultural differences. There are a small number of human pheromones (sex attractants) and the consumer product industry of perfumes, food and drinks invests billions of dollars in research. Anthropology still has much to learn about our sense of smell in relation to behavioral and social cues, evolutionary history, mate choice, food decisions, and overall health.^[10]

Little has been suggested concerning the functions of the human olfactory system with research centred on olfactory malfunction (onosmia), olfactory senses of mammals, and analysis of evidence for particular theories. Functions may frequently be associated with other senses especially taste but have been placed in three broad areas: ingestive behaviour (detection of undesirables, energy-rich or other desirable or undesirable foods, appetite regulation),, environmental hazard avoidance (association especially with with fear and disgust), and social communication (sexual signals, fitness detection, immune-chemical signals and biological compatibility, maternal odours, emotional contagion, fear an stress).^[11] There is also the possible evolutionary historical purpose of navigation.^[12]

Association of smell with particular kinds of plants or their parts is best known through its links to taste. Though the sweet-smell of flowers like roses and jasmine is assumed to be attracting insects when the flowers are ready for pollination, its function in humans is not known, if indeed there is one: it is unlikely that humans ever acted as pollinators.

Evolutionary psychology

The field of evolutionary psychology is only just opening up but it has been realised for some time that our feelings and emotions are the bridge between modern humans and our primal ancestors, mostly those who hunted, scavenged, and gathered food on the African plains more than a million years ago. Our legacy of emotions and affects comes from the past in ways that we are only just beginning to explore.^[6]

Biophilia

We know what it feels like to be influenced by our instincts, to feel sexual attraction and empathy … or hunger when we get a whiff of our favourite cooking. Perhaps more controversially biologist and author Ed Wilson in his book Biophilia (1984) postulated a natural sense of affinity for other forms of life. He defined biophilia as ‘the urge to affiliate with other forms of life’. Of course we may feel nurturing sentiments when we see small or cuddly animals but Wilson had a more general idea in mind, more like an emotional response to landscape: the way we can stand in awe of our surroundings. Ed Wilson was suggesting that this was something more than being simply impressed by the scale of nature, it ran deeper than that, it was like an evolutionary echo within our consciousness of nurturing environments of the past. A difficult hypothesis maybe but a plausible one worth exploring.

Since the publication of this book more thought has been given to the topic. Is there more than just decorative value in having indoor plants or flowers and plants in hospital rooms, can green space in cities provide relief from the multitude of manmade objects, and have therapeutic and restorative effects that are a consequence of more just visual variety? Can we feel deprived of natural landscapes or plants, and does greenery relieve the physiological and psychological effects of stress? Are plants a crucial consideration for our quality of life?.^[12]

Clearly a case like this can be built into an attractive ‘just so’ story with little evidential foundation. Biophilia is perhaps ‘… ‘not an attribute with a strong penetrance’ … more likely … ‘to be shaped … by cultural factors and individual peculiarities’. There is some evidence that women have a stronger affective response to plants than men. Could this be because women were much more involved with plant collection and processing during the EEA when men were out hunting or the more recent cultural tradition of women being in charge of domestic affairs? It makes a nice story but difficult to say if it is anything more than this.^[13]

Plant commentary & sustainability analysis

We have much to learn about our the way plants have been incorporated into our genetic make-up over the long period of plant-primate coevolution. Among our likely plant-related biological traits can be included: bipedalism; our dentition and digestive structures and processes; our senses of taste and sight, especially our colour vision; and our psychological appreciation of nature and landscapes.

Plant science can play its part in the 21st century enterprise of understanding more fully the biological basis of our human nature. Findings in this field can help us better understand and manage, among other things, our diets and psychological integration with the natural world.

Key points

Among the human genetically-based traits that are presumed to be causally associated with plants present in our environment of evolutionary adaptation are:
 

• Humans are totally dependent on plants while plants are only incidentally dependent on humans

• Human dependencies were established before the emergence of Homo sapiens through the plant creation of atmospheric oxygen, and the provision of the food energy that facilitates organic order and organization that temporarily resists universal entropy, the tendency to disorder

• Among the human biological traits that are adaptations to a world of plants can be included: bipedalism, the form of our dentition, herbivory, the nature of our diet and digestive system.

• Of special note is the way that plants have influenced the evolution of our senses: the binocular and colour vision of our sight; the sweet, sour, and bitter aspects of our taste;

• Most mammals are lactose intolerant. Our human lactose tolerance is the outcome of 10,000 years of agriculture and our adaptation to cows’ milk

• Among the set of human psychological predispositions is our biophilia, the stimulus and exhilaration we feel among nature and wide natural landscapes

• Plants have a direct influence on our moods and behaviours, the most obvious examples being the psychotropic plant drugs – not only the obvious ones like opium, cocaine, and alcohol-based drinks, but also tea and coffee. However, it seems unlikely that these drugs have influenced our genetic makeup

• Understanding the biological history of the human diet can help us deal with modern dietary problems. Carnivores, for example, do not develop hardening of the arteries, even with the consumption of large quantities of fats and cholesterol. Does this indicate a low meat intake evolutionary diet?

• The primate diet must include vitamin C, probably a consequence of an evolutionary diet of plants and fruit.

• Leaf-eaters (folivores), like gorillas, need to ferment their main food in both stomach and gut and this resulted in their increase in general bodyweight due to modifications in the digestive tract which was unnecessary for, say, frugivores (fruit-eaters).

• A wide variety of plants must be included if the diet is to provide all the essential amino acids.

First published on the internet – 1 March 2019

. . . 24 July 2020 substantive revision
. . . 3 June 2023 added addition to bipedalism
. . . 30 June 2023 – minor revision

La Bouqueria Market – Spain
Courtesy Wikimedia Commons