As seen above, our results indicate the complete opposite of the evolutionary model of medical systems based on the concept of a structural core that was predicted by Ferreira Júnior and Albuquerque [2]. The composition of important medicinal plants is not supported by the assumptions of specific adaptive long-term factors. Other more dynamic factors must also be considered. Our results allow us to affirm that the composition of plants from the structural core is not in a constant adaptive search for crystallization in the memory and recall of individuals in free lists. Moreover, at the population level, it has a very “volatile” lifespan from a temporal point of view. Nevertheless, we can also say that the importance of medicinal plants can be completely dependent on the current and historical temporal contexts.

According to the structural core model [2], the composition of medicinal plants considered most important by people is the result of medical system processes that involve efficacy, availability, the type of disease treatment, and social learning about plant resources. Therefore, while these factors are very important for the formation of the structural core, the change in the composition of plant species over time may be influenced by other factors of the system.

Several studies in the literature show how these processes can modulate behaviors that involve the knowledge and use of the plants considered to be important. Casagrande [30], for example, showed that factors such as availability and frequency of use, as well as the social and family roles that a person plays in society, directly influence their therapeutic choices. In a meta-analysis on the use value of useful plants, Gonçalves et al. [44] showed that factors, such as the local availability of plant resources, influence the differential use of firewood. This is because people may deliberately choose to use the plants available within their environment over others. Despite some opposing evidence [45], knowledge about medicinal plants of greater use value seems to be unaffected by phytosociological vegetation metrics [44]. Santos et al. [46] showed that plants considered important for different uses in the treatment of common diseases are strongly related to people's perception of them as effective for the treatment or cure [6, 47, 48].

The structural core model predicted that these important medicinal plants assemble all the necessary characteristics to act as protagonists in the treatment of diseases in medical systems, because they are perceived as most effective and/or available. It was expected that this set of information would accumulate over the long term, thereby creating a referential “basic kit” for the population over time.

However, as evidenced by our results, the structural core hypothesis was refuted. Such a “basic kit” did not exist, at least in the sense of targeting the specialization of medical systems through long-term cultural evolution. Perhaps we were not able to detect any pattern due to our time frame of 2 years of analysis. Although the structural core hypothesis was refuted, we believe that the only way to answer this question would be to invest in future long-term longitudinal studies to verify patterns in the composition of plant species considered important for the treatment of diseases over time.

Our findings indicate that the composition of the structural core is significantly more dynamic than that predicted by the model. It is always good to remember that most ethnobotanical studies, and socio-cultural and ecological data collection, are carried out at a specific point in space and time. Therefore, the 2015 model did not consider that the formation of the composition of important medicinal plants must have processes aimed at solving short-term spatial/temporal context challenges in social-ecological systems, in addition to those for long-term ones. Different disease episodes could greatly affect an individual’s patterns of remembering and using medicinal plants in daily life [49,50,51]. Our results indicate that there must be zones in the knowledge structure of medicinal plants that have very flexible compositions in the face of temporal variations.

The first question we can ask about this result is, “What are the factors that modulate this non-conservative temporal plasticity?” The second is whether a dynamic and plastic structural core adds an adaptive advantage to social-ecological systems. Furthermore, “Is the composition of important medicinal plants a contingent context-dependent phenomenon in medical systems?” Answering these questions will not be easy because there are many dimensions of systems to consider (such as socio-environmental, ecological, socio-cultural, and political) [52].

We believe that the explanation of our results is related to the hypothesis that important medicinal plants, based on their structural core, are knowledge structures created as a result of solving everyday short-term medical challenges. Therefore, the importance of medicinal plants must be a contingency phenomenon. The only way to test this hypothesis would be to carry out long-term studies with replicated experimental designs aimed at investigating the factors behind the formation of important medicinal plants both in context and over time. Although our study initially discarded the idea of the “basic kit,” it is expected that clear patterns may be found with 5 or 6 years of annual follow-ups. Only then will we be able to confirm or deny questions such as if there are conserved zones of the structural core with significant variations in the composition of important plants. The natural dynamics of ecological availability and the epidemiological context may be guiding the species composition of important plants.

Thus, to understand whether ecological availability is somehow related to the plasticity of the structural core over time, in periodic collections we must investigate the location and quantity of each of the important medicinal plants. Will they be the ones that are most commonly found in people's medicinal backyards? Will they be the most abundant ones in adjacent or exploited forests? Are they fully available to the community? Or are they anywhere near the studied communities? The fundamental method would be to simultaneously collect the same response variables (e.g., structural core and local preference) and explanatory variables (e.g., ecological availability) to investigate the patterns of the context, and also collect the minimum measurements of the same factors in the same way annually in long-term ethnobotanical research.

Epidemiological context can also be one of the main factors of medical systems in the modulation of a short-term adjusted and plastic structural core. In the original model of the structural core [2], it is argued that the composition of the most popular medicinal plants reflects the long-term evolutionary process in the treatment of common local diseases. Therefore, this “adjustment” of the composition of plants would be directed toward diseases that are always present in social-ecological systems. Therefore, if this composition adjustment is long term, 2 years of space/time variation should essentially capture the same set of recalled medicinal plants. However, our data suggest the opposite. We believe that what explains the plastic variation in the structural core, as captured by our measurements and relative to the local epidemiological context of the population at the time of the interviews, is how common the disease in question is [is it historical, seasonal, or newly established] and how frequently medicinal plants are used for their treatment.

Nevertheless, we must consider that, in addition to frequency, diseases have a specific severity level and latency period associated with them [3, 53,54,55]. Despite the severity of diseases inducing a low total richness of the knowledge of medicinal plant species for their treatment, because people tend to experience fewer new cure alternatives for illnesses with a high risk of death, these “idiosyncratic” medicinal plants may be related to the specific therapeutic resilience of the interviewed individual [54]. Furthermore, previous experience with these types of diseases can affect the individual’s recall rate of important medicinal plants.

Another variable of medical systems associated with the epidemiological context, which is the frequency of the use of plant resources, may also explain the plasticity of our results. Whether the diseases are common or uncommon, it is expected that the frequency of use of plant resources will indirectly influence the composition of the structural core. Casagrande [30], for example, directly verified the influence of the frequency of medicinal plant use on the formation of the composition of important medicinal plants [measured by salience]. In this way, Casagrande empirically showed that the more the plant is used, the more remembered the plant becomes. Priority is given to remembering those plant resources that are re-experienced daily or are significantly important to the person interviewed [49,50,51]. Reyes-Garcia et al. [51], however, showed that we can have plants rarely reported in interviews that are frequently used, and plants that are often reported in interviews and not used much. Our point here is that the frequency of use of medicinal plants in episodes of diseases may be modifying the rate of medicinal plant prioritization in a way that is not expected by the theoretical models of the structural core.

Thus, it is expected that over a 2-year period, individuals and populations may have several episodes of different diseases, which may be common or severe and historical or emerging. Moreover, the frequency of use of medicinal plants in each type of treatment varies. Accumulatively, this is what we believe may be influencing the plasticity of the structural core composition over time. Future studies that seek to better understand the short-term evolutionary dynamics of the composition of important plants, from the perspective of the frequency and severity of diseases, and the frequency of plant species use, could investigate measuring these epidemiological variables through methods such as residential therapeutic calendars, a survey of health posts, and individual perception [56].

Another explanatory hypothesis of the functioning of the structural core model proposed by Ferreira Júnior and Albuquerque [2] is that adults are the fundamental learning pathways for the consolidation of a child’s knowledge about important things in the environment. As the structural core would be the population reference for adaptive information to deal with diseases that circulate in social-ecological systems, it was expected that important medicinal plants among adults would also be the most important among the younger generations of the community. However, our similarity data refuted this hypothesis. The first question we can ask regarding this is why medicinal plants considered important by adults do not seem to exert a referential influence on those seen as important for children. It is important to point out that it is more plausible to consider that children seem to share knowledge that is different from that of adults [57, 58]. In addition, it is noteworthy to mention that we do not have the effect of the influence of the factors mentioned above for the first hypothesis [temporal variation], because all the interviewees, children and adults, were inserted in the same social-ecological space/time contexts.

We must ask ourselves, “How do children learn and reason about medical and medicinal plant knowledge through their development in social-ecological systems?” The formation of the memory of children has already been widely debated in the recent literature on cognitive psychology [59, 60]. Nevertheless, for many decades, the study of children's cognition has always been neglected in scientific ethnobotany/ethnobiology discussions, based on the assumption that children are mere assistants in the process of acquiring information from the environment. However, recent studies have shown that the knowledge of children can be specific to their day-to-day activities and, when viewed from a group perspective, may even have a quality of their own as a type of “children's culture” [57, 58]. Empirical research also suggests that the prevalence of vertical transmission can be overestimated in self-reports about the learning model [57]. Moreover, because children learn a lot from other children, this indicates the importance of horizontal transmission in the formation of knowledge and child practice [61].

Several ethnobotany studies have discussed the role of parents and guardians in the cultural learning of children and adolescents. These studies have shown the importance of this vertical information transmission route for the formation of knowledge among young people [39, 57, 62]. They have also shown that medicinal plants that are either easier to use or easier to find close to homes are generally those that are incorporated into the knowledge of children, whereas medicinal plants from the forest are better known by adults [63]. It is worth noting that the knowledge of children is closely associated with specific aspects of social-ecological systems in which children and adolescents develop themselves. Examples of such are the different societies in which important knowledge and practices of medical systems are learned early on by children and teenagers [39, 64]. This suggests the role of adult facilitation in the cultural learning of valuable information. Notably, there is also the deliberate action by local healers of not sharing all the crucial information they know about their medicinal skills with younger healers. This is done to maintain their status as healer gurus who “know about all plants,” and, in doing so, they seek prestige and future consultations [65].

If we consider the evolutionary point of view of the acquisition of cultural knowledge and practices, compared to our phylogenetic cousin primates, humans develop the competencies of cultural skills long after weaning and the age of first reproduction [66]. This means that, compared with our relatives the primates, we have a very long childhood. This phenomenon is very costly from an energy perspective, as it depends on extensive support in food and care from several other members of the society [relatives or not] [66]. Nevertheless, it is seen in the literature that it is fundamental to allow long periods of social learning about the numerous codes and signs within the culture. This creates more episodes between the search for testing hypotheses, innovations, and learning to imitate the ability of adults in survival and reproduction [67].

The above-mentioned discussion shows that cultural learning in childhood goes through different stages in its development. On the one hand, it is heavily influenced by adult references who are relatives, as well as by pre-established rules of gender and social dynamics in places where children and adolescents grow up [63, 68, 69]. On the other hand, they have the autonomy to actively search for solutions and acquire knowledge about things in the environment to share with their peers between games and other joint activities [62]. Our results on the non-similarity of important medicinal plants between children and adults in the studied community may reflect a child’s long process of acquiring knowledge and cultural practices. This process is marked by questions of trial-and-error, testing of hypotheses, and imitation, which is characteristic of this phase of development. These results may also relate the factors related to the act of children sharing aspects of a culture [67, 69]. In children culture, the factors for considering the importance of a medicinal plant may have nothing to do with those considered by adults.

As a brief case study, of the plants contained in the structural core of children and adolescents in our community, only two medicinal plants were categorized as those that are a part of the children’s structural core; these are “ameixa” and “capim santo” (lemongrass). “Ameixa” is the medicinal plant with the highest importance among adults, both in 2017 and 2019 if we consider our entire sample. It is also the one with the highest importance among the adults residing only in Muquém, which would explain why it also appears as the plant with the highest importance value among children. However, “capim santo” is the 24th most important medicinal plant among the adults of Muquém, which falls far from the first nine that make up its structural core (Table 2). The explanation for this pattern may be related to the fact that this medicinal plant is exotic, taken as a tea, and is popularly known in the region for the treatment of stomach complications, especially for children as it is considered “weaker.” Perhaps, the treatment events or perceived efficacy of these two plants are more common in communities related to children, especially in the administration of “capim santo” tea.