A growing body of empirical evidence shows that experiences of nature provide people with diverse psychological benefits, including improved cognitive function and mental health. While our understanding of the proximate causes of these positive psychological responses of humans to nature has advanced, the ultimate (evolutionary) drivers behind them remain poorly understood.
In this study, we provide a summary of several widely accepted evolutionary hypotheses originating from Biophilia, reviewing their challenges and limitations. Subsequently, we propose a new evolutionary psychological hypothesis—the greenery hypothesis—that integrates recent findings in evolutionary psychiatry with many experimental results unexplained by existing hypotheses/theories.
Our proposed hypothesis states that humans have adapted to periodic severe drought and re-watering cycles by developing both negative and positive psychological responses to the absence or presence of greenery within the landscape as cues to optimise their own behavioural activity.
The greenery hypothesis holds the potential to yield several insights into the fundamental understanding of human psychological responses to nature exposure with significant implications for various related fields, including psychiatry, urban planning, and biodiversity conservation and restoration.
4 GREENERY AS A CRUCIAL CUE FOR DROUGHT AND RE-WATERING
we propose a hypothesis positing that humans perceive the presence or absence of greenery as a cue to detect a significant environmental change, specifically, drought. Long-term water scarcity resulting from drought would have profound ecological impacts on the fitness of all organisms, humans included (Vicente-Serrano et al., 2020). Extended periods of drought can lead to the withering of many edible plants and the death and dispersal of herbivorous animals, crucial food resources for hunter-gatherer humans. Thus, the alternation between extreme dry and wet periods has been suggested as a crucial factor in early human evolution (Maslin et al., 2014).

Indeed, severe drought would have worsened the nutritional status of humans, sometimes resulting in massive mortality, acting as a strong selection pressure on human physical and psychological traits. Reports indicate that during severe droughts, people in hunter-gatherer societies in arid and semi-arid regions experience higher mortality rates due to famine and infectious diseases (Awuondo, 1987; Jones & Schwitalla, 2008; Silberbauer, 1980). In an environment degraded by severe drought in the EEA, the adaptive benefits from foraging behaviour are expected to be lower than those in a normal environment. Therefore, psychological and behavioural responses that suppress foraging effort, relative to the normal environment, are considered to be adaptive.

During such extreme adverse environments, natural selection may favour psychological responses, such as depression, to minimise behavioural activity. In fact, experimental evidence supports this notion, demonstrating that severe starvation triggers severe depression (Lasker, 1947; Phillipou et al., 2018). Even in modern society, several studies suggest that drought increases adverse mental health outcomes through various several causal pathways, especially in rural areas (Vins et al., 2015). Negative psychological responses, such as depression, may play an adaptive role by contributing to the suppression of wasteful activity during deteriorative environmental conditions caused by drought.

Arguably, the most evident environmental signal of severe drought for humans is loss of greenery in the landscape. read further from the article