Evolutionary Aesthetics: What Makes the Umbrella Thorn Acacia Beautiful?
Natural landscapes can capture our imagination with their beauty. Through art, photography, and personal experience, it's clear we have an aesthetic response to nature's scenery,
But when we admire nature, we may be unconsciously expressing desires that were programmed by millenia of human history.
The earliest humans most likely developed in Eastern Africa, near the Great Rift Valley of Kenya. Much of our evolution occurred in this type of ecosystem—tropical African savanna, made up of plains and scattered trees. Our ancestors thrived in this environment, and we became who we are because of it.
Other apes are quadrupedal, using four limbs to move, and their bodies are well-built for climbing trees in a forest. But as climate change turned woodlands into grasslands, walking upright on two legs became a more efficient posture for early hominids. It takes up to 75% more energy for chimpanzees to “knuckle-walk” on the ground, compared to a walking human.
The hominids also found new resources to survive. On a savanna, much vegetation grows at a low height, directly available to ground-dwelling creatures. Because this supports large populations of grazing and browsing animals, more meat is produced in savannas compared to forests. In this environment, the new bipedal, omnivorous primate species began a path of evolution towards Homo sapiens.
Could our ancestral home give us an innate visual preference for savannas? Consider that the backyards and parks we cultivate often take the shape of tree-studded grassland. That would be an example of evolutionary aesthetics, the theory that what we find beautiful is based on what’s best for our survival. Specifically, we’re more attracted to savannas because we’re best-adapted to live in them.
A study by Balling & Falk (1982) looked at people’s preferences between images of natural landscapes, which represented five different biomes: tropical rainforest, deciduous forest, coniferous forest, savanna, and desert. Participants varied in age and were asked to rate the images on a six-point scale, judging how much they would like to visit or live in the landscape.
Interestingly, elementary school children in the study had the strongest preference for savanna over the other biomes. Older participants found savanna to be as attractive as the type of environment they were most familiar with. It’s possible children are more likely to favor savanna-like environments because they have less lived experience in other biomes.
A different study by Synek and Grammer (1998) posed a similar question. They asked children from Vienna, both prepuberty and postpuberty, to rate computer-generated landscape images. The images ranged in tree density and height differences (hilliness).
The results partially replicated those of the 1982 study. Before puberty, children significantly preferred the savanna-like images with fewer trees and flatter ground. However, after puberty, children were overall more partial to mountainous landscapes with many trees, similar to Vienna. The researchers suggested that in older individuals, evolutionary preference for their home environment may be stronger simply because they were able to survive in it through childhood; thus, it must be a suitable human habitat.
Of course, many variables play into a person’s perception of natural beauty. Evolutionary aesthetics is by no means deterministic, but there is compelling evidence that it can influence our preferences.
The Umbrella Thorn Acacia
If savanna is the prototypical human landscape, then the umbrella thorn acacia (Vachellia tortilis), native to African plains, may be the prototypical tree. Though in very dry conditions it may be little more than a wiry shrub, in a healthy savanna ecosystem, it’s a medium to large, well-branched tree with an umbrella-like canopy.
As part of a study (Heerwagen & Orians, 1993), evolutionary biologist Gordon H. Orians photographed many specimens of the umbrella thorn acacia, hypothesizing that the beauty of a particular tree’s shape is related to how functional it would’ve been for early hominids.
He suspected that trees would be considered more attractive if they had lower trunks (easier to climb), more layered canopies (providing different views for a lookout), and broader canopies (better shelter from sun or rain). He asked participants to rate the attractiveness of each tree on a six-point scale.
The overall results supported his hypotheses. Highly-rated trees had low trunks and more layered, broad canopies, with canopy broadness proving the most significant variable.
For the umbrella thorn acacia—more than other tree species—evolutionary aesthetics may play a noticeable role in our aesthetic judgments, since our ancestors interacted with trees like it in the East African savanna.
A separate study (Sommer & Summit, 1995) on aesthetic tree preferences used computer-drawn tree shapes (of no particular species) with variations in canopy size, trunk height, and trunk width. It found that participants preferred moderately dense canopies and trunks that branched off close to the ground—again, an ideal tree to climb and peer out over the savanna. High trunks and skimpy or very dense canopies were rated poorly.
If you imagine a beautiful tree, what’s the first image that comes to your mind? Maybe your personal experiences have led you to favor a douglas fir, weeping willow, or coconut palm. But if the silhouette of the umbrella thorn acacia tickles your aesthetics, your hunter-gatherer ancestors could be the reason.
Vachellia tortilis seeds are available for purchase from The Seedy Business.
Balling J.D. and Falk, J.H. (1982), Development of visual preference for natural environments, Env. & Behav., 14:1, 5 – 28.
Heerwagen, J.H. and Orians, G.H. (1993). Humans, habitats, and aesthetics, In: Kellert, S.R. & E.O. Wilson (Eds), The Biophilia Hypothesis, Island Press.
Leakey, L. S. B. (1963), Very early East African Hominidae and their ecological setting. In African ecology and human evolution, Edited by: Howell, F. C. and Bourlière, F., 448–457.
Orians, G. (1986), An ecological and evolutionary approach to landscape aesthetics. In: Penning-Rowsell, E.C. & D. Lowenthal, Landscape Meanings and Values, Allen & Unwin, London.
Rodman, P. S. and McHenry, H. M. (1980), Bioenergetics and the origin of hominid bipedalism. Am. J. Phys. Anthropol., 52: 103-106.
Raichlen, D. A., Pontzer, H. & Sockol, M. D. (2007), Chimpanzee locomotor energetics and the origin of human bipedalism. Proc. Natl Acad. Sci., 104 (30): 12265-12269.
Sommer, R. and Summit, J. (1995), An exploratory study of preferred tree form. Env. & Behav., 27:4, 540- 557.
Syneck E (1998) Evolutionary aesthetics: visual complexity and the development of human landscape preferences. Diss, University of Vienna, Vienna