Three Case Studies on Unusual Primate Evolutionary Ideas
Evolutionary Biology | Jasmine Gunton
What do hippos, bats, and snakes have in common? Yes, they are all animals, yet they are also all involved in some strange ideas concerning primate and human evolution.
Introduction
By now, we have all (hopefully) accepted that humans are very closely related to other primates. To be specific, humans likely evolved from the ancestors of chimpanzees (Pan troglodytes) [1]. With the advent of this popular scientific theory filtering into society, many wondered: But where did the primates come from? Most evolutionary anthropologists will claim that primates evolved from creatures similar in appearance to the modern-day colugo (Order: Dermoptera), a small gliding mammal [2-3]. Nevertheless, there currently exists some strange fringe theories that are as amusing as they are complex. In this article, we will analyse three case studies of unusual primate evolutionary ideas that have been thrust into the scientific community (with little acknowledgement/ welcome). This article is not trying to spread pseudoscience; it is simply an entertaining look into alternate evolutionary ideas. When viewing a new scientific idea, I encourage the audience to think critically about its validity in relation to the current scientific consensus. Additionally, I wanted to clarify that when I use the word ‘theory’ in this article, I am not referring to its traditional scientific definition. Instead, in this article, a ‘theory’ is analogous to an idea or hypothesis.
Aquatic Apes
To summarise, the aquatic ape theory (AAT) claims that modern humans evolved from the great apes by adapting to an aquatic environment [4]. The primary ‘evidence’ given to this theory is the evolution of hairlessness and bipedalism in humans. These two traits would have been advantageous for swimming and collecting shellfish on the seabed [5]. It has been argued that human hairlessness is analogous to the traits seen in hippos (Hippopotamus amphibius) and whales (Infraorder: Cetacea), mammals that made the evolutionary transition from the land to the water. In 1942, anthropologist Max Westenhöfer suggested that human-held traits, including webbed fingers and the regression of smell, also supported the AAT [6]. One cannot deny that seafood is an important food source for many communities, and fishing is still implemented on large scales. Recently, some anthropologists have hypothesised a shore-based diet scenario emphasising the human requirement for iodine. Iodine-deficiency illnesses are often seen in inland habitats that do not have regular access to seafood. Hence this scenario implies that humans evolved from aquatic ape-like ancestors [7]. Today the vast majority of anthropologists and adjacent academics reject the AAT, claiming that it constitutes an ‘umbrella hypothesis’[8]. In this case, an umbrella hypothesis can be defined as an idea that explains certain features of a species as a result of a single adaptive breakthrough. There are many issues with the AAT, including that its ideas are inconsistent with the fossil record and that inferring aquatic behaviour from traits such as subcutaneous fat and hairlessness is an excessive reach. Anthropologist Henry Gee points out that humans likely only began to eat seafood in large quantities around 200 kya, far after the human species had emerged [9]. The critique of AAT by academics can be nicely summarised by a quote from biologists Caroline Pond and Dick Colby, describing AAT as “speculative, theoretical and in many places so imprecise as to be misleading”.
Bat Primates
The flying primate theory (FPT) takes a drastic turn from the previously mentioned theory. It postulates that the megabat order Megachiroptera is a sister group to primates [11]. In other words, this theory suggests that bats are the closest relative to primates, not colugos. The FPT supports a phylogenetic scenario of diphyletic origins, in which megabats can be considered ‘flying primates’ and are less closely related to microbats (Microchiroptera). Recent genetic studies support a monophyletic scenario in which megabats and microbats should be considered as one order: Chiroptera, and that they are not closely related to primates [12]. First, we will consider the ‘evidence’ in support of the FPT. A 2000 study in support of FPT argues that the complex visual system of the bat genus Pteropus is similar to that of early primates [13]. Additionally, perceived differences in the neurology of microbats and megabats imply that megabats are more closely related to primates [14]. The main evidence against FPT comes from modern molecular studies focusing on the genetic differences between bats and primates. A 1992 study found that parsimony analysis of gene sequences supported the monophyletic hypothesis [12]. Additionally, a presentation from 2001 found that most morphological evidence supported bat monophyly and that bat DNA was distinct from species known to be closely related to primates, such as tree shrews (Order: Scandentia) and colugos [15]. Most zoologists agree that bats are monophyletic and that megabats are not ‘flying primates’.
Snakes Snakes Snakes
The third evolutionary theory we will discuss here has arguably more legitimacy than the AAT or FPT. This hypothesis, which we will refer to as the snake detection theory (SDT), suggests that the threat of snakes (Order: Squamata) to early primates shaped our brain structure and vision. In a 2006 study, anthropologist Lynne Isbell proposed that the presence of snakes helped result in the evolution of several neurological features in humans, namely orbital convergence, visual specialisation, and brain expansion [16]. In early primates, the threat of predation would have selected for the appearance of orbital convergence. In later primates, the threat of being poisoned by venom would have acted as further selection for this trait [17]. In other words, snakes may be the reason why our eyes are angled closer together rather than on the side of our heads. The other traits, visual specialisation and brain expansion, would have helped early primates and humans to detect and avoid snakes. A 2011 study tested the SDT by analysing whether orbital convergence was more likely to be found in species that regularly dealt with snakes. However, the results of this study were inconsistent with SDT [17]. It seems likely that the evolution of human stereoscopic vision would have helped with detecting snakes but was not caused by snakes. This is a slightly depressing conclusion, so I will now attempt to amuse you with the results of a similar study on snakes and humans. In 2008, it was found that when shown four visual stimuli (snakes, flowers, frogs, and caterpillars), both children and adults detected snakes more rapidly than the other three stimuli [18]. Other studies have found that snakes are deeply embedded into human behaviour systems designed to evade predators [19]. From these studies, it appears that it can not yet be disproven that snakes had some role in human evolution.
Conclusion
There will always be mysteries associated with evolutionary biology, as we cannot physically go back and have a look. Whilst we gather evidence that allows us to garner a greater understanding of primate evolution, it is delightfully fun to explore all the unusual hypotheses we come up with. I want to emphasise again that the ‘theories’ presented in this article should be taken with a grain of salt. I have discussed them to entertain and encourage readers to think about some cool aspects of anthropology. The emergence of humans is like nothing the Earth has ever seen; it is only normal to be interested in how we came to be.
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Jasmine is a third-year Bachelor of Advanced Science (Honours) student specialising in Ecology. She is interested in researching areas in insect ecology and ecological restoration. This year she is also a part of the Science Scholars programme.