Hum vs. anthr
Other vs. anthr
The results obtained with respondent subgroups based on total number of authored peer reviewed publications and total number of authored popular science publications are not shown, because they were not associated with significantly different ( p < .05) means in any comparisons.
*** p < .001; ** p < .01; * p < .05.
Overall scientific experience (as measured with the number of scientific publications authored) had no effect on the scores given to either the dryland or the water‐related hypotheses (Table 2 ). However, the more knowledge the respondents had on human evolution specifically (self‐assessed familiarity with the hypotheses, number of scientific publications on human evolution or experience in teaching human evolution), the lower the scores they gave to the water‐related hypotheses. Among biologists, those who knew more about human evolution were more critical than the less knowledgeable ones, and (paleo)anthropologists were more critical than human biologists with the same self‐assessed knowledge level.
When the dryland hypotheses were split into two groups depending on whether they were based on behavioral arguments or environmental adaptation, both groups obtained rather similar results. The main difference was that the behavioral hypotheses received somewhat higher average credibility scores, which reflects the fact that 10 of the 12 most popular hypotheses were based on behavior (on the other hand, so were the four least popular hypotheses).
To visualize the differences in opinion among the (paleo)anthropologists and representatives of other fields, we repeated the ordination of the hypotheses for each of the four respondent groups separately. In accordance with the fact that most respondents were biologists, the ordination based on the biologists’ data only (Figure 2 c) was very similar to the ordination based on all respondents (Figure 2 a). The ordination based on (paleo)anthropologists’ views (Figure 2 b) differed especially in relation to the hypotheses for bipedalism: Hypotheses that explained bipedalism by foraging, tool use, or carrying were very far removed from the main cloud and toward the opposite side than the water‐related hypotheses. In addition, the average credibility scores given to the water‐related hypotheses were among the lowest of any hypotheses. This contrasted with the situation in the ordination based on human biologists’ data (Figure 2 d), in which the water‐based hypotheses had intermediate credibility scores.
The hypotheses differed clearly from each other in the frequencies of different credibility scores, but there were some similarities in the overall pattern among those six traits for which three or more hypotheses were evaluated (Figure 4 ). None of the hypotheses received the “very likely” score from more than 46% of the respondents, but most traits had at least one hypothesis that was considered “very likely” by more than 23% and likely (either “very likely” or “moderately likely”) by 72%–90%. Many of the intermediately popular hypotheses divided the respondents rather evenly between those who found them likely and those who found them unlikely (the latter referring to the scores “very unlikely” and “moderately unlikely” combined).
Credibility scores given by survey respondents to hypotheses that aim to explain the evolutionary origin of specific human traits. The hypotheses are sorted in order of decreasing popularity as estimated by the percentage of respondents who scored them likely (i.e., either “very likely” or “moderately likely”). Descriptions of the hypotheses as they were given in the survey are shown in Table 1
A causal relationship between articulate speech and descended larynx was accepted by most respondents, but there was no consensus on the direction of the causality. That the larynx descended because this was required by articulate speech was found likely by 84% and very likely by 43%. At the same time, that the evolution of speech was triggered by the descended larynx was found likely by 61% and very likely by 18%. In fact, 36% of the respondents scored both directions as equally likely.
Traits in the category “other” had only one explanatory hypothesis each in the survey, and this was water‐related. All of these hypotheses received many more “very unlikely” than “very likely” scores. However, four hypotheses (that baby swimming, profuse sweating, diving ability, and magnitude of diving reflex evolved as adaptations to a semi‐aquatic way of life) received so many “moderately likely” scores that the percentage of respondents who found them likely was slightly larger than the percentage who found them unlikely (Figure 4 ).
Details on how the hypotheses were scored by respondents representing different fields of expertise are shown in Figure 5 . In accordance with the statistical test results, most hypotheses received rather similar scores from respondents of all fields of expertise. However, (paleo)anthropologists were clearly more critical than representatives of the other fields in relation to several hypotheses, including: that nakedness evolved to avoid ectoparasites, that the big brain evolved because warfare caused pressure for higher intelligence, and that any traits evolved as adaptations to swimming or diving.
Frequencies of credibility scores given to hypotheses aiming to explain different traits (columns) by respondents of different fields of expertise (rows). In each panel, the answers are, from left to right, “very likely,” moderately likely,” “no opinion,” “moderately unlikely,” and “very unlikely.” Hypotheses that have been included in the aquatic ape hypothesis are shown in shades of blue and green. Those dryland hypotheses for which the opinions of anthropologists and other expertise groups clearly diverged are shown in magenta. The other hypotheses are in shades of brown, with darker colors given to hypotheses that received higher average credibility scores in the survey
There was a lot of variation among the traits in how many of the proposed explanations the respondents found convincing (Figure 6 ). For any one trait, 33%–64% of the respondents did not find any of the proposed hypotheses “very likely,” while 19%–38% found exactly one and 8%–45% more than one. Ten respondents (0.8%) explained that they did not score any of the hypotheses as likely, because they do not believe that humans have evolved at all (most of them explicitly referred to special creation by God).
The number of hypotheses (colors) proposed to explain each human trait (rows) that each respondent found very likely (left panel) or likely (either very likely or moderately likely; right panel). The total number of hypotheses included in the survey is shown after the name of each trait
The survey asked respondents’ opinions on twenty critical arguments that have been presented against the aquatic ape hypothesis. For most arguments, the modal response was “no opinion,” especially among those 43% of the respondents who had never heard of AAH before. Nevertheless, some arguments were clearly more frequently agreed with than others (Figure 7 and Table 3 ). The most widely accepted critique was that not all aquatic mammals have naked skin, so hairlessness cannot be considered an aquatic adaptation. In the other extreme, less than 3% of the respondents fully agreed and less than 12% mostly agreed with the critique that AAH is unscientific or not worthy of attention for the reasons given; in most cases, the number of respondents who strongly disagreed with these critiques was larger than the number who mostly or fully agreed.
The degree to which respondents representing different expertise fields agree with critique presented against the aquatic ape hypothesis. The full description of each point of critique can be found in Table 3
Points of critique presented against the aquatic ape hypothesis (AAH). The abbreviations are used in Figure 7 , and the full text is copied verbatim from the survey
Abbreviation | Critique |
---|---|
Hairy aquatics | Not all aquatic mammals have naked skin, so hairlessness cannot be considered an aquatic adaptation. |
Not parsimonious | AAH is less parsimonious than other proposed hypotheses: It has to explain both how human traits evolved in water, and how they were retained after return to land. |
Unnecessary | AAH is not needed, because all human traits can be explained by terrestrial scenarios. |
Coincidence | Humans may be similar to aquatic mammals in some traits, but this is only a coincidence and has no evolutionary relevance. |
No skeletal adaptations | AAH is not supported by fossil evidence, because this shows no skeletal adaptations to an aquatic environment. |
Determinism | A major problem with AAH is that it is based on extreme environmental determinism. |
Nonaquatic fossils | AAH is contradicted by the fossil record, because this suggests a permanently nonaquatic environment. |
Less consistent | AAH is internally less consistent than other proposed hypotheses. |
Apes swim | According to AAH, humans should swim better than apes and have more streamlined bodies, but they do not. |
Not enough time | There has not been enough time for an aquatic phase. |
Comparative anatomy | AAH is merely an exercise in comparative anatomy, not a scientific hypothesis. |
Conflicts evolution | AAH conflicts with what is known about evolutionary processes in general. |
Timing unknown | AAH lacks credibility, because its proponents do not agree on when and where the supposed aquatic phase took place. |
Simplistic | AAH is too simplistic to be taken seriously. |
Not peer‐reviewed | AAH can be ignored, because it was not published in a peer‐reviewed journal, and because it is mostly discussed in forums other than scientific journals. |
False evidence | AAH lacks credibility, because the evidence presented in its favor is false. |
Not professionals | AAH can be ignored, because its main proponents are not professionals in the field of human evolution. |
Pseudoscience | AAH is pseudoscience comparable to creationism. |
Cannot predict | AAH is unscientific, because it cannot make predictions. |
Feministic | AAH is unscientific, because it has been used in feministic argumentation. |
The main results of our survey can be summarized as follows: (1) There was no general agreement among the respondents on why any of the uniquely human traits have evolved: None of the proposed hypotheses was universally either accepted or rejected. (2) For any individual trait, the percentage of respondents who found none of the hypotheses “very likely” was between >30% (bipedalism) and >65% (nakedness). (3) In general, opinions on the credibility of the hypotheses were independent of a person's background (gender, age, field of expertise, degree of scientific experience), but (paleo)anthropologists were clearly more critical than representatives of other fields. (4) The hypotheses that mention adaptation to swimming or diving as an explanatory factor were found much less credible by (paleo)anthropologists and slightly more credible by human biologists than by biologists and representatives of other fields. (5) Most respondents were critical about the aquatic ape hypothesis (AAH), but only a small minority considered it to be unscientific.
Of course, all conclusions based on the survey data must be considered tentative only, because the response rate was very low, and it is possible that the results are biased. Members of some subgroup might have been more likely to respond than members of some other subgroup, and the average credibility scores given to the different hypotheses by the respondents may not be representative of the opinions of all scientists in the background population. However, it is unlikely that a lack of general agreement on the drivers of trait evolution or such a clear difference in opinion between (paleo)anthropologists and others could have emerged just as a result of biased sampling.
Our results did not reveal a set of explanations that would collectively provide a coherent and popular scenario for the origin of all (or even many) human traits. Indeed, some of the hypotheses that had almost equal and rather high average credibility scores explained the same trait, whereas for other traits, no hypothesis emerged as particularly popular. Against this background, it is interesting that almost half of the respondents fully or mostly agreed with the statement that the aquatic ape hypothesis “is not needed, because all human traits can be explained by terrestrial scenarios”.
The lack of agreement on why humans evolved the traits we have today is very obvious in our results: No hypothesis was universally accepted, and for most traits, there were several almost equally popular alternative hypotheses rather than one that would generally be considered superior to the others. None of the hypotheses received the score “very likely” from more than half of the respondents or obtained an average credibility score higher than 4.26 (of 5). For hairlessness, the most popular hypothesis was thought to be “very likely” by only 16% of the respondents, and its average credibility score (3.48) was closer to 3 (which is the limit between being considered more likely than unlikely) than to 4 (moderately likely). In addition, for only two of the traits (subcutaneous fat layer and descended larynx), the most popular hypothesis was found at least moderately likely by almost all respondents at the same time as the next most popular hypothesis was found clearly less likely. This may partly reflect the fact that fewer alternative hypotheses have been proposed for these traits than for many of the others included in the survey.
Importantly, lack of agreement did not reflect just ignorance on the topic among nonspecialists, because the responses were, in general, very similar between anthropologists and respondents representing other fields of science. In fact, anthropologists were even more skeptical about all hypotheses than representatives of the other fields were. In other words, outsiders were slightly more convinced that the proposed hypotheses are plausible than those who work in the field. Maybe anthropologists (especially paleoanthropologists) are more systematically trained to be wary of just‐so‐stories (explanations of past events and processes backed up by little or no evidence) than students in nearby fields are. It is also possible that outsiders are somewhat less likely to question hypotheses proposed within an unfamiliar field. This could be because they do not feel qualified to do so, or because they have not heard of the debates that draw attention to the weaknesses of the hypotheses.
Our results conform with the widespread belief that professionals in the field of human evolution are more critical toward the aquatic ape hypothesis (AAH) than outsiders are (Langdon, 1997 ; Bender et al., 2012 ; see also nonscientific sources such as Hawks, 2005 ; Moore, 2012 and Wikipedia: Aquatic Ape Hypothesis: Talk). However, this did not seem to be due to overall scientific ignorance, because how respondents assessed the credibility of the hypotheses proposing adaptation to swimming or diving was independent of both their overall scientific experience level and how they assessed the credibility of the other hypotheses. Interestingly, those whose main field of expertise is human biology had the most positive attitudes toward the water‐related hypotheses, giving them an average credibility score that was as much as 0.9 units higher (on a 1–5 scale) than the average score given by (paleo)anthropologists.
The difference in average opinion between (paleo)anthropologists and other scientists can be interpreted in two opposite ways. On the one hand, those who know the field of human evolution best may be best positioned to make a justified evaluation of the validity of the alternative hypotheses. On the other hand, prior knowledge may induce one to reject unconventional hypotheses offhand merely because they challenge the established paradigms of a field (Bender et al., 2012 ; Klayman, 1995 ). Obviously, the two interpretations lead to opposite conclusions on whether or not the critical attitude of the (paleo)anthropologists can be taken as evidence that AAH is flawed. In our survey, a vast majority of the respondents who had an opinion on the issue disagreed with the statement that AAH can be ignored because its main proponents are not professionals in the field of human evolution. This was the case both overall and within each field of expertise separately, although the proportion of respondents who agreed with the statement was higher among (paleo)anthropologists than among representatives of the other fields.
In this context, it is also interesting that the respondents’ assessment of the credibility of the water‐related hypotheses did not depend on the number of scientific papers they had authored. This indicates that established scientists are no more likely to reject or accept these hypotheses than junior scientists are—unless their scientific experience relates directly to the field of human evolution. A vast majority of the respondents disagreed with the critique that AAH is unscientific. Of course, this does not mean that they would consider the explanations proposed by AAH to be correct, and indeed, all the hypotheses related to AAH received relatively low credibility scores (although not as low as the least popular dryland hypotheses).
If, for the sake of argument, we accept the most popular explanation for each trait to be the correct one, a scenario of evolution by internal drive emerges: The large brain evolved because complex social organization required higher intelligence, the subcutaneous fat layer evolved to serve as an energy reserve for the developing brain, articulate speech evolved because there was social pressure for elaborate communication, the larynx descended because this was required by articulate speech, bipedalism evolved to make the use of tools and weapons easier, and nakedness evolved to avoid overheating when hunting. For most traits, the next most popular explanation was not far behind in popularity. Most of these were also based on inherent drivers, but sometimes in the opposite temporal sequence (e.g., articulate speech was triggered by the descended larynx; large brain evolved because it was required by articulate speech). We found this result disturbing, because the overwhelming popularity of hypotheses based on inherent drivers gives the impression that human evolution is generally thought to have been goal‐directed. This would be in conflict with the current understanding (explained in every evolutionary biology textbook) that evolution has no foresight.
Overall, the survey revealed no general agreement among the respondents: None of the proposed hypotheses on why specific uniquely human traits have evolved was universally either accepted or rejected. Nevertheless, identifying and quantifying what is not generally known and agreed upon can be useful in itself, as it may help to focus future research on answering the most important open questions. Clearly, there is still a long way to go before the question “why are humans so different from other primates” has been answered in a comprehensive and generally satisfactory way.
Conflict of interest.
None declared.
HT designed and conducted the survey and led the writing. All authors discussed the results and planned the data analyses together. The R code used to analyze the data and draw the figures was written by MT with contributions from JT.
Acknowledgments.
We thank Carlos Peña for writing the code to extract respondents’ email addresses from the Internet; Mirkka Jones, Kalle Ruokolainen, and Timo Vuorisalo for comments that helped to improve the survey questions; and Jouko Tuomisto for comments on the manuscript.
Tuomisto H, Tuomisto M, Tuomisto JT. How scientists perceive the evolutionary origin of human traits: Results of a survey study . Ecol Evol . 2018; 8 :3518–3533. https://doi.org/10.1002/ece3.3887 [ PMC free article ] [ PubMed ] [ Google Scholar ]
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