Evolutionary origin of human traits

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Evolutionary origin of human traits is a variable page about the topic. For a survey study looking at researchers' opinions about this topic, see How scientists perceive the evolutionary origin of human traits: results of a survey study

Question

Why and how did humans become so different from other apes?

Rationale

Hypotheses

There are conflicting hypotheses to explain why the traits that so clearly distinguish humans from other primates originally evolved. One idea is that the ancestors of humans came to live in a different kind of environment than the ancestors of chimpanzees and gorillas, and hence experienced different selection pressures and obtained a suite of unique traits as adaptations to the new environment. What that new environment was and which selection pressures were most important has been debated, however, and a number of hypotheses based on ideas other than environmental adaptation have also been proposed. To date, general discussion on the topic seems mostly to have focused on finding merit or flaws in one hypothesis at a time. The purpose of this page is to provide a structured forum for the general evaluation and comparison of the different hypotheses on human origins.

Below you can find a list of hypotheses explaining different traits that separate humans from other great apes. They were a part of a survey done on people's understanding about the reasons behind human traits, answered by paleontologist, biologists, medical professionals, and other people with professional understanding about evolution (human or in general) or how human bodies work. Some analysis done on the survey answers can be found below. Some hypotheses much discussed in literature in the last years are missing from the survey (and thus he list below), such as the postural feeding hypothesis, stating bipedalism originally arose as a feeding posture and not as a way of locomotion, and the expensive tissue hypothesis, which suggests the energy needed for building a big brain came from reducing some other expensive tissue in our body, namely the gut. A literary review was done on the same hypotheses asked about in the survey to find out whether the understanding about human evolution the answerers had was the same that could be gotten from scientific literature published in the last decades. Some analysis on the literary review results is found on this page.

The short version of the hypotheses in the list below are the ones used to refer to them in the analysis.

Hypotheses and their shorter versions

For short	Hypothesis
Bipedalism
Energy efficiency	When covering long distances on the ground, walking or running erect on two legs is energetically more efficient than walking or running on four legs.
Thinner branches	In the canopy, walking erect facilitates using multiple supports (as in orangutans) and hence makes it possible to move on thinner branches than when brachiating or moving quadrupedally.
Wading	In a littoral habitat, walking erect allows wading in deeper water with the nostrils above the surface (apes cross water bodies bipedally), and the same posture increases streamlining when swimming and diving for food (as in penguins).
Thermoregulation-b	Walking erect helps in thermoregulation in the savanna by exposing less skin to the midday sun and more skin to cooling wind.
Better view	Walking erect makes it possible to see above the savanna grass and hence spot danger from further away.
Foraging	Walking erect makes foraging more efficient, because hands are not needed for locomotion.
Carrying food	Walking erect makes it easier for a male to carry high-quality food such as meat to the female and infants.
Carrying baby	Walking erect makes it possible for a female to carry its offspring in its arms.
Tool use	Walking erect makes it easier to use tools and weapons.
Genitals	Walking erect is favored by sexual selection, as it makes the genitals more visible.
Big brain
Meat	A shift in diet towards eating more meat triggers encephalization, because meat is rich in energy.
Fish	A shift in diet towards eating more fish and other seafood triggers encephalization, because seafood is rich in both energy and the omega-3 fatty acids that are an essential component of brain tissue.
Cooking	The use of fire triggers encephalization, because cooking increases the nutritional value of plant foods.
Social	Complex social organization causes pressure for greater intelligence and hence triggers encephalization.
Collaborative hunting	Collaborative hunting causes pressure for greater intelligence and hence triggers encephalization.
Language	Spoken language causes pressure for greater intelligence and hence triggers encephalization.
Warfare	Warfare causes pressure for greater intelligence and hence triggers encephalization.
Neoteny	Encephalization is a secondary effect of neoteny (the retention of juvenile features into adulthood), which is advantageous when specialized adult morphology adapted to one environment has become maladaptive in a new environment.
Bipedalism-b	Encephalization is triggered by bipedalism, which changes the blood circulation and provides a cooling mechanism for the larger brain.
Nakedness	Encephalization is triggered by nakedness, which provides a cooling mechanism for the larger brain.
Nakedness
Skin contact baby	Direct skin-to-skin contact strengthens the emotional bond between a female and its nursing offspring.
Skin contact sex	Direct skin-to-skin contact makes sex more enjoyable, and is favored by sexual selection.
Cleanliness	In animals that feed messily on carrion, naked skin stays cleaner than hairy skin (or feather-covered skin as in vultures).
Ectoparasites	In mammals that live in permanent nests, naked skin helps to avoid a high ectoparasite load.
Drag	In mammals that live partly or entirely entirely in water, fur is often lost because it causes drag when swimming but fails to provide efficient insulation when wet (e.g. walrus, hippopotamuses, dolphins).
Thermoregulation-h	In mammals that hunt on the savanna, naked skin dissipates heat more efficiently and reduces the risk of becoming overheated.
Large mammal	Large mammals can regulate their body temperature without investing in hair, and humans are relatively large compared to other primates.
Clothes	Once the use of clothes has become common, fur becomes unnecessary.
Subcutaneous fat
Energy supply	In conditions of variable food supply, subcutaneous fat can store energy for times of food scarcity, and in infants it secures the development of the large brain.
Insulation	In wet conditions, subcutaneous fat provides more efficient insulation than hair does, and it makes swimming easier by increasing buoyancy and streamlining of the body.
Savannah thermoregulation	Subcutaneous fat is an adaptation to thermoregulation in the savanna, together with nakedness and sweating.
Body shape	Subcutaneous fat defines the body shape and its evolution is driven by sexual selection.
Descended larynx
Articulation	Articulate speech requires a descended larynx, because this makes it possible to produce a wider variety of sounds.
Impressive voice	A descended larynx makes the voice stronger and more impressive, and can evolve through sexual selection (as in the males of some deer).
Diving-l	A descended larynx can evolve as an adaptation to diving (as in some aquatic mammals), because it makes it possible to close the air passages when under water and to inhale rapidly through the mouth when surfacing.
Speech
Larynx	Speech is triggered by the descended larynx, which allows making a wider variety of sounds.
Diving-s	Speech requires voluntary breath control, which can evolve as an adaptation to diving. In water, visual and olfactory cues are inadequate and therefore liable to be replaced by vocal communication (as in whales).
Bipedalism-s	Speech requires voluntary breath control, which can evolve after bipedalism frees breathing from the constraint posed by the mechanics of locomotion.
Reassurance	Speech provides a means for females to reassure their offspring who have to be put down while foraging.
Sociality	Social pressure for more elaborate communication triggers evolution of speech.
Hunting	Collective hunting requires a means of effective communication and therefore triggers evolution of speech.
Culture	Transmitting cultural tradition (e.g., how to cope with unusually severe droughts) from one generation to the next requires a means of effective communication and therefore triggers evolution of speech.
Other traits that could be explained by an aquatic phase in human evolution
Baby swimming	Human babies can be taken for a swim long before they can walk. They are comfortable in water and capable of holding their breath when submerged.
Nose	Unlike apes, humans have an arched nose and flexible nostrils. These help prevent water from entering the respiratory tract when diving.
Smell	Humans have a relatively weak sense of smell, as aquatic mammals often do.
Webbing	Humans have partial webbing between their fingers and toes. Webbed feet are common among semi-aquatic animals (such as otters and ducks), but are not found in non-human primates.
Eccrine glands	Cooling sweat is excreted from eccrine glands in humans but from apocrine glands in other primates. Apocrine glands could have lost their thermoregulatory function in human ancestors during a period when dip-cooling replaced sweat-cooling.
Sweating	Humans sweat more profusely than any other primate. Since this can lead to fatal loss of water and electrolytes in a few hours, the trait probably evolved in conditions of abundant water and salt supply.
Diving-o	Compared to other primates, humans are stronger swimmers and can dive both deeper and further.
Apnea	The diving reflex (slowing down of heartbeat and oxygen usage in water) increases the resistance of the brain to apnea, and its magnitude in human divers is comparable to that in semi-aquatic mammals such as otters and beavers.
Fond of water	Compared to other primates, humans are unusually fond of immersing themselves in water. This is manifested in the popularity of beach holidays, swimming and bathing.

The answerers' opinions should be used as continuous [0,1] variables in such a way that each answer is transformed into its quantile of all answers to that question. Then neural networks, Bayesian belief network analyzers (such as B-source), or other statistical tools can be applied.

For single answer analyses, multinomial regression models should be applied.

In the following sections, we will present results about support to different hypotheses explaining the traits studied. We have two sources of information and three graphs for each hypothesis: Survey to authors of relevant journals, with ca. 1200 responses; and authors of articles about human evolution and the traits of interest (272 individual authors). The survey graphs on the left show the repondents' views on how likely each hypothesis is. The middle graphs show the same information separated by the expertise of the respondent (and for visibility, we assumed the X axis to be continous so that we could draw probability densities). The graphs on the right show the support from article authors in the literature review. Each hypothesis has one row of graphs, and they are ordered by the trait the hypotheses explain.

Bipedalism

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Brain

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Nakedness

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Subcutaneous fat

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Larynx

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Speech

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Other traits

Other traits that could be explained by a waterside hypothesis

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Survey

The graph above shows the results for correlations done from the survey answers between all different hypotheses. In most cases the correlations were quite small, and almost always positive. There are only a few negative correlations in these answers, and in them the correlations are very small, -0.14 being the strongest negative correlation. The most notable correlations are between the hypotheses that suggest a semi-aquatic ancestor. If one believes in one, then all the water-relates hypotheses seem likely, and if one doesn't, they seem unlikely. Interestingly, eating of fish and other sea food enabling brain growth does not correlate with the rest of the water-related hypotheses, even though if fish was needed for brain growth, it indicades humans lived on shores, fishing and wading, thus probably spending a significant time in water, which is also the basis for all the other waterside hypotheses. The fish hypothesis does however have a correlation with eating meat being a major step toward bigger brains, as well as cooking the food, which makes it possible to get more energy from it.

The graph below shows the results for principal components analysis done on the survey data. All the water-related hypotheses (except eating fish) are at the top right corner, and the rest are at the bottom. This means those people who have an opinion about one water-related hypothesis are more or less the same opinion about the rest of them. What people thought about the water-related hypotheses doesn't seem to have much of a connection to their opinions about the rest of the hypotheses.

For other hypotheses it seems that the idea that brain developed as a response to hairlessness convinces the same people as the hypotheses of hairlessness evolving to enhance skin contact and bipedalism to show off genitals. Completely different people were the people who found it convincing that the brain developed to enable speaking or hunting and that the subcutaneous fat layer developed as an energy supply.

The sizes for the dots tell about how likely the hypothesis was found to explain the trait in the survey. The bigger the dot, the more likely people thought it was. The numbers from the scaling were gotten by calculating the average answer (1 being very likely and 5 being very unlikely), and subtracting that from 5 (to get the likeliest ones with the highest values instead of the lowest).

Literary review

Hidden below you can find a summary of the articles found in a literary review on the evolution of human traits done in the summer 2016 to find out, which hypothesis on which human traits are currently discussed in scientific articles, and which according to them seem the most likely ways the human traits have evolved. F, A and N in the last column stand for For, Against and Neutral, according to which point of view is argued in the article about the hypothesis in question.

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Literary review summary(-) Obs Article Year Type Hypothesis Trait For Against Neutral 1 Ko 2015-1 2015 Review article Better view Bipedalism A 2 Videan & McGrew 2002 2002 Research article Better view Bipedalism A 3 Bender et al. 2007 2007 Review article Better view Bipedalism A 4 Sutou 2012 2012 Essay Carrying baby Bipedalism F 5 Watson et al. 2008 2008 Research article Carrying baby Bipedalism A 6 David-Barret & Dunbar 2016 2016 Research article Carrying food Bipedalism A 7 Ko 2015-1 2015 Review article Carrying food Bipedalism F 8 Videan & McGrew 2002 2002 Research article Carrying food Bipedalism F 9 Carvalho et al. 2012 2012 Comment Carrying food Bipedalism F 10 Sutou 2012 2012 Essay Carrying food Bipedalism F 11 Lieberman 2011 2011 Essay Energy efficiency Bipedalism F 12 Rodman & McHenry 1980 1980 Research article Energy efficiency Bipedalism F 13 David-Barret & Dunbar 2016 2016 Research article Energy efficiency Bipedalism N 14 Sockol et al. 2007 2007 Research article Energy efficiency Bipedalism F 15 Leonard & Robertson 1995 1995 Comment Energy efficiency Bipedalism F 16 Leonard & Robertson 2001 2001 Comment Energy efficiency Bipedalism F 17 Isbell & Young 1996 1996 Review article Energy efficiency Bipedalism F 18 Pontzer et al. 2009 2009 Research article Energy efficiency Bipedalism F 19 Pickford 2002 2002 Review article Energy efficiency Bipedalism F 20 Halsey & White 2012 2012 Research article Energy efficiency Bipedalism A 21 Wang et al. 2003 2003 Research article Energy efficiency Bipedalism A 22 Steudel 1996 1996 Research article Energy efficiency Bipedalism A 23 Taylor & Rowntree 1973 1973 Comment Energy efficiency Bipedalism A 24 Steudel-Numbers 2003 2003 Research article Energy efficiency Bipedalism A 25 Crompton et al. 1998 1998 Research article Energy efficiency Bipedalism A 26 Wheeler 1984 1984 Review article Energy efficiency Bipedalism N 27 Carey & Crompton 2005 2005 Research article Energy efficiency Bipedalism N 28 Videan & McGrew 2002 2002 Research article Foraging Bipedalism F 29 David-Barret & Dunbar 2016 2016 Research article Thermoregulation-b Bipedalism N 30 Wheeler 1984 1984 Review article Thermoregulation-b Bipedalism F 31 Morgan 1993 1993 Review article Thermoregulation-b Bipedalism A 32 Porter 1993 1993 Comment Thermoregulation-b Bipedalism F 33 Wheeler 1994 1994 Comment Thermoregulation-b Bipedalism F 34 Wheeler 1991-1 1991 Research article Thermoregulation-b Bipedalism F 35 Wheeler 1991-2 1991 Research article Thermoregulation-b Bipedalism F 36 Ruxton & Wilkinson 2011 2011 Research article Thermoregulation-b Bipedalism A 37 Ko 2015-1 2015 Review article Thermoregulation-b Bipedalism A 38 Chaplin et al. 1994 1994 Research article Thermoregulation-b Bipedalism A 39 Bender et al. 2007 2007 Review article Thermoregulation-b Bipedalism A 40 Roberts & Thorpe 2014 2014 Review article Thinner branches Bipedalism F 41 Stanford 2006 2006 Research article Thinner branches Bipedalism F 42 Crompton et al. 2010 2010 Review article Thinner branches Bipedalism F 43 Sigmon 1971 1971 Research article Thinner branches Bipedalism F 44 Stanford 2002 2002 Brief communication Thinner branches Bipedalism F 45 Senut 2006 2006 Book section Thinner branches Bipedalism F 46 Thorpe 2009 2009 Research article Thinner branches Bipedalism F 47 Tuttle 1981 1981 Thinner branches Bipedalism F 48 Preuschoft 2004 2004 Review article Thinner branches Bipedalism F 49 Thorpe et al. 2007-1 2007 Report Thinner branches Bipedalism F 50 Thorpe et al. 2014 2014 Debate Thinner branches Bipedalism F 51 Crompton et al. 2003 2003 Thinner branches Bipedalism F 52 Gebo 1996 1996 Research article Thinner branches Bipedalism A 53 Begun et al. 2007 2007 Comment Thinner branches Bipedalism A 54 Thorpe et al. 2007-2 2007 Comment Thinner branches Bipedalism F 55 Wheeler 1984 1984 Review article Tool use Bipedalism A 56 Harcourt‐Smith 2007 2007 Book section Tool use Bipedalism A 57 Bender et al. 1997 1997 Wading Bipedalism F 58 Morgan 1993 1993 Review article Wading Bipedalism F 59 Wrangham et al. 2009 2009 Research article Wading Bipedalism F 60 Niemitz 2010 2010 Review article Wading Bipedalism F 61 Kuliukas 2002 2002 Research article Wading Bipedalism F 62 Kuliukas 2009 2009 Review article Wading Bipedalism F 63 Kuliukas et al. 2009 2009 Research article Wading Bipedalism F 64 Bender et al. 2007 2007 Review article Wading Bipedalism F 65 Isler & van Schaik 2014 2014 Review article Collaborative hunting Brain F 66 Carmody & Wrangham 2009 2009 Research article Cooking Brain N 67 Joordens et al. 2014 2014 Research article Fish Brain F 68 Broadhurst et al. 2002 2002 Review article Fish Brain F 69 Broadhurst et al. 1998 1998 Review article Fish Brain F 70 Cunnane 2007 2007 Comment Fish Brain F 71 Cunnane et al. 2007 2007 Short report Fish Brain F 72 Cunnane & Crawford 2003 2003 Review article Fish Brain F 73 Cunnane et al. 1993 1993 Research article Fish Brain F 74 Cunnane & Crawford 2014 2014 Research article Fish Brain F 75 Verhaegen 1995 1995 Brief communication Fish Brain F 76 Braun et al. 2010 2010 Research article Fish Brain F 77 Carlson & Kingston 2007-1 2007 Research article Fish Brain A 78 Carlson & Kingston 2007-2 2007 Comment Fish Brain A 79 Langdon 2006 2006 Review article Fish Brain A 80 Lakatos & Janka 2008 2008 Language Brain F 81 Brown & Greenhood 1991 1991 Review article Language Brain F 82 Falk 1980 1980 Review article Language Brain F 83 Previc 2002 2002 Brief communication Meat Brain F 84 Speth 1989 1989 Research article Meat Brain A 85 Ibraimov 2007 2007 Review article Nakedness Brain N 86 Shea 1989 1989 Review article Neoteny Brain A 87 Sandel et al. 2016 2016 Research article Social Brain F 88 Shultz & Dunbar 2010 2010 Research article Social Brain F 89 Shultz & Dunbar 2007 2007 Review article Social Brain F 90 Navarrete et al. 2011 2011 Letter Social Brain F 91 Dunbar 1992 1992 Research article Social Brain F 92 Dunbar 1998 1998 Research article Social Brain F 93 Dunbar 2003 2003 Review article Social Brain F 94 Allman 1999 1999 Book section Social Brain A 95 Falk 1980 1980 Review article Warfare Brain A 96 Pitt 1978 1978 Review article Warfare Brain F 97 Pawlowski 1999 1999 Body shape Fat N 98 Cunnane & Crawford 2003 2003 Review article Energy supply Fat F 99 Wells 2006 2006 Research article Energy supply Fat F 100 Correia et al. 2004 2004 Research article Energy supply Fat F 101 Rebora 2010 2010 Review article Insulation Fat A 102 Wheeler 1984 1984 Review article Savannah thermoregulation Fat F 103 Pawlowski 1998 1998 Savannah thermoregulation Fat F 104 Pawlowski 1999 1999 Savannah thermoregulation Fat F 105 Rantala 2007 2007 Review article Clothes Hairlessness A 106 Pagel & Bomder 2003 2003 Letter Clothes Hairlessness N 107 Pagel & Bomder 2004 2004 Book section Clothes Hairlessness N 108 Rebora 2010 2010 Review article Clothes Hairlessness A 109 Kushlan 1985 1985 Review article Clothes Hairlessness N 110 Glass 1966 1966 Letter Clothes Hairlessness F 111 Hershkovitz 1966 1966 Letter Clothes Hairlessness A 112 Brace et al. 1966 1966 Letter Clothes Hairlessness A 113 Rantala 2007 2007 Review article Drag Hairlessness A 114 Pagel & Bomder 2003 2003 Letter Drag Hairlessness A 115 Rebora 2010 2010 Review article Drag Hairlessness A 116 Wheeler 1985 1985 Review article Drag Hairlessness A 117 Rantala 2007 2007 Review article Ectoparasites Hairlessness F 118 Pagel & Bomder 2003 2003 Letter Ectoparasites Hairlessness F 119 Rantala 1999 1999 Discussion Ectoparasites Hairlessness F 120 Dean & Siva-Jothy 2012 2012 Letter Ectoparasites Hairlessness F 121 Pagel & Bomder 2004 2004 Book section Ectoparasites Hairlessness F 122 Dunn 1966 1966 Comment Ectoparasites Hairlessness A 123 Rebora 2010 2010 Review article Ectoparasites Hairlessness A 124 Giles 2011 2011 Review article Skin contact baby Hairlessness F 125 Giles 2011 2011 Review article Skin contact sex Hairlessness F 126 Rantala 2007 2007 Review article Skin contact sex Hairlessness A 127 Wheeler 1984 1984 Review article Thermoregulation-h Hairlessness F 128 Ruxton & Wilkinson 2011 2011 Research article Thermoregulation-h Hairlessness F 129 Ibraimov 2007 2007 Review article Thermoregulation-h Hairlessness F 130 Rantala 2007 2007 Review article Thermoregulation-h Hairlessness A 131 Pagel & Bomder 2003 2003 Letter Thermoregulation-h Hairlessness A 132 Rebora 2010 2010 Review article Thermoregulation-h Hairlessness A 133 Wheeler 1985 1985 Review article Thermoregulation-h Hairlessness F 134 Schwarts & Rosenblum 2005 2005 Research article Thermoregulation-h Hairlessness F 135 Lupi 2008 2008 Review article Thermoregulation-h Hairlessness F 136 Wheeler 1996 1996 Comment Thermoregulation-h Hairlessness F 137 Wheeler 1992 1992 Research article Thermoregulation-h Hairlessness F 138 Ebling 1985 1985 Review article Thermoregulation-h Hairlessness F 139 Kushlan 1985 1985 Review article Thermoregulation-h Hairlessness F 140 Brace et al. 1966 1966 Letter Thermoregulation-h Hairlessness F 141 Amaral 1996 1966 Review article Thermoregulation-h Hairlessness N 142 Davidson 2003 2003 Review article Articulation Larynx F 143 Shprintzen 2003 2003 Comment Articulation Larynx A 144 Nishimura et al. 2006 2006 Research article Articulation Larynx A 145 Frey et al. 2011 2011 Impressive voice Larynx F 146 Frey et al. 2015 2015 Research article Impressive voice Larynx F 147 Hombert 2010 2010 Book section Impressive voice Larynx F 148 Fitch & Reby 2001 2001 Research article Impressive voice Larynx F 149 Fitch 2002 2002 Review article Impressive voice Larynx F 150 Apicella & Feinberg 2009 2009 Research article Impressive voice Larynx N 151 Apicella et al. 2007 2007 Letter Impressive voice Larynx N 152 Falhman & Schagatey 2014 2014 Review article Diving-o Other F 153 Schagatey 2011 2011 Book section Diving-o Other F 154 Verhaegen 1993 2013 Review article Nose Other F 155 Mladina et al. 2009 2009 Review article Nose Other N 156 Verhaegen 1995 1995 Brief communication Smell Other F 157 Verhaegen et al. 2007 2007 Book section Smell Other F 158 Wheeler 1991-2 1991 Research article Sweating Other A 159 Wheeler 1992 1992 Research article Sweating Other A 160 MacLarnon & Hewitt 1999 1999 Research article Bipedalism-s Speech A 161 Stout & Chaminade 2012 2012 Review article Culture Speech F 162 Corballis 2009 2009 Book section Culture Speech F 163 Verhaegen 1995 1995 Brief communication Diving-s Speech F 164 Verhaegen et al. 2002 2002 Opinion Diving-s Speech F 165 MacLarnon & Hewitt 1999 1999 Research article Diving-s Speech A 166 Lieberman 1973 1973 Review article Hunting Speech N 167 Shprintzen 2003 2003 Comment Larynx Speech A 168 Nishimura et al. 2006 2006 Research article Larynx Speech A 169 Falk 2004 2004 Review article Reassurance Speech F 170 Brown & Greenhood 1991 1991 Review article Sociality Speech F 171 Dunbar 1993 1993 Sociality Speech F 172 Hamilton 1974 1974 Review article Sociality Speech F 173 Kurland & Beckerman 1985 1985 Sociality Speech F 174 Locke 2001 2001 Review article Sociality Speech F 175 Ko 2015-2 2015 Review article Sociality Speech N

There are may histograms drawn from the review data to show how many of the found articles support or argue against all of the different hypotheses. They also show which hypotheses are the most talked about in the literature. Another set of histograms were drawn from the amount of authors writing the articles about each of the hypotheses. For this, all the authors for all articles were listed and plotted the same way as he articles, being for, against or writing something neural about the hypothesis. Before drawing the histograms the data was processed so that one author was counted only once to support or oppose a certain hypothesis, even if they had authored more than one article on the topic. You can see the histograms by trait from the authors below. Green color means supporting, red opposing and blue writing something neutral about the topic. All of the histograms can be found here and here. Note that not all of the hypotheses in the survey were discussed at all in any of the articles in the literature review.

The lower graph above shows relations between writers (small dots) and hypotheses (bubbles), or how the different writers wrote about hypotheses. Green lines mean supporting, red lines mean arguing against, and grey lines mean writing something on the topic, but not being clearly for or against the hypothesis. It shows us which hypotheses are discussed by the same authors, and whether the author supported both, opposed both, or whether they're hypotheses that one person doesn't think likely to both be true.

Several observations can be made from the graph. First, most authors only discussed a single hypothesis. The maximum number of different hypothesis discussed by a single author was six ----#: . CHECK THE DATA --Jouni (talk) 12:48, 4 September 2016 (UTC) (type: truth; paradigms: science: comment), but few authors discussed more than two hypotheses.

Second, some areas, notably nakedness, stimulated many authors to discuss several possible hypotheses. Another similar area was bipedalism, where, interestingly, authors supporting thinner branch hypothesis simultaneously opposed energy efficiency hypothesis. However, in the survey data these two hypotheses were not correlated.

Third, there are some isolated hypotheses such as neoteny, culture, and impressive voice, where the authors of these topics did not discuss any of the other hypotheses, and also none of the other authors discussed these hypotheses.

It can be concluded that there is no single study that would systematically look at the evolution of important human traits and evaluate strengths and weaknesses of related hypotheses.

Student survey

Hypotheses and reasonings related to bipedalism.

A questionnaire was sent to students to study whether they found the reasoning of each hypothesis convincing.

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Student survey(-) Obs Trait Hypothesis Reasoning For Against Neutral 1 Bipedalism Better view You see further when on two legs F 2 Bipedalism Carrying items Chimps carry food in hands F 3 Bipedalism Carrying items Females prefer males that can carry food F 4 Bipedalism Carrying items Hairless mothers had to carry babies F 5 Bipedalism Carrying items Carrying baby on either side uses energy A 6 Bipedalism Carrying items First tools date 1.5 M years after bipedalism A 7 Bipedalism Energy efficiency Knuckle walk is least energy efficient F 8 Bipedalism Energy efficiency Bipedalism is at least as efficient as tetrapedalism F 9 Bipedalism Energy efficiency Easy to develop efficient bipedalism F 10 Bipedalism Energy efficiency For chimps bipedalism is less efficient than on knuckles A 11 Bipedalism Energy efficiency Bipedalism is not more efficient than tetrapedalism A 12 Bipedalism Energy efficiency Knees-bent bipedalism is not efficient A 13 Bipedalism Foraging Current primates walk on two legs when eating from bushes F 14 Bipedalism Foraging First apes have structures for forging but not for long-distance walk F 15 Bipedalism Foraging On two legs you can reach higher F 16 Bipedalism Foraging During foraging, bipedalism is efficient F 17 Bipedalism Foraging There was no reason to focus on standing-height foods A 18 Bipedalism Thermoregulation-b Smaller surface area F 19 Bipedalism Thermoregulation-b Longer operation times on savannah F 20 Bipedalism Thermoregulation-b No other savannah animals are bipedalists A 21 Bipedalism Thermoregulation-b There are easier ways to thermoregulate A 22 Bipedalism Thermoregulation-b Humans did not actually develop on savannah A 23 Bipedalism Thinner branches This is how orangutangs walk F 24 Bipedalism Thinner branches Chimps cannot extend their legs like humans and orangutangs F 25 Bipedalism Thinner branches Human scapula resembles that of orangutangs F 26 Bipedalism Thinner branches Chimp climbing is like bipedalism F 27 Bipedalism Thinner branches Many attachment points help moving on thinner branches F 28 Bipedalism Thinner branches First erect humans had thumb-like pollux F 29 Bipedalism Thinner branches Wrist bone association with knuckle walk is false F 30 Bipedalism Thinner branches If gorilla and chimp ancestors walk on knuckles, so did human ancestors A 31 Bipedalism Thinner branches Orangutangs are poor walkers on two legs. A 32 Bipedalism Thinner branches If human-oranguntang ancestors walked on trees, that should be seen in chimps and gorillas A 33 Bipedalism Tools use Bipedalism helps carrying tools around F 34 Bipedalism Wading Apes walk on two legs when wading F 35 Bipedalism Wading Two other primate species waded F 36 Bipedalism Wading Shallow waters offer a lot of food F 37 Bipedalism Wading Boyance helps bipedalism F 38 Bipedalism Wading Bipedalism makes body streamlined for swimming F 39 Bipedalism Wading Primates usually don't go to water A 40 Bipedalism Wading From trees to ground is more parsimonious than trees-water-ground A

+ Show code- Hide code library(OpasnetUtils) library(ggplot2) library(igraph) dat <- opbase.data("Op_en5865",subset="Student survey") #order <- c("F", "N", "A") #dat$Result <- factor(dat$Result, levels=order) for(i in 1:ncol(dat)) {dat[[i]] <- as.character(dat[[i]])} edg <- data.frame( from = c(dat$Reasoning, dat$Trait), to = rep(dat$Hypothesis, 2), Etype = c(rep("If then", nrow(dat)), rep("Makes relevant", nrow(dat))), FAN = c(dat$Result, rep("", nrow(dat))) ) edg <- edg[!duplicated(paste(edg$from, edg$to)) , ] ver <- data.frame( name = c(dat$Reasoning, dat$Trait, dat$Hypothesis), Vtype = rep(c("Reasoning", "Trait", "Hypothesis"), each = nrow(dat)) ) ver <- ver[!duplicated(ver$name) , ] dag <- graph.data.frame(edg, directed = TRUE, vertices = ver) colo <- c("Red", "Blue", "Pink", "Orange", "Brown", "Yellow", "Purple") plot( dag, vertex.label.cex = 0.8, vertex.size = ifelse(V(dag)$Vtype == "Hypothesis", 6, 3), vertex.label.family = "Helvetica", vertex.color = ifelse(V(dag)$Vtype == "Hypothesis", colo[as.factor(V(dag)$Trait)], "SkyBlue2" ), vertex.label = V(dag)$name, #ifelse(V(dag)$Vtype == "Hypothesis", V(dag)$name, NA), edge.color = c("Gray", "Red", "Green")[as.factor(E(dag)$FAN)], edge.arrow.size = 0.3, layout = layout.fruchterman.reingold, asp = 0.7 )

Calculations

calc1

+ Show code- Hide code library(OpasnetUtils) library(ggplot2) # Creates a sample data a. a <- data.frame( Occupation = rep(c("Biology", "Paleontology"), each = 3), Question = c("A", "B", "C"), Answer = sample(1:5, size = 180, replace = TRUE) ) # Creates a simple histogram with font size 24. # Then makes a grid of these histograms based on different values of Occupation * Question ggplot(a, aes(x=Answer)) + geom_histogram() + theme_grey(base_size = 24) + facet_grid(Occupation~Question, scale='free')

• Data version 1: raw data in Excel from Webropol: free text columns removed [1]
• Data version 2: cleaned with this code [http://en.opasnet.org/en-opwiki/index.php/Special:R-tools?id=

number of X column (1-25): number of fill column (empty = all): Encryption key: + Show code- Hide code library(OpasnetUtils) library(ggplot2) # Fetch the stored raw data (requires a password to open it). objects.get("Gygi7pqJFrngDdbx") data <- objects.decode(etable, key) # Print and plot examples of the cleaned data. titles <- c( "- ", "- ", "- ", "- ", "Anthropology or archaeology", "Biology (animal physiology, anatomy or morphology) ", "Biology (ecology) ", "Biology (evolution) ", "Biology (genetics or molecular biology) ", "Biology (other, please specify) ", "Geology ", "Human cardiovascular or respiratory system ", "Human musculoskeletal system ", "Human nervous system ", "Human nutrition ", "Other aspects of human biology (please specify) ", "Paleoanthropology ", "Paleontology ", "Other, please specify ", "Peer reviewed articles in scientific journals ", "Articles or books targeted at the general public ", "Peer reviewed articles in scientific journals ", "Articles or books targeted at the general public ", "- ", "- ", "When covering long distances on the ground, walking or running erect on two legs is energetically more efficient than walking or running on four legs. ", "In the canopy, walking erect facilitates using multiple supports (as in orangutans) and hence makes it possible to move on thinner branches than when brachiating or moving quadrupedally. ", "In a littoral habitat, walking erect allows wading in deeper water with the nostrils above the surface (apes cross water bodies bipedally), and the same posture increases streamlining when swimming and diving for food (as in penguins). ", "Walking erect helps in thermoregulation in the savanna by exposing less skin to the midday sun and more skin to cooling wind. ", "Walking erect makes it possible to see above the savanna grass and hence spot danger from further away. ", "Walking erect makes foraging more efficient, because hands are not needed for locomotion. ", "Walking erect makes it easier for a male to carry high-quality food such as meat to the female and infants. ", "Walking erect makes it possible for a female to carry its offspring in its arms. ", "Walking erect makes it easier to use tools and weapons. ", "Walking erect is favored by sexual selection, as it makes the genitals more visible. ", "A shift in diet towards eating more meat triggers encephalization, because meat is rich in energy. ", "A shift in diet towards eating more fish and other seafood triggers encephalization, because seafood is rich in both energy and the omega-3 fatty acids that are an essential component of brain tissue. ", "The use of fire triggers encephalization, because cooking increases the nutritional value of plant foods. ", "Complex social organization causes pressure for greater intelligence and hence triggers encephalization. ", "Collaborative hunting causes pressure for greater intelligence and hence triggers encephalization. ", "Spoken language causes pressure for greater intelligence and hence triggers encephalization. ", "Warfare causes pressure for greater intelligence and hence triggers encephalization. ", "Encephalization is a secondary effect of neoteny (the retention of juvenile features into adulthood), which is advantageous when specialized adult morphology adapted to one environment has become maladaptive in a new environment. ", "Encephalization is triggered by bipedalism, which changes the blood circulation and provides a cooling mechanism for the larger brain. ", "Encephalization is triggered by nakedness, which provides a cooling mechanism for the larger brain. ", "Direct skin-to-skin contact strengthens the emotional bond between a female and its nursing offspring. ", "Direct skin-to-skin contact makes sex more enjoyable, and is favored by sexual selection. ", "In animals that feed messily on carrion, naked skin stays cleaner than hairy skin (or feather-covered skin as in vultures). ", "In mammals that live in permanent nests, naked skin helps to avoid a high ectoparasite load. ", "In mammals that live partly or entirely in water, fur is often lost because it causes drag when swimming but fails to provide efficient insulation when wet (e.g. walrus, hippopotamuses, dolphins). ", "In mammals that hunt on the savanna, naked skin dissipates heat more efficiently and reduces the risk of becoming overheated. ", "Large mammals can regulate their body temperature without investing in hair, and humans are relatively large compared to other primates. ", "Once the use of clothes has become common, fur becomes unnecessary. ", "In conditions of variable food supply, subcutaneous fat can store energy for times of food scarcity, and in infants it secures the development of the large brain. ", "In wet conditions, subcutaneous fat provides more efficient insulation than hair does, and it makes swimming easier by increasing buoyancy and streamlining of the body. ", "Subcutaneous fat is an adaptation to thermoregulation in the savanna, together with nakedness and sweating. ", "Subcutaneous fat defines the body shape and its evolution is driven by sexual selection. ", "Articulate speech requires a descended larynx, because this makes it possible to produce a wider variety of sounds. ", "A descended larynx makes the voice stronger and more impressive, and can evolve through sexual selection (as in the males of some deer). ", "A descended larynx can evolve as an adaptation to diving (as in some aquatic mammals), because it makes it possible to close the air passages when under water and to inhale rapidly through the mouth when surfacing. ", "Speech is triggered by the descended larynx, which allows making a wider variety of sounds. ", "Speech requires voluntary breath control, which can evolve as an adaptation to diving. In water, visual and olfactory cues are inadequate and therefore liable to be replaced by vocal communication (as in whales). ", "Speech requires voluntary breath control, which can evolve after bipedalism frees breathing from the constraint posed by the mechanics of locomotion. ", "Speech provides a means for females to reassure their offspring who have to be put down while foraging. ", "Social pressure for more elaborate communication triggers evolution of speech. ", "Collective hunting requires a means of effective communication and therefore triggers evolution of speech. ", "Transmitting cultural tradition (e.g., how to cope with unusually severe droughts) from one generation to the next requires a means of effective communication and therefore triggers evolution of speech. ", "Human babies can be taken for a swim long before they can walk. They are comfortable in water and capable of holding their breath when submerged. ", "Unlike apes, humans have an arched nose and flexible nostrils. These help prevent water from entering the respiratory tract when diving. ", "Humans have a relatively weak sense of smell, as aquatic mammals often do. ", "Humans have partial webbing between their fingers and toes. Webbed feet are common among semi-aquatic animals (such as otters and ducks), but are not found in non-human primates. ", "Cooling sweat is excreted from eccrine glands in humans but from apocrine glands in other primates. Apocrine glands could have lost their thermoregulatory function in human ancestors during a period when dip-cooling replaced sweat-cooling. ", "Humans sweat more profusely than any other primate. Since this can lead to fatal loss of water and electrolytes in a few hours, the trait probably evolved in conditions of abundant water and salt supply. ", "Compared to other primates, humans are stronger swimmers and can dive both deeper and further. ", "The diving reflex (slowing down of heartbeat and oxygen usage in water) increases the resistance of the brain to apnea, and its magnitude in human divers is comparable to that in semi-aquatic mammals such as otters and beavers. ", "Compared to other primates, humans are unusually fond of immersing themselves in water. This is manifested in the popularity of beach holidays, swimming and bathing. ", "AAH conflicts with what is known about evolutionary processes in general. ", "A major problem with AAH is that it is based on extreme environmental determinism. ", "AAH is not needed, because all human traits can be explained by terrestrial scenarios. ", "AAH is merely an exercise in comparative anatomy, not a scientific hypothesis. ", "Not all aquatic mammals have naked skin, so hairlessness cannot be considered an aquatic adaptation. ", "Humans may be similar to aquatic mammals in some traits, but this is only a coincidence and has no evolutionary relevance. ", "According to AAH, humans should swim better than apes and have more streamlined bodies, but they do not. ", "AAH lacks credibility, because the evidence presented in its favor is false. ", "AAH is not supported by fossil evidence, because this shows no skeletal adaptations to an aquatic environment. ", "AAH is contradicted by the fossil record, because this suggests a permanently non-aquatic environment. ", "AAH lacks credibility, because its proponents do not agree on when and where the supposed aquatic phase took place. ", "There has not been enough time for an aquatic phase. ", "AAH is too simplistic to be taken seriously. ", "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. ", "AAH is internally less consistent than other proposed hypotheses. ", "AAH is unscientific, because it cannot make predictions. ", "AAH is unscientific, because it has been used in feministic argumentation. ", "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. ", "AAH can be ignored, because its main proponents are not professionals in the field of human evolution. ", "AAH is pseudoscience comparable to creationism. ", "- ", "Articles in scientific journals ", "Books by Elaine Morgan ", "Books by other authors ", "Articles or programs in popular media (press, TV, radio) ", "University courses on human evolution ", "Personal communication from someone who knew the hypothesis ", "Blogs or other personal web pages ", "Wikipedia ", "AAH was rejected as implausible. ", "AAH was mentioned or described, but no opinion on its validity was expressed. ", "AAH was found more plausible than alternative hypotheses. " ) # oprint(head(data)) X <- colnames(data)[X] graph <- function(data, x, fillcol) { fillname <- colnames(data)[fillcol] my_title <- wrapper(titles[fillcol], width = 50) out <- ggplot(data, aes_string(x = x, weight = 1, fill = fillname)) + geom_bar(position = "fill", na.rm = TRUE) + theme_grey(base_size = 24) + labs(title = my_title) return(out) } wrapper <- function(x, ...) paste(strwrap(x, ...), collapse = "\n") if(is.null(fillcol)) fillcol <- 26:108 # If not defined, show all result columns as graphs. for(i in fillcol) { print(graph(data, X, i)) }

Histograms

A code for drawing histograms from the literary review data showing the number of articles and and authors supporting and arguing against different hypotheses. You can see all the histograms by clicking "Run code".

• Model run 13.9.2016

+ Show code- Hide code library(OpasnetUtils) library(ggplot2) library(igraph) summary_data <- opbase.data("Op_en5865",subset="Literary review summary") authors <- opbase.data("Op_en5865",subset="Literary review authors") authors_summary <- merge(summary_data, authors,by="Article") # Merged tables authors_short <- authors_summary[,c("Trait","Hypothesis","Result.x","Result.y")] # Irrelevant columns taken out authors_uniq <- unique(authors_short) # A list of authors only defending of attacking one hypothesis once. #ggplot(data=summary_data, aes(summary_data$Result)) + geom_histogram() #ggplot(summary_data, aes(Result, fill = Hypothesis)) + geom_histogram() # eri hypoteesit eri värillä #ggplot(summary_data, aes(Hypothesis, fill = Result)) + geom_histogram() # F/A/N eri värillä hypoteeseittain. hypothesis.order <- c("Tool use","Foraging","Better view","Energy efficiency","Carrying baby","Carrying food","Thermoregulation-b","Wading","Thinner branches","Genitals","Social","Language","Collaborative hunting","Warfare","Meat","Neoteny","Fish","Cooking","Bipedalism-b","Nakedness","Thermoregulation-h","Ectoparasites","Large mammal","Clothes","Drag","Cleanliness","Skin contact sex","Skin contact baby","Energy supply","Savannah thermoregulation","Insulation","Body shape","Articulation","Impressive voice","Diving-l","Sociality","Hunting","Culture","Larynx","Bipedalism-s","Reassurance","Diving-s","Baby swimming","Sweating","Diving-o","Apnea","Nose","Fond of water","Smell","Webbing","Eccrine glands") authors_uniq$Hypothesis <- factor(authors_uniq$Hypothesis, levels=hypothesis.order) ## histograms per trait in articles for(i in unique(summary_data$Trait)){ trait2 <- ggplot(summary_data[summary_data$Trait == i,], aes(Hypothesis, fill = Result)) + geom_histogram() + labs(x="", y="Count", title= paste(i, "in articles")) + theme_grey(base_size=24) print(trait2) } ## histograms per trait by author for(i in unique(authors_uniq$Trait)){ authors2 <- ggplot(authors_uniq[authors_uniq$Trait == i,], aes(Hypothesis, fill = Result.x)) + geom_histogram() + labs(x="", y="Count", title= paste(i, "by author")) + theme_grey(base_size=24) print(authors2) } order <- c("F", "N", "A") authors_uniq$Result.x <- factor(authors_uniq$Result.x, levels=order) summary_data$Result <- factor(summary_data$Result, levels=order) ## Histograms wrapped by trait in articles for(i in unique(summary_data$Trait)) { trait <- ggplot(summary_data[summary_data$Trait == i,], aes(Result)) + geom_histogram(fill="blue") + labs(x = "", y = "count", title = paste(i, " in articles")) + theme_grey(base_size=24) + facet_wrap(~Hypothesis) print(trait) } ## Histograms wrapped by trait by authors for(i in unique(authors_uniq$Trait)) { authors <- ggplot(authors_uniq[authors_uniq$Trait == i,], aes(Result.x)) + geom_histogram(fill="blue") + labs(x="", y="Count", title = paste(i, " by author")) + theme_grey(base_size=24) + facet_wrap(~Hypothesis) print(authors) } ###Summary graphs authors_all <- ggplot(authors_uniq, aes (Hypothesis, fill = Result.x)) + geom_histogram() + labs(x="", y="count") + theme_grey(base_size=20) + theme(axis.text.x=element_text(angle=90,hjust=1,vjust=0.4)) authors_all + facet_wrap(~Trait, ncol=4, scales="free_x") #all <- ggplot(summary_data, aes(Hypothesis, fill=Result)) + geom_histogram() + labs(x="", y="count") #all + facet_wrap(~Trait, ncol=4, scales="free_x) ## All hypotheses separately for(i in unique(authors_uniq$Hypothesis)) { authors_uniq$weight <- ifelse(authors_uniq$Hypothesis == i, 1, 0) temp <- ggplot(authors_uniq, aes(x=Result.x, weight=weight)) + geom_bar(fill="blue") + labs(x = "", y = "count", title = paste(i, " by authors")) + theme_grey(base_size=24) + coord_cartesian(ylim = c(0, 25)) print(temp) } #oprint(odag) #?odag #odag() #authors_for <- authors_uniq[authors_uniq$Result.x == "F",] #oprint(authors_for) #authors_against <- authors_uniq[authors_uniq$Result.x == "A",] #oprint(authors_against) edg <- data.frame( from = authors_uniq$Result.y, to = authors_uniq$Hypothesis, Trait = authors_uniq$Trait, FAN = authors_uniq$Result.x ) ver <- rbind( data.frame( name = authors_uniq$Result.y, Trait = NA ), data.frame( name = authors_uniq$Hypothesis, Trait = authors_uniq$Trait ) ) ver <- ver[!duplicated(ver$name) , ] ver$Type <- ifelse(ver$name %in% authors_uniq$Hypothesis, "Hypothesis", "Author") dag <- graph.data.frame(edg, directed = TRUE, vertices = ver) colo <- c("Red", "Blue", "Pink", "Orange", "Brown", "Yellow", "Purple") plot( dag, vertex.label.cex = 0.4, vertex.size = ifelse(V(dag)$Type == "Hypothesis", 6, 3), vertex.label.family = "Helvetica", vertex.color = ifelse(V(dag)$Type == "Hypothesis", colo[as.factor(V(dag)$Trait)], "SkyBlue2" ), vertex.label = ifelse(V(dag)$Type == "Hypothesis", V(dag)$name, NA), edge.color = c("Red", "Green", "Gray")[as.factor(E(dag)$FAN)], edge.arrow.size = 0.3, layout = layout.fruchterman.reingold, asp = 0.7 )

See also

How scientists perceive the evolutionary origin of human traits: results of a survey study

Human evolution hypotheses described in Wikipedia:

• Aquatic ape hypothesis
• Endurance running hypothesis
• Gathering hypothesis
• Hunting hypothesis
• Hypotheses explaining bipedalism
• Killer ape hypothesis

Keywords

Human evolution, Bipedalism, Brain, Hairnessless, Subcutaneous fat, Speech