The human opposable thumb: with reason neurologist Frank R. Wilson called it ‘the Twenty-Four-Karat thumb’. Because thumb studies in primates have revealed that only humans have ‘perfect opposable thumbs’. But there is more, because new evidence indicates that the opposable thumb has a direct correlation with self-awareness in the primate family!
Scientists say that there are two things that have given rise to man’s dominance as a species: one is the opposable thumb, the other is self-awareness. Interestingly, scientists also developed a test to assess ‘self-recognition’ (which is in psychology known as a first milestone in the development of self-awareness in young children) via the so-called ‘mirror test’.
The video below shows how this ‘mirror test’ has also been used to assess the capability for self-recognition in primates. And the table below displays a strong correlation between self-recognition and the opposable thumb in the primate family tree (+ a the bottom a picture is presented summarizing the hand characteristics seen in the listed primate species).
The video ‘Evolution of the human hand’ – presents a detailed picture of how modern science perceives the evolution of the human hand in time. The video is sort of based on Darwin’s evolution theory, but the details were delevered by experts in anthropology who studied how the hand shape, finger length & palmar creases evolved during the past 1.8 million years.
The video demonstrates how the ‘early’ humanoid hands (and primates) are typically featured with 3 or more ‘complete transverse creases’ (multiple simian lines), which are positioned horizontal in the hand + two major vertical lines. While at the end of the video displays a typical human hand featured with only 2 curved, oblique positioned ‘primary palmar creases’ (heart line and head line) + one major vertical line (life line).
And the differences between the human hand and the hands of primates served as a model for the evolution of the human hand in time – see below the hands of a man compared to the hands of a baboon, orangutang & chimpanzee.
Another important figure in the history of medical science was the Scottish surgeon John Hunter, who turned the attention of science from the structure of hands to it’s function:
“Structure is the intimate expression of function”
- John Hunter, Scottish surgeon (1728-1793) -
More details about the evolution of other features of the human hand are presented in the articles:
Hand shape varies between the two sexes: males typically have larger, relatively broader hands, while females typically have smaller, relatively narrower hands. But hand shape also varies among the various populations around the world: in Asia the avarage hand shape is relatively narrower than in European & North American countries. And there is even a link between hand shape & intelligence!
But in order to understand these patterns properly, one first has to understand the relations ship between hand shape & body length. Because in general, all longer populations in the world (males, Europeans & North Americans) typically display a relatively broad hand shape, while all small populations (females, Asians) typically display a relatively narrow hand shape.
A good measure to describe the shape of the hand is the so-called ‘hand index’, which is defined as the ratio between the ‘hand width’ (= palm width measured at the metacapals) vs. the ‘hand length’ (= the distance between the tip of the middle finger and the distal wrist crease).
The average ‘hand index’ in human kind is close to 0.44, and is typically much higher than the ‘hand index’ seen in primates, which is typically (far) below 0.40 (though in gorillas – the largest of all primate species – the ‘hand index’ is higher than 0.40).
NOTICE: Finger length can also be measured relative to ‘hand length’ & ‘hand width’, but that topic will be discussed later.
HAND SHAPE & SEXE:
In males the ‘hand index’ is typically higher than 0.44, and measures above 0.45 are often seen. While in females the ‘hand index’ is typically lower than 0.44, and measures below 0.43 are not rare at all. These sexe differences are for a large part the result of the body height differences between males and females.
HAND SHAPE & RACE:
The average ‘hand index’ among the various races differs significantly, and is typically lower among asians. In people from China does not vary a lot from the average of human kind, but among for example people from Japan & India the ‘hand index’ is typically close to 0.43 or even lower. At least partly these differences are explained by racial differences in body height.
HAND SHAPE & IQ:
A high hand index typically correlates with a low IQ. And this link between hand shape & IQ has been confirmed in quite a few studies among various types of populations.
In a 1980 study in the former Yugoslavia reported among 540 men a negative correlation between hand index & all 10 measures for IQ.
Regarding the sexe differences, one has to be aware of the earlier mentioned point that tall populations typically have a higher ‘hand index’. The fact that women have a lower ‘hand index’ compared to men, is largely neutralized by the fact that women are smaller than men. And therefore there one should not associate this sexe difference with IQ differences among the sexes (because so far there is no evidence for that at all).
And finally there is evidence that when the ‘hand index’ is corrected for body height, then this appears to explain a significant part of the IQ differences that are typically seen between the nations of the world. Though this issue has not been studied thoroughly.
HIGH OR LOW ‘HAND INDEX’ IN INDIVUALS:
The above describes patterns for hand shape implicate that regarding the implications of a high or low ‘hand index’ indivuals, one always has to consider sexe & race before jumping into conclusions!
Time for a hand shape palm reading… what is your ‘hand index’?
|‘Whorls’ are a common features in the hands of many primate species!
What are the major differences between the hands of primate species and the human?
• 1 – Primates usually have a shorter thumb than humans – the thumb of the macaque (see photo on the left) does not rearch out behond the distal border of the handpalm.
• 2 – Primates usually have a lower ‘2D:4D digit ratio’ than humans – the hand of the macaque is featured with a much longer ring finger (digit 4) than the pointer finger (digit 2).
|• 3 – Primates usually have more fingerprint- and palmar whorls than humans – the hand of the macaque is featured with 5 palmar whorls.
• 4 – Primates always have a lower ‘ridge density’ than humans.
• 5 – Primates usually have (various) palmar transversal creases, a.k.a. ‘simian lines’ – the hand of the macaque has one ‘simian line’.
SUGGESTIONS FOR FURTHER READING:
PHOTO: Impression from the back of the hand of a macaque:
November 14, 2009
Research at the universities of Liverpool and Oxford into the finger length of various primate species has revealed that cooperative behavior is linked to exposure to hormone levels in the womb!
The sientists have used finger ratios as an indicator of the levels of exposure to the hormone and compared this data with social behaviour in primate groups.
The team found that Old World monkeys, such as baboons and rhesus macaques, have a longer fourth finger in comparison to the second finger, which suggests that they have been exposed to high levels of prenatal androgens. These species tend to be highly competitive and promiscuous, which suggests that exposure to a lot of androgens before birth could be linked to the expression of this behaviour.
Other species, such as gibbons and many New World apes, have digit ratios that suggest low levels of prenatal androgen exposure. These species were monogamous and less competitive than Old World monkeys.
The results show that Great Apes, such as orangutans and chimpanzees, expressed a different finger ratio. The analysis suggests that early androgen exposure is lower in this groups compared to Old World monkeys. Lower androgen levels could help explain why Great Apes show high levels of male cooperation and tolerance.
HOW CAN WE UNDERSTAND THESE RESULTS?
Primate finger length researcher Emma Nelson explains:
“It is thought that prenatal androgens affect the genes responsible for the development of fingers, toes and the reproductive system. High androgen levels from a foetus or mother during pregnancy, may alter gene function and lead to subtle changes in relative digit length and the functioning of the reproductive system. Finger ratios do not change very much after birth and appear to tell us something about how very early androgens affect adult behaviour, particularly behaviour linked to mating and reproduction.”
ILLUSTRATION: A comparison of the human hand with primate hands reveals that only the human hand is featured with a long opposable thumb!
SUGGESTION FOR FURTHER READING:
• Finger length linked with social behavior!
In may 2009 Dr Roland Ennos and his team at The University of Manchester presented fingerprints findings with a surprizing outcome. Fingerprints do not help primates grip, as previously thought. Instead, a fingerprint actually reduces the friction needed to hold onto flat surfaces.
Dr Ennos disproved the long-held assumption that fingerprints help primates to grip with a simple machine, three strips of perspex and the right hand of Masters student Peter Warman. They tested the student’s grip on every finger + thumb at three different widths of perspex as the machine pulled the perspex strips down via a weight in a plastic cup. The researchers also tested grip at three different angles by bending both the fingers + the thumb. This wide range of testing conditions allowed them to separate pressing force from the contact area and overcome any confounding variables.
The results indicate that fingertips act more like rubbers than hard solids; their friction coeficient fell at higher normal forces and friction was higher when fingers were held flatter against wider sheets and hence when contact area was greater.
WHY DO WE HAVE FINGERPRINTS?
“The experiment was so simple, this discovery could have been made 100 years ago; but scientists make assumptions and tend to look at complicated things instead.
My preferred theory is that they allow the skin to deform and thus stop blistering. That is why we get blisters on the smooth parts of our hands and feet and not the ridged areas: our fingerpads, palms and soles.”
SUGGESTIONS FOR LEARNING MORE ABOUT FINGERPRINTS: