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 primate hands family tree shows how dominant behavior in primates can be linked with hand structure. For example, gorillas & baboons are known to belong to the most dominant/aggressive primate species and both species have a hand structure that is different from other primate species!
It is rather fascinating to see that both species (gorilla + baboon) actually have a hand structure that reminds us of the typical hand differences seen between males and females, incuding: short fingers, broad palms & a low 2D:4D digit ratio.
These biological & evolutionary patterns appear to explain why in nearly all primate species males tend to dominate females. Except for the bonobos of course, females tend to collectively dominate males by forming alliances and use sexuality to control males – interestingly, a few years ago a study reported that bonobos have a rather human-like 2D:4D digit ratio (close to 0.94)… which is illustrative for their rather high emotional intelligence. The Bonobo-society has been described as ‘extraordinarily peacefull’.
This illustrates that Sir Charles Bell was very right about the hand representing capacity, and modern science has still a long way to go in order to understand properly how the structure of the hand corresponds with behavior!
|‘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!