Martijn van Mensvoort -  Hand Research


APRIL 6, 2013

About Opposable Thumbs, 1D:5D digit ratio & Self-Awareness!

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!

Only the human hand is capable to make digit axis rotation resulting full pad contact between the thumb and index finger (the great human apes have imperfect opposable thumbs, where full pad to pad contact is not possible). What humans also have that no other animal species has is self-awareness.
It is this trait, a function of our higher intelligence, that is the single most important reason for our ascent to the dominant species on Earth. Therefore the opposable thumb & the capability of self-awareness make a unique 'human' pair.
Can the 1D:5D finger ratio serve as a marker for human self-awareness?

National Geographic video fragment: 'Self-recognition in apes'.
By looking at the differences and similarities between apes and us we may find the secret ingredient that makes apes apes, and humans humans. We take it for granted, but self-recognition is a fundamental building block of human society. We exist as individuals, each with our own unique features.

Scientists believe self-recognition is essential for our survival. We can live in large groups because we recognize similar features to our own in others!

Art by Koko the talking gorilla.

Piece of art by 'Koko', the talking gorilla.

Show a monkey a mirror, and it thinks it's another monkey: it attacks.
But how do our close relative the great apes react when faced with their image?

In 1970 psychologist Gordon Gallup Jr. developed an experiment test to determine whether an animal possesses the ability to recognize itself in a mirror. The 'Mirror Test' is now known as the primary indicator of self-awareness in non-human animals; it is said to mark entrance to the mirror stage by human children (at the age of about 18 months) in developmental psychology.

Bonobos, chimpanzees, orangutans have passed the mirror test. Initially, it was thought that gorillas were unable to pass the test, but there are now several well-documented reports of gorillas passing the test (such as Koko - the gorilla that made the piece of art on the left).

The National Geographic video above describes the full story! NOTICE: Non-primate species that have managed to pass the 'Mirror Test' are: dolphins, orcas, elephants & European magpies.


Opposable thumbs are a signature feature of the primate family, and played a large role in the ancient humans' invention and use of tools And the long, opposable human thumb became in science known as a symbol for human evolution.

"The movement of the thumb underlies all the skilled procedures
of which the hand is capable."

[John Napier in 'Hands', 1993]

An animal species is said to have opposable thumbs if the thumb is capable of bending in such a way that it can touch all the other digits on the hand. Opposable thumbs are shared by many primates, including most simians, and some prosimians (see picture below).

The human thumb shows a combination of four characteristics:
Comics by Chuckleduck: just give those opposable thumbs a chance.

Comics by Chuck Lea Duck:
'just give those opposable thumbs a chance'.
1) The human thumb rotates perfectly 'opposable' to all 4 fingers (digits), and only the human hand is capable to make digit axis rotation resulting full pad contact between the thumb and index finger. No other primates have this perfect characteristic (great apes & old world monkeys have imperfect opposable thumbs, and tufted capuchins are known to have a limited precision grip).

2) The human thumb has a low setting, and the human thenar & hypothenar start at a likewise proximal location to the wrist. This characteristic is only seen in human apes (in old- and new world monkeys the thenar is set more distally to the wrist; the ulnar side of their hands is more developed).

John Napier's power grip & precision grip. 3) The human thumb is long: usually it is longer than the pinky and it typically extends beyond the distal end of the palm. No other primates have this combined characteristic (some prosimians & new world monkeys have a thumb extending beyond the distal end of the palm, but in those species that is more due to a higher setting).

4) The human thumb has greater breadth in the distal phalanx than in the proximal phalanx, which makes a better pad-to-pad contact possible with the other fingers.

These four characteristics represent key-elements of the human 'twenty-four karat opposable thumb' (a term coined by neurologist Frank R. Wilson in his book 'The Hand' - 1998). The combination results in the capability to make the so-called 'power grip' and 'precision grip' - which were described in detail by physician John Napier in 'Hands' (1980).

Now, does the human opposable thumb correlate with self-awareness?

(Read further about how the opposable relates to self-awareness below)

(The drawn hands inside the picture above origin from A. H. Schultz, The Life of Primates, Universe Books, 1969)

Primatologists and hand research pioneers J. Napier and P. Napier defined opposable thumbs as:

"A movement by which the pulp surface of the thumb is placed
squarely in contact with - or diametrically opposite to - the
terminal pads of one or all of the remaining digits."


Scientists recognized that self-recognition is essential for our survival, and the opposable thumb is known to play a key-role in human survival at the evolutionary battleground with animals featured with a body much stronger than the human body! It appears not a matter of whether there is a link, the unanswered question is: how strong is the link?

The table below shows that there is a strong correlation between (im)perfect opposable thumbs and the (partial) capability of self-recognition via the 'Mirror Test' (which is believed by scientists to represent the primary indicator of self-awareness).

The table also presents an overview of 4 other significiant aspects of the thumb in each of the major primate family species, including: the so-called 'opposability index' (an index developed by Napier which describes the full hand ratio between digit 1 and digit 2), thumb setting, thumb extension & the thumb-pinky ratio (= D1:D5 ratio).

PBS Kids 'opposable thumbs' song.
All five thumb aspects appear significant for the result of the 'Mirror Test'. However, all five non-human ape species display the combination of imperfect opposable thumbs + a low set thumb; but only the great human apes (bonobo, chimpanzee, gorilla & orangutan) have the capability to pass the 'Mirror Test'!

Interestingly, even though the absolute length of the gibbon's (imperfectly) opposable thumb is relatively high, due to it's low setting it does not reach beyond the distal termination of the palm as seen in the human hand. But gibbons are not capable to pass the 'Mirror Test' (only 3 out of 12 gibbon species have shown some form of self-awareness, but no gibbon has ever passed the 'Mirror Test').

Only humans have the combination of the 5 thumb aspects featured in the table; and it is rather fascinating to see that the bonobo's thumb approaches the typical characteristics of the human thumb regarding in all 5 thumb dimensions!

(Read further about thumb markers for human awareness)

mirror test?
Perfect opposable
index > 60?
Low set
Thumb extends
beyond palm?
1D:5D digit ratio
> 1.00?




Yes (65)



Yes (1.11)

_Human apes:
Bonobos Yes Imperfect No (>55?) Yes Almost No (>90?)
Chimpanzees Yes Imperfect No (43) Yes No No (0.81)
Gorillas Some Imperfect No (48) Yes No No (0.81)
Organutans Yes Imperfect No (40) Yes No No (0.60)
Gibbons No, some partial Imperfect No (47) Yes No No (0.92)
_Old World Monkeys:
Macaques No, some partial Imperfect No (54) No No No (0.69)
Baboons No Imperfect No (58) No No No (0.80)
Guenon monkeys No Imperfect No: 51 No No No (0.69)
Longnose monkeys No Imperfect No (45) No No No (0.58)
Surili monkeys No Imperfect No (42) No No No (0.48)
Colobus monkeys No No No (48) No No No (<0.10)
_Old World Monkeys:
Capuchin monkeys No, but partial Imperfect Yes (61) No Almost No (0.89)
Squirrel monkeys No Imperfect Yes (61) No Almost No (0.86)
Tamarin monkeys No No Yes (69) No Almost No (0.71)
Night monkeys No Imperfect Yes (65) No Almost No (0.70)
Spider monkeys No No No (30) No No No (<0.10)
Tarsiers No No Yes (67) No Yes No (0.83)
_Non tarsier prosimians:
Lemurs No No, only pseudo Yes (??) Yes No No (0.??)
Aye-ayes No No, only pseudo Yes (??) Yes No No (0.??)
Bush babies No No, only pseudo Yes (81) Yes No No (0.71)
Lorises No No, only pseudo Yes (78) Yes Almost No (0.86)
Slow lorises No No, only pseudo Yes (55) Yes No No (0.72)
Potos No No, only pseudo Yes (60) Yes No No (0.73)

The 1D:5D digit ratio.

The 1D:5D finger ratio:
A thumb-marker for human self-awareness?

Though 'perfect opposable thumbs' became already for a long time known as a typically human characteristic, the data inside the table indicates there is one other thumb-dimension that appears to play a significant role: the 1D:5D digit ratio (= the ratio measured in the inner hand between absolute external thumb lengt vs. absolute external pinky finger length).

Only in the human hand is the (externalized) thumb length longer than the (externalized) length of the pinky finger.

This results in the so-called 'human digital formula': 3D > 4D > 2D > 1D > 5D.

(Simians typically have Sorrel's digital formula: 3D > 4D > 2D > 5D > 1D;
while prosimians typically have this digital formula: 4D > 3D > 5D > 1D > 2D)

Very little is known about the 1D:5D digit ratio; however, if the 1D:5D ratio is higher than 1.00... it can for sure be described as a typically human characteristic!

[NOTICE: The popular 2D:4D digit ratio can not serve as likewise 'evolutionary' marker, because various primate species typically have a 2D:4D digit ratio higher than the average human 2D:4D digit ratio, which is close to 0.95 (e.g. gibbons, capuchins & tarsiers often have a 2D:4D digit ratio that is often even much higher than 1.00)]

(More about the hand characteristics in primates...)




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