Changes in our hands and fingers were a side-effect of
changes in the shape of our feet and bipedalism freed the hand to evolve for
other purposes. Our ancestors did not have to use our forelimbs for locomotion
any more and a group of chimpanzee-like apes began to throw rocks and swing
clubs at adversaries, and this behaviour yielded reproductive advantages for
millions of years, driving natural selection for improved throwing and clubbing
prowess. The two fundamental human handgrips, first identified by J. R. Napier,
and named by him the ‘precision grip’ and ‘power grip’, represent a throwing
grip and a clubbing grip, thereby providing an evolutionary
explanation for the two unique grips, and the extensive anatomical remodelling
of the hand that made them possible[1].
The fingers, metacarpal and carpal bones of the chimpanzee
hand are elongated, but in typical primate fashion the thumb is small, weak and
relatively immobile The third and fourth metacarpals, which absorb the highest
compression during knuckle-walking, are especially robust [2]. Both proximal
and middle phalanges are curved toward the palm to withstand stress from
gripping limbs during arboreal locomotion. The finger tips are cone-shaped, and
lack broad apical tufts [2,3]. Thumb phalanges and metacarpals are slender and
short and the intrinsic muscles of the thumb, underlying the thenar region of
the palm, are small [4]. With this hand the chimpanzee’s grip is designed to
hang from trees – a hook grip. Because the thumb is weak and short, its distal
phalanx is relatively immobile and its distal pad cannot be opposed to those of
the fingers and it cannot generate a firm pinch or squeeze.
The human thumb in contrast is longer and stronger, the palm
and fingers are shorter, and the fingers have lost their curvature. The distal
phalanges have gained large apical tufts which support broad, palmar,
fibrofatty pads that distribute pressure during forceful grasping and whose
deformation accommodates the pads to uneven surfaces [2,3]. Apart from
thickening of the fifth metacarpal and enlargement of its base, the balance of
strength and robustness has shifted radially, to the thumb, second and third
fingers [2,5]. The thumb metacarpal articulates with the carpals in a saddle
joint which in combination with remodelling in the metacarpal–phalangeal joint
allows its distal pad to be placed against those of the other fingers,
providing full opposability [6]. The intrinsic thumb muscles are larger and
three new muscles have evolved in humans to add strength and control to thumb
movements. The flexor pollicis longus muscle, absent in chimpanzees, is the
most powerful thumb muscle in humans. It flexes the distal phalanx of the thumb
and maintains the orientation of its pad toward the fingers against pressure.
Also new are the deep head of the flexor pollicis brevis and the first volar
interosseous muscle [7]. Another unique attribute of the human hand is finger
rotation. When the fingers are flexed, they rotate towards the central axis so
that the fingertips can meet the tip of the thumb. The metacarpal–hamate
articulation permits supination of the fourth and fifth metacarpals, whereas
the heads of the second and third metacarpals allow pronation of the proximal phalanges
[2]. These modifications have given the human hand the two new grips – the
precision grip and the power grip.
Taking this concept of evolution a step further let us see
how we are designing a robotic hand. HANDLE (Developmental pathway towards autonomy
and dexterity in robot in-hand manipulation) is a Large Scale "Integrated
Project" funded by the European Union within The Seventh Framework
Programme FP7, in which nine European institutions, coordinated by the Pierre
and Marie Curie University of Paris (France), participate to build a robotic
arm and hand. . The objective of the project is to create a robotic hand that
can reproduce the abilities and movements of a human hand in order to achieve
the optimal manipulation of objects. The research carried out by the HANDLE
project's partners are in the area of visual perception, motion planning, new
sensors, acquisition of motor skills using artificial intelligence techniques,
etc.[8]
When trying to recreate the movements of a human hand with a
robotic system, there are several complex problems that must be resolved. In
the first place, there is a lack of space, the human hand is incredibly
complete, which makes it a challenge to try to put all of the necessary pieces
into the robotic hand and to integrate all of the actuators that allow for
mobility similar to that of a human hand. Again there are currently no sensors in
the market that are small enough to be integrated into the device so that it
can have sensitivity similar to that of a human hand and, thus, be able to make
precise movements.
Shadow Robot Compny’s Shadow Hand is perhaps the closest robot Hand to the human hand[9]. It
provides 24 movements, allowing a direct mapping from a human to the robot. The
hand contains an integrated bank of 40 Air Muscles which make it move and
produce precision grip and power grip. The Air Muscle is a simple yet powerful
device for providing a pulling force. It behaves in a very similar way to a
biological muscle. When actuated with a supply of compressed air, they contract
by up to 40% of its original length. The air muscles are compliant, which
allows the Hand to be used around soft or fragile objects. This hand can be
fitted with touch sensing on the fingertips, offering sensitivity sufficient to
detect a single small coin or manipulate delicate objects such as fruit and
eggs.
Finally, although the researchers may manage to make a
perfect robot from the mechanical and sensorial point of view, without
intelligence elements the device will not be able to function autonomously nor
adapt its movements and control to the characteristics of the objects, such as
their geometry, texture, weight or use.
So this only goes to suggest that evolution of the hand
alone did not help the human species. The associated development of the human
brain was invaluable.
References:
- Young RW. Evolution of the human hand: the role of throwing and clubbing. J Anat. 2003 January; 202(1): 165–174.
- Susman RL. Comparative and functional morphology of hominoid fingers. Am. J. Phys. Anthropol. 1979;50:215–236. [PubMed]
- Napier JR. Studies of the hands of living primates. Proc. Zool. Soc. London. 1960;134:647–657.
- Marzke
MW. Evolution of the hand and bipedality. In: Lock A, Peters A, editors. Handbook
of Human Symbolic Evolution.
- Marzke MW, Shackley MS. Hominid hand use in the Pliocene and Pleistocene: evidence from experimental archaeology and comparative morphology. J. Human Evol. 1986;15:439–460.
- Napier JR. The form and function of the carpo-metacarpal joint of the thumb. J. Anat. 1955;89:362–369. [PMC free article] [PubMed]
- Susman RL. Fossil evidence for early hominid tool use. Science. 1994;265:1570–1573. [PubMed]
- http://www.handle-project.eu/
- http:/www.shadowrobot.com
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