54 | New Scientist | 4 April 2020
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KEITHERSK
IN
E/ALAMYLend a hand
Is there any evolutionary
advantage to humans being
right or left-handed? Does
handedness exist in other animals?Steve Jacques
Head of Anatomy,
Leicester Medical School, UK
Hand or paw preferences are seen
in a wide variety of species from
other great apes to cats, dogs and
amphibians, frequently favouring
the right but not always. Even
fish have been observed to have
a preference for one pectoral fin
over the other, in this case the left.
The reason is often assumed to
be that the right hand is controlled
by the left hemisphere of the
brain, which is also dominant
for language in about 95 per cent
of us. However, this explanation
is questionable because most
left-handers are left-hemisphere
dominant for language as well.
An explanation that appeals
to me is the idea that hand
preference arises not from
asymmetries in the brain but
from asymmetries in the body.
We tend to think that the body is
functionally symmetrical, in other
words that right and left limbs can
be considered equivalent, albeit
mirror images. But this isn’t
actually the case. Consider the
movement known to anatomists
as “supination” – rotation of the
forearm so the palm of the hand
faces upwards. If you supinate the
right forearm, this is a clockwise
rotation, whereas supination
of the left is anticlockwise.
The opposite movement,
rotating the arm so that the
palm faces down, is known
a pronation. Supination is a
more powerful movement than
pronation because it is assisted
by the biceps muscle, whereas
pronation isn’t. This is probably
why most screws and other
threaded fixings require
clockwise turning to tighten
them, because most people
would rely on the more
powerful supination of the
right arm to do this.Could it be that our ancestors
needed to engage in a clockwise
rotation of their forearms to carry
out an activity, and the fact that
this is more powerful on the right
facilitated this and thus favoured
the right arm? Might it have been
the case that once tools requiring
clockwise rotation started to
become more commonplace,
cultural inheritance started to
interact with genetic factors
leading to an evolutionary shift
in favour of right-handers? We
can’t know for sure.David Muir
Edinburgh, UK
The fact that approximately
85 per cent of the population is
right-handed indicates that there
is an evolutionary advantage. The
fact that 15 per cent is left-handed
signifies that being so isn’t
particularly disadvantageous.
I used to feed a pair of carrion
crows, and the female would
skip to the food leading with
her left foot and hold the scrap
with the same foot to eat it.
The male would lead with his
right and feed with the right.This “handedness”, “footedness”,
or more correctly, laterality isn’t
uncommon in birds.Chris Daniel
Colwyn Bay, Conwy, UK
There is inconclusive evidence in
the scientific literature about the
advantages of being left-handed.
However, some studies show that
left-handers have a greater facility
for languages but perform more
poorly in maths.
I am left-handed and have a
career in science and engineering,
but my otherwise identical twin
brother is right-handed and a
professional linguist. If the above
studies are correct then we have
both been in the wrong jobs.Linda Dow
Berkeley, California, US
Parrots have handedness, or
rather footedness. They are
zygodactyl, which means theyhave two toes facing forwards and
two facing backwards. Most birds
have three going forwards and
one backwards. The orientation
of the toes makes parrots good at
climbing and picking things up
and holding them. They often
pick up a large piece of food
and hold it in one foot while
eating it a piece at a time. Most
are left-footed. Whichever foot
a bird uses is almost always the
same. My cockatoo is left-footed.Aural enhancement
Do people with bigger ears
hear better?The editor of this page,
Julia Brown, writes:
The function of the outer ear, or
pinna, is to amplify and channel
sound into the ear canal. Bigger
ears will do this slightly more,
but since the pinna amplifies
sounds by only about 15 decibels
( for comparison a whisper is about
30 decibels), any change is too
slight to make much difference.
More important is the shape
of the pinna. Its folds have evolved
to specifically amplify frequencies
of the human voice. The pinna is
also important for pinpointing the
height of a sound, as researchers
at the University of Montreal in
Canada found when they played
sounds to volunteers, first without
and then with silicone moulds
pressed into their outer ears. The
moulds made it more difficult for
people to tell whether a sound
source was above or below them.
However, after a week of
wearing the moulds, everyone
regained their ability to pinpoint
a sound, indicating that their
brains had learned to process
the new sound pattern. ❚This week’s new questions
Reptilian skill How are lizards able to walk upside down?
Vritant Kumar, Nalanda, IndiaTime to rhyme Why do we appreciate rhyming words in
songs and poetry? How long can one “hold” a sound while
waiting for a rhyme? Rod Tranchant, East Wittering, UKWhat enables a lizard
to walk upside down
on the ceiling?