Elsevier

Medical Hypotheses

Volume 94, September 2016, Pages 108-111
Medical Hypotheses

Handedness in man: The energy availability hypothesis

https://doi.org/10.1016/j.mehy.2016.06.018Get rights and content

Summary

More than 90% of the human species are right handed. Although outwardly our body appears symmetrical, a 50/50% lateralization in handedness never occurs. Neither have we seen more than 50% left handedness in any subset of the human population. By 12–15 weeks of intrauterine life, as many as 6 times more fetuses are noted by ultrasound studies to be sucking on their right thumbs. Distinct difference in oxygenation leading to dissimilar energy availability between right and left subclavian arteries in place by week 9 of life may hold the clue to the lateralization of hand function and eventually, the same in the brain. We know there is a higher incidence of left handedness in males, twins, premature babies and those born to mothers who smoke. They may represent a subset with less distinct difference in oxygenation between the 2 subclavian arteries during the fetal stage. This hypothesis if correct not only closes the gap in understanding human handedness and lateralization but also opens a vista for new research to focus on in utero tissue energy availability and its impact on outcome in life.

Introduction

Many theories have been put forth to explain why more than 90% of the human species are right handed [1]. None however, has adopted an energy driven approach to look at the subject. Over the last few decades detailed research has given us new insights and a better understanding of embryogenesis and fetal growth [2]. We now understand that the low levels of oxygen in the developing embryos stimulate the production of hypoxia inducible factors which can impact the development of organ structure including blood vasculature [3]. The difference seen in the vasculature between both subclavian arteries in early fetal stage may hold the clue to the phenomenon.

Section snippets

Difference in oxygenation between the right and left subclavian arteries

The fetus gets its oxygen and energy supply through the mother via the placenta and umbilical vein. The oxygen and energy are then pumped out by the fetal heart to the various organs. By 9 weeks of gestation, the 6 pharyngeal arches are already reduced to the adult form where the 4th arch predominates [4]. The left ventricle by then is in place to pump the slightly more oxygenated blood to the aorta preferentially perfusing the coronary arteries and the three great vessels of the aorta. The

The hypothesis

All fetuses go through the 9 months in a fairly hypoxic environment having to draw its oxygen supply from the outside environment through the mother. The longer journey for oxygen to arrive at the tissues of the fetus sees the death annually of approximately 3 million fetuses as stillbirths compared to half a million parturients during their pregnancies [11]. There are other evidences of the hypoxic environment – the two main ones are the high hemoglobin all babies are born with as part of the

Making sense of the literature with regards to left handedness, right handedness, non-right handedness

The determination of handedness is not confined to the assessment of hand preference for a single process although the most obvious task that is focused on by the lay public is writing. For the last 45 years we have two very definitive handedness inventory where hand preference of up to 10 tasks are determined to categorize handedness in a person [23], [24]. Many handedness studies suffer from failure to use these inventories and interpretation of their results therefore becomes suspect [17].

The

Testing of the hypothesis

We can potentially use blood level oxygen dependent MRI and Doppler ultrasound to determine if there is a difference in oxygenation between the 2 subclavian arteries [28] and correlate it to the thumb sucking preference of the fetus. It could be used to confirm if indeed those fetuses noted to be expressing left thumb sucking has less oxygenation difference between the 2 arteries compared to those who are right thumb sucking.

The hypothesis can also alternatively be tested by collecting enough

Implications of the hypothesis

The hypothesis does not seek to replace all the existing relevant studies especially with regards to how the primary cilia causes body asymmetry [35], [36]. It is aimed at filling the gap in thinking with regards to how the body asymmetry, especially with respect to the fetal cardiovascular system, brings about handedness [5], [34].

The hypothesis also aims to raise the awareness that oxygenation and energy availability may ultimately be the “controller” of handedness and other lateralized

Author contribution

Y.K. Chan conceived the hypothesis, researched extensively and wrote the paper. P.S. Loh discussed, contributed and prepared the manuscript for submission.

Author information

Y.K. Chan is a Professor of Anesthesiology in University of Malaya, Malaysia and author of the Acute Care book series who is a partially corrected left handed obstetric anesthesiologist who teaches physiology, believes the world revolves around energy and has keen interest on the impact of oxygen in energy transformation. She has been a long time thinker on the phenomenon of left handedness.

P.S. Loh is a Senior Lecturer of Anesthesiology in University of Malaya, Malaysia. She is a right handed

Conflict of interest

Authors declare no conflicts of interest.

Acknowledgements

None.

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