‘Homo’ is the only primate whose tooth size decreases as its brain size increases

Andalusian researchers, led by the University of Granada, have discovered a curious characteristic of the members of the human lineage, classed as the genus Homo: they are the only primates where, throughout their 2.5-million year history, the size of their teeth has decreased alongside the increase in their brain size.

The key to this phenomenon, which scientists call “evolutionary paradox”, could be in how Homo’s diet has evolved. Digestion starts first in the mouth and, so, teeth are essential in breaking food down into smaller pieces. Therefore, the normal scenario would be that, if the brain grows in size, and, hence, the body’s metabolic needs, so should teeth.

However, in the case of Homo, this has not been the case, according to scientists in an article recently published in the journal BioMed Research International. The main author of the study, researcher Juan Manuel Jimenez Arenas, from the University of Granada’s Department of Pre-History and Archaeology, points out that “This means that significant changes must have occurred in order to maintain this trend”.

A change in diet, incorporating a higher amount of animal food, must have been one of the keys to this phenomenon. The quality leap in Homo’s diet, through a greater intake in animal proteins, fats and certain olio-elements, is essential for a correct working and maintenance of the brain. On a similar note, a larger brain allows greater social and cultural development, which, at that time, led to the achievement of important technological innovations.

In order to validate this theory, the researchers evaluated the relationship between the size of post-canine teeth and the volume of the endocranium in a wide set of primates, among which were found the main representatives of Homo fossils. “Before we started the study, it was well known that, throughout the evolution of humans, tooth-size diminished and brain-size increased. We have established that they are two opposing evolutionary trends that have been linked for 2.5 million years, when our first ancestors within the Homo genus first appeared on the evolutionary stage”.

Genetic Study

The study’s authors also relate these changes to the inactivation of gene MYH16, linked to temporalis musculature, which fell in size approximately 2.4 million years ago. This would do away with an important barrier for encephalization (a hypertrophied temporalis musculature prevents the development of the cranial dome). Likewise, they analyzed their relationship with the inactivation of gene SRGAP2, which helps towards the evolution of the neo-cortex, playing a principal role in human brain development.

This study has been carried out thanks to the collaboration of Juan Manuel Jiménez Arenas, along with three renowned lecturers and researchers from the University of Malaga: Paul Palmqvist and Juan Antonio Pérez Claros, from the Dept. of Ecology and Geology, and Juan Carlos Aledo, from the Dept. of Biochemistry and Molecular Biology.

Header Image : Left: Upper and lower views of cranium OH5 (Paranthropus boisei). Right: Same views of Homo Sapiens cranium.

Contributing Source : University of Granada

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  • Sherelle Elizabeth Rippy

    Interesting removing 7 of my teeth based on someone elses (imposters) dental exam when I was in Army Active Duty on Dec 14, 1999 at Ft. Jackson, SC doesn’t count. I didn’t have any dental exam for the military nor while in the military before Dec 14, 1999 even though my dental records say different. Who’s counting, truly not the military, I’m not sure the military really knows how may soldiers and staff it has because of the fact it knowingly allows identity fraud and crimes associated with fraud.

  • Phil Blank

    You shou.d have renewed your military ID card a long time ago!
    They removed your SS# from your card about 6 years ago!
    I’ve never had any theft from the military regarding my ID.
    And I went in back in the 70s, and yes, they did my dental in boot camp.

  • Phil Blank

    So we better not get any smarter or we’ll be drinking all our meals in the future!

  • Cynde Fetherston

    Sherelle Elizabeth Rippy: WHAT has that got to do with this?