Homo is the genus of hominids that includes modern humans and species closely related to them.
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 a study 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, pointed 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, researchers said.
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," researchers said.
"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," researchers added.
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.