When did we say goodbye to the monkeys? Because of one small mutation, the human jaw weakened - and the brain grew * Prof. Yoel Rek: It is more likely to assume that the brain grew first, and only then did the chewing system shrink
The news agencies and Yuval Dror, Haaretz, and voila!
Right: human skull; Left: Chimpanzee skull. A mutation that occurred about 2.4 million years ago
By Yoel Rak
Recently, American researchers reported on the discovery of a mutation that caused a reduction in the mass of the jaw muscles of the human ancestors about 2.4 million years ago. Muscle contraction, the researchers argued, allowed the human brain to grow. But a much more logical scenario is that things happened just the opposite
The story of the biological development of the genus Homo - we, Homo sapiens, are the last in the chain of species, and the only ones to survive - is summed up "in total" in the change of two body systems, which can be summed up briefly and simply: the volume of the brain is increasing and the chewing system, with all its components, is shrinking.
The other significant change that took place in the history of the development of the human branch (hominid family) since it split from that of the chimpanzee - the transition to upright walking - predated the emergence of the genus Homo by many years and in any case far preceded the beginning of the growth of the brain. The adoption of upright walking occurred very early, near the point of split from the chimpanzee branch, about 7-5 million years ago. In other words, almost the entire branch, with all its numerous and later sub-branches, was long ago equipped with this special movement mechanism.
All this is known to us today thanks to the testimony of the fossils. This reality, by the way, was not the one that Darwin saw before his eyes. In No Fossils, Darwin proposed to link upright walking to an increase in brain volume, considering two systems that depend on each other and mutually stimulate each other's development: freeing the hands from supporting the body and making them available to the brain's ability required its growth, and on the other hand, a larger brain demanded even more the freeing of the hands.
Big muscles, small brain
It is almost self-evident that the increase in brain volume and the contraction of the masticatory system have a far-reaching effect on the general shape of the skull. The increase in the volume of the brain results in the growth of the brain box, the rounding of its outline and the straightening and advancement of the forehead above the eye sockets (Figure 1). Increasing the volume results in an increase in the maximum width of the skull to far above its base, far above the level of the ear openings. On the other hand, the contraction of the chewing system and the reduction of the size of the jaws is expressed in the sinking of the jaws into the face. The prominent nose and chin - one of the distinctive signs of the Homo sapiens species - are nothing more than a by-product of this sedimentation.
In chimpanzees, on the other hand, as in many other monkeys that have not undergone the described change, the volume of the brain remains small and the jaws are large and prominent. The brain box, therefore, is flat and low. The forehead, viewed from the side, is almost completely horizontal. And when viewed from behind, the skull box has a bell-like outline, the maximum width of which is low, near the base of the skull, at the level of the ear openings. On the other hand, the size of the dental arches causes the center of the face to protrude forward, until there is not even a hint of a chin and a protruding nose.
This combination of a small brain volume and a large chewing system (and massive muscles that activate it) results in the fact that the chimpanzee's lateral muscles, which are attached to both sides of the brain box like large fans (their job is to lift the lower jaw and bring the lower teeth together with the upper ones), meet with each other at the top of the skull . Sometimes, especially in males, the disproportion between the size of the masticatory system and the width of the brain is so great, that a bone spur grows on the top of the skull; Its purpose is to increase the surface area of the bone that serves as an outpost for the fibers of the fan giant muscle.
Of course, humans are also equipped with these muscles, but their modest size from here, and the huge volume of the brain from here, mean that a long distance separates the muscle fan on the right side from the one on the left. The muscles only attach to the side of the braincase and never meet at its apex. Furthermore, a microscopic examination of the cross section of the muscle fibers revealed a long time ago that a certain part of the fibers in humans are thin and "degenerate" compared to those of the chimpanzee or other monkeys.
It is clear then that a major change in the chewing system occurred sometime along the line of human development. Until recently, fossils were the only ones to provide the answer. These taught us that a long period of time in the evolutionary history of man was characterized by creatures with a primitive chewing system such as that of the chimpanzee. The dental arches of these early hominids were long and the masticatory muscles that drive the jaw system were also, as expected, developed and large (we know this based on the scars they left on the bone). From the top of the skull, not only was it a common phenomenon, but their size often also required the development of the carbula of the bone at the top of the skull. This anatomy is clearly visible in an important group of skulls belonging to the famous "Lucy" species, a species known as "Australopithecus afarensis" (Figure 2). The anatomy of the skull of members of this species was so primitive that it is customary to refer to them as "chimpanzees that walk upright".
Most of what we know today about "Australopithecus afarensis" originates from a wonderful geological site in the north of Ethiopia, in the area known as "Afar". The fossil remains of Australopithecus are from geological layers that are between 3 and 4 million years old. In these layers there is not even a trace of durable tools. The tiny brain volume of the Australopithecus (about 500 cc, similar to chimpanzees) was not enough to produce them. Up the sequence, in younger geological layers (2.3 million years), many stone tools (albeit simple and crude, as expected from such ancient tools) were discovered near the remains of the hominid who was probably responsible for their production. Although only the palate and the dental arch adjacent to it survived from this hominid, they were enough to prove that they are significantly shorter than the dental arch that characterizes the earlier species at the same site (Figure 3).
There is no doubt that this jaw already belongs to a species of Homo ("Homo abilis") and is the earliest evidence of the contraction of the chewing system. Therefore, it is easy to prove that the contraction event took place in the period of time between 3 million years before our time - the time of the presence of the last Australopithecus afarensis at the site - and 2.3 million years before our time, the age of the layers containing the short jaw and the simple stone tools.
anatomical transformation
These findings recently made headlines again, but this time the interest in them comes from a new and surprising direction. In a study published a few weeks ago in the journal "Nature", Hansel Stedman and his colleagues from the School of Medicine at the University of Pennsylvania report on the discovery of a mutation in a human gene, which is behind the "degeneration" of the chewing muscles and the reduction of their mass (the gene before the mutation was the one that chimpanzees, many other monkeys and early hominids had subject to its influence). The researchers were able to calculate when the mutation occurred: they estimate that it happened about 2.4 million years ago - a date that is consistent with the conclusions based on the fossil findings.
The discovery of the researchers from the University of Pennsylvania is impressive, but they did not stop there and offered a naive interpretation that arouses the most criticism of their research (although it does not detract much from its quality). The researchers attribute much greater importance to the refinement of the muscles than might be expected. With the reduction of the mass of the masticatory muscles and the reduction of the area of their grip on the skull, the barrier to the increase in brain volume was removed, in their opinion. They consider the covering of the brain box by large masticatory muscles, which meet at its apex, the main barrier to the development of the brain - a kind of helmet, which limits growth. According to them, the accidental mutation, which resulted in the reduction of the muscles, was the main factor in the removal of that oppressive helmet.
Why is this interpretation probably wrong? First, as the individual develops, the brain reaches its peak size long before the chewing muscles complete their development; In other words - long before they could be a limitation. But far beyond that, another logical scenario that is more in line with the way evolution works is much more plausible and logical than the proposed scenario. According to this scenario, the changes occurred in the opposite order: the increase in brain volume and the use of tools that followed this transformation resulted in the fact that a significant part of food processing (cutting, crushing and crumbing) was done not by the teeth and jaws (this was their role since time immemorial) but outside the mouth, on by the tools.
In such a situation, having a large and bulky chewing system suddenly becomes an oppressive disadvantage. Indeed, anyone who finds a way to reduce the system that has become redundant, becomes a distinct advantage. It should be noted that the process is Darwinian - it is not the reduction of the use of the system that leads to its reduction. Accidental mutations are what make the change possible, thanks to the benefit they bring in the refinement of the chewing system. The researchers were able to find one mutation, and in a sophisticated way they also provided the time when it occurred, but there is no doubt that this is not the only point in the matter. The contraction of the chewing system on all its components (muscles, jawbones, face and teeth) is a fundamental functional anatomical transformation, which is not caused by a change in just one gene.
The author is a professor of human anatomy and evolution at the Tel Aviv University School of Medicine
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The man from the monkey is allowed - one mutation
27/3/04
Humans are descended from monkeys, according to the theory of evolution. But what was the evolutionary event that caused them to separate? This question has been with many researchers for many years, and now a group of researchers from the USA claims that they have the first clue that may lead to an answer. According to them, a mutation in one of the genes shared by humans and monkeys caused the jaw muscles to weaken and shrink. This reduction created additional space in the skull for the brain and led to its growth - something that allowed humans to develop and advance, compared to the great apes. "It's an interesting theory. It is difficult to prove it with certainty, but it is thought provoking", says Prof. Avitar Nebo, head of the Institute of Evolution at the University of Haifa.
The study, published yesterday in the scientific journal "Nature", was written by a group of biologists and plastic medicine experts from the University of Pennsylvania and the Children's Hospital of Philadelphia. The researchers tested a family of genes known as "MYH", including a certain gene called "MYH16". This gene is responsible for creating the protein "myosin", which is the main protein of muscle fibers. When they tested the gene in the jaw muscles of great apes (gorillas, chimpanzees, etc.), they discovered that it is fully expressed. However, when they tested the gene in the jaw muscles of humans, they discovered that it appears with a mutation that prevents the accumulation of protein in the jaw muscles - and which causes the jaw muscles of humans to weaken compared to those of monkeys.
From here on, the researchers believe, a fascinating evolutionary process began. According to them, there is a connection between the development of the muscles or their weakening and the structure of the bones. The weakening of the jaw muscles reduced the bones of the lower jaw and freed up space in the skull. The mind expanded into the space that was vacated, growing and enriching the abilities of human beings. Calculations made by the researchers indicate that the mutation occurred about 2.4 million years ago - a date that is consistent with other findings that at that time the ancient humans began to use tools.
"The researchers are making a very big generalization," says Prof. Nebo. "Their finding regarding the mutation is interesting, and it is probably accurate. But the question arises whether it really explains the evolutionary changes that have occurred in humans. There is a lot of substance in the theory, but on the other hand, there is no confidence that it describes what really happened, since it is guilty of fundamental assumptions on which there is no consensus." For example, Nevo claims, calculating the period of time in which the mutation occurred is problematic.
Other experts in the field of evolution also differ in their opinions regarding the new theory. "The claim made by the researchers is against all the foundations of evolution. Mutations of this type must have had much smaller consequences than described," said Prof. Evan Lovejoy from Kent University in Ohio, he said in an interview with the AP news agency.
On the other hand, Prof. Milford Wolfoff from the University of Michigan, who deals with biological anthropology, is more impressed: "This is simply great research. Before us is a joint flow of genetic evidence with tools created 2.4 million years ago."
Prof. Pete Carey, who specializes in developmental biology at a research institute in Sydney, Australia, wrote an editorial accompanying the study. He hopes that his publication will cause a new wave of research that will deal with a genetic comparison between humans and great apes. "Some have argued that such a comparison between the human genetic load and that of the chimpanzees would reveal a difference of about 40 million chemical bases. The work of identifying the differences that were directly related to our becoming human can be terrifying in its magnitude." However, according to him, work of this kind has already begun, and it may open a new window for us to understand the way in which the human race developed.
They know evolution - the rise of man
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