In the following fascinating story it would seem that a mutation to gene MYH16 about 2.4 million years ago might have been responsible for releasing a hitherto vice-like grip on human skull-size. Is this the crucial mutation that led to homo sapiens? Probably not. Once the human cranium had the potential to grow larger, there would have to have been other mutations that forced it to do so.
But, given that the human genome is only a few score genes’ difference from that of the chimp and gorilla (which had already shot off in different evolutionary directions 4 million years previously) then it looks as though the MYH16 is one of perhaps only a handful of very special genes that took the hominid line into its final stretch.
Though it’s possibly not the final stretch as far as we’re concerned. Given the rapidly changing environment — technological rather than environmental in the last 12,000 years since agriculture — evolution will be proceeding apace as it’s always done.
Homo cinsini, Australopithecuslar arasından sivrilten faydalı mutasyon bulunmuş. Dikkat edersen Australopithecusların kafatasları daha çok diğer maymun türlerine benziyor. Oysa 2.4 milyon yıl önce sahneye bir anda giren Homo Habilislerden itibaren bütün Homo cinsi üyelerinin kafası insansı. Hatta Australopithecuslarda 350 cc olan kafatası hacmi, Homo Habilis’ten itiraben bir anda 700 cc oluyor. Homo cinsini ayıran mutasyon bulunmuş. MYH16 genindeki bir mutasyon sonucunda Homo cinsi, Australopitheculardan ayılmış. Bu gen çene kaslarında ağır miyozin proteinlerini kodluyormuş. Çok güçlü çene kasları, kemiklerin embriyonik dönemde gelişmesini ve kafatas hacminin artmasını enelliyormuş. Bu gendeki bir mutasyon sonucunda, bu gen protein üretimini durdurmuş, çene kaslarının küçülmesi, kafatasının genişlemesine ve beynin büyümesine izin vermiş. Diğer maymunların hepsinde aktifmiş bu gen. İnsanlardaysa artık bir psödogen olmuş. Ama maymunlara güçlü çene veren gen hala DNA’mızda, ama inaktif halde. Bu mutasyon 2,5 milyon yıl önce olmuş. Zaten 2.4 milyon yıldan itibaren Homo Habilisler buluyoruz. Bu insana giden yoldaki en önemli mutasyonmuş.
WHEN WE BECAME HUMAN: HAS THE MUTATION GENE BEEN FOUND?
A crucial change in the genetic make-up of an ape that lived 2.5 million years ago turned a small-brained, heavy-jawed primate into the direct ancestor of modern humans.
Scientists have discovered the precise mutation in a key gene found in all primates that allowed the evolution of man’s large braincase, the single most important attribute which sets humans apart from the rest of the animal kingdom.
The gene is still responsible for producing the relatively large jaw muscles of all monkeys and apes but the mutation has rendered it useless in humans, enabling our braincase to become larger.
The scientists believe that the loss of heavy jaw muscles which were attached to the skull was a crucial event that permitted human ancestors to acquire a larger brain and, with it, the attributes of human consciousness and language.
A study into a gene responsible for generating the muscles of the face and jaw found that it is active in all non-human primates such as macaque monkeys, gorillas and chimpanzees — the closest living relative of humans.
The scientists also found that the gene, called MYH16, in human DNA but, to their surprise, they discovered that it is no longer working because of a mutation that took place about 2.4 million years ago — a date that was estimated from a molecular “clock” of mutation rates in human DNA.
Nancy Minugh-Purvis, the director of anatomy at the University of Pennsylvania in Philadelphia, said that the date coincides with the known age of when the human family — the genus Homo — first began to emerge from its apelike ancestors.
“Whether this led to the evolution of the human line itself is a very difficult question to answer,” Dr Minugh-Purvis said. “We do know that the mutation occurred approximately 2.4 million years ago and that only 100,000 or 200,000 years later the earliest members of the genus Homo appears in the fossil record.
“I don’t know that we can say this led to the evolution of language but I might point out that one of the tiny muscles of the soft palate — the tensor veli palatini – which is used in speech belongs to the group of muscles of the head which are altered by this mutation.”
Dr Minugh-Purvis added: “This is the first time such a genetic difference between humans and chimps has been successfully correlated to anatomical differences between our two species and to anatomical changes in the hominid fossil record.”
Anthropologists have long argued over the factors that led to the evolution of a large-brained ape, and have thought for many years that it must be associated with anatomical changes to the skull which led to finer, less robust jaw muscles.
Monkeys have chewing and biting muscles that are nearly 10 times as large and powerful as those used by humans. But it means that the muscles need strong attachments to the skull, which is thought to constrain the size of the braincase.
If a mutation occurred that suddenly reduced the size of these muscles, it would allow the braincase to become larger over time but then the problem arises over what the small-jawed ape would have been able to eat.
Pete Currie, a developmental biologist in Sydney, says in the journalNature that the mutation may have coincided with a change in diet, such as eating soft meat, which did not need big, crunching jaws.
“These findings suggest a seductive hypothesis — that a decrease in jaw-muscle size produced by inactivation of MYH16, removed a barrier to the remodelling of the hominid cranium [braincase] which consequently allowed an increase in the size of the brain,” Dr Currie said.
“At first glance, such a proposition looks far-fetched. Surprisingly, however, altering the size of different muscles can produce dramatic alterations in the bones to which they attach.” Dr Minugh-Purvis said that the study is breaking new ground in terms of understanding human evolution. “Humans and chimps differ in a fairly limited number of genes,” she said. “However, as the MYH16 mutation illustrates, even a tiny mutation can have enormous consequences in terms of altering the biology and behaviour of an organism.”
Hansell Stedman, a professor of surgery who worked with Dr Minugh-Purvis, said that the research involved a collaboration between several scientific disciplines. “Around the lab, we jokingly call this the ‘room for thought’ mutation, space we had to involve scientists from several disciplines to make sense of the possible domino effects,” Professor Stedman said.
“In other words, we had to do a lot of experiments to connect the dots…. Then, in looking at the modern and fossil skulls, it dawned on us that we just might have to look ‘outside the box’ to appreciate the real significance of the initial findings.”
The Independent — 25 March 2004