The evolution of humans from apes may have been the result of a chancemutation of the jaw, writes Dick Ahlstrom.
An inherited muscle "disease" caused by a single damaged gene 2.4 million years ago could be the trigger that allowed humans to evolve from the apes. While the apes remained largely unchanged, the mutation may have allowed our brain size to grow rapidly, resulting in the new kid on the evolutionary block, Homo sapiens.
Researchers at the University of Pennsylvania School of Medicine were studying myosin, the main contractile protein that makes up muscle tissue. Unexpectedly they discovered one small mutation that knocked out an entire myosin gene. It may also have put our ancient ancestors on the path towards human form.
Prof Hansell Stedman and Dr Nancy Minugh-Purvis and colleagues describe their startling work today in the journal, Nature. It provides convincing evidence that the mutation gave us smaller jaw muscles, and as a consequence allowed our skull size to increase and our brain volume to grow.
Anthropologists have long debated how humans evolved from ancestors that had large jaw muscles and small brains. The discovery of a damaged myosin gene, MYH16, which in apes controls the growth of muscles involved in biting and chewing, may now provide the answer.
The researchers initially didn't know they were even dealing with a gene, given computer driven human genome studies hadn't recognised it as a gene. Once isolated, the research team took muscle samples from modern humans right around the world, from Africa, South America, Western Europe, Iceland, Japan and Russia.
All modern humans had the same mutation in MYH16, a change that rendered the gene non-functioning. The mutation was not present however in the DNA of seven species of non-human primates including our close ancestor, chimpanzees.
Interestingly, the researchers also discovered that while the MYH16 gene was there and trying to do its job in humans, as evidenced by the transcription of MYH16 messenger RNA from the DNA, it did not deliver the muscle-controlling myosin protein because of the mutation.
The authors suggest those with the mutation were left with "diseased" jaw muscles that delivered a weaker bite. Jaw muscles and their bony attachments in apes and monkeys are much larger and more powerful than in humans.
The chewing muscles of the macaque are nearly 10 times as large as in humans, because it has working MYH16, the authors say.
The ape brain case by necessity is exceptionally bony in order to handle the pressure exerted by the jaw muscles, as a result blocking the growth in size of the brain.
The mutation reduced this pressure, with the wimpy mutant bite reducing forces on the brain case in turn allowing it to grow and expand.
The researchers compared gene sequences across a range of species in an attempt to read the "molecular clock" that allows scientists to date the arrival of genetic mutations.
They calculated that the mutation appeared in a hominid ancestor about 2.4 million years ago.
Not long afterwards, about two million years ago, the first of the less-muscled, larger-brained skulls of the earliest known examples of the genus Homo start to appear in the fossil record.
"The coincidence in time of the gene-inactivating mutation and the advent of a larger braincase in some early Homo populations may mean that the decrease in jaw-muscle size and force eliminated stress on the skull, which released an evolutionary constraint on brain growth," states Dr Minugh-Purvis.
This raises the "intriguing possibility" that a single gene mutation was enough to trigger the evolution of Homo, paving the way to our larger brains, the authors add.