Genes influenced a large part in how we became, us. But, the story isn’t complete without learning about how tiny variations in a single gene can bring about massive differences between species, over a period of a few million years.
Gene switches have been covered previously on Mind Brunch. But, we face an uphill task in studying those in humans due to the inability to analyze the function of gene switches in the embryo stage. So, which genes among the 16 million base pairs control the processes in us? That’s a tough question to crack, but we can help ourselves in this regard. How? Well, we are a lot in number. The sheer size of human population and access to medical facilities and related data enables us to chase those rare anomalies, through whom we can hope to find those crucial genes. A couple of examples are covered in the next section.
The muscles in the human jaw
Human jaws are smaller compared to those of gorillas, chimpanzees or our hominin ancestors. A muscle called temporalis attaches the jaw to the skull. A gene MYH16 (encodes a protein called Myosin Heavy Chain 16) has been found to be responsible for this muscle. That is because, this protein is a key factor in building fibers, in those muscles that are used for forceful contraction (chewing). Found in both primates and humans, a mutation has rendered this gene inactive in humans. This has resulted in a drastic reduction of fibers in this muscle within humans.
Further research has shown that this mutation occurred somewhere between 2.1 and 2.7 million years ago, a time when the present genus of Homo was starting out. As with associated effects of a single change (like in bipedalism), less stronger muscle meant no need for larger bones. The jawline of humans traces back to this mutation, the reason why we have reduced lower jaws. Not just that, this lighter bone structure could have played an important role in developing speech. It is yet to be ascertained whether this mutation was a result of another physiological change or the initial trigger to subsequent changes.
Modern medical technologies allow researchers to perform s=real-time scanning to observe which areas of the brain react to a given stimulus and thus have been able to identify the gene that plays a crucial role in speech. This gene has been named FOXP2. The protein that this gene sequences, controls other genes that eventually bring about the speech. A single change in this gene puts it out of action and affected patients are affected by an impairment of speech and language. This gene again is observed in other species and the closeness is surprising. In the 716 positions that his protein operates in, humans and mice differ only in 4. The number reduces to just 3 places for Orangutans and 2 for Gorillas and chimpanzees.
Unlike the earlier gene that was discussed, the evolution of FOXP2 may have been a result of small variations in DNA length that accumulate over time. When one such variant is selected naturally, it reduces the overall variance by causing a ‘sweep’, the entire process aptly named “Selective sweep”. This may explain the development of speech and language in humans, resulting from several mutations of this gene being selected throughout the past 200 thousand years. Another aspect of the gene FOXP2 is it has been found to play a crucial role in the development of mammalian brains, possibly regulating the development of specific regions within the brain.
Slow transitions vs. Rapid changes
If the evolution of the two features resulting from the genes explained above is anything to go by, then it is that evolution is a long and tedious process. It should be enough for us to drop any ideas about ‘sudden’ changes happening within a generation. Another aspect is the inter-relational and inter-dependence of smaller features, characters accruing over time to bring about a permanent development. The sum total of genes that work together as well as independently to contribute to a character is called “Genetic architecture”.
MYH16 and FOXP2 show the magnitude of work that goes behind the discovery and charting the work of a single gene and its effects on our biology. Many of the features might have developed in some ancestors who are related directly to modern humans or a distant relative species that has gone completely extinct. Such is the journey of gene mutations, subtle physiological changes, natural preferences that take a winding route to get to this present point in time. All these discoveries also help equally in course correcting our own story of evolution and existence. Evo-Devo apart from widening the scope of earlier fields has laid the foundation for an exciting, new journey in studying our evolution.
This series is influenced by chapter ‘A beautiful mind: The making of Homo sapiens’ of the book Endless forms most beautiful by Sean Carroll.
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If you would like to learn more about Evolutionary Developmental Biology you can head over to this link: https://evolution.berkeley.edu/evolibrary/article/evodevo_01