All except one of these guys from the past had a strong jawline:
And he just doesn't look very healthy and has a strong open mouth posture. Probably somewhat malnourished and grew up mouth breathing. Modern Somali men are indeed pretty weak-jawed on average but that probably has more to do with greater gracility being caused by poorer modern nutrition Trading in meat with organs like liver and raw milk products for processed foods. Even most Somali women don't seem quite built the same anymore. You don't see as many 175cm, decent profiled Iman/
Waris oids out there anymore as opposed to the shorter, narrow-faced, chubby cheeked look.
You are correct in your assessment in that it's not a question of genetics but of lifestyle change. The last guy looks like he might have had irritation causing him to mouth breathe. Despite that his face looks more normal than most mouth breathers today.
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Poor breathing habits (such as over breathing, laboured breathing, chaotic chest breathing, taking big breaths vs. deep breathing, frequent throat clearing, frequent sighing, open mouth breathing or posture be it while awake and or sleeping).
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Poor tongue placement at rest (not fastened to the tongue most of the time).
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Poor chewing habits (only on one side of mouth, not chewing good thoroughly (meaning incomplete breakdown of food and possible digestive issues.
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Poor body posture i.e. constantly sloping head/forward head body posture due to extending the neck from frequent phone or computer usage.
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Processed food vs. harder more unprocessed foods. (Bariis and Baasto are a part of urban Somali diet and were not eaten in the rural areas). Processed sugar was scarce and seed-based vegetable oils were non-existent. I don't even think Somalis ate wheat to be honest. Many also have gluten sensitivities and intolerance but don't know.
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Change in child rearing and family planning. Somalis used to consider it sunnah to breast feed children for a minimum of
two years. And I suppose breast feeding especially done long term bodes well with jaw development in infants (who have more plastic faces). And children were actually spaced out and planned more so. They didn't have them back-to-back but spaced a minimum of 2-3 years. I think they stopped this practice and do things at random. Since they abandoned their old agropastral ways of life. Child spacing is underemphasized but it improves the health outcomes of children by allowing the mother to replace nutrient stores from pregnancy and improves the health outcomes of children.
- Aggregate exposure to greater chemical and environmental toxins. This has had untold implications. We are essentially stewing, bathing and breathing in them.
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Sedentary lifestyle. Somalis have a tendency to develop more fat to lean muscle mass especially if they are sitting around and not moving as they were intended. This is the case especially for women. They are at risk of being fat or skinny fat if they don't pay attention to their diets and activity levels.