While I think the idea of adult Paleolithic hunter-gatherers regularly dying at age 30 can be laid to rest (sadly, I reckon that particular misconception has an impressive life expectancy), last week’s post on the Gurven-Kaplan paper brings up another question: if the human potential lifespan of 68-78 years, or roughly seven decades, is an evolved, inherent, even genetic trait, what is the evolutionary justification for its selection? Where is the advantage?
The classic Darwinian view is that selection of traits revolves almost entirely around fertility. Once an animal can no longer produce offspring, it has no “reason” to go on living. Since a genes’ survivability ultimately comes down to reproduction, whether an individual can have kids is the primary determinant of viability.
Speculation on ancestral lifespan is fun and potentially illuminating, but I think examining living, albeit imperfect, examples of modern hunter-gatherers offers greater insight. Sure, the environment has changed, wild food sources have shrunk in diversity and availability, and modern civilization has encroached and meddled and disrupted, but the few remaining hunter-gatherer populations exhibiting relatively untouched traditional lifestyles represent the most promising window into what life actually looked like and how long it lasted for our ancestors. Luckily, a couple of researchers – Gurven and Kaplan – had the bright idea to look at ethnographic studies on actual, living HG populations and analyze the available data on actual lifespan and mortality therein. They found some interesting stuff.
It has become an article of faith among, well, basically everyone, that our ancestors lived short, brutal lives. What are they touting as the average lifespan these days – 35 years old or so? I’ve heard anything between 25 and 40 years. The common counter is that infant mortality rates were higher than they are today, thus skewing the average. It’s also often pointed out that a relatively benign accident or illness by today’s standards – a broken arm, a rolled ankle, or a minor infection – could have prematurely ended Grok’s life. And that these cases say nothing about Grok’s potential to live 70+ years. The “short and brutal” meme has wedged itself in the public psyche, and it’s going to take a lot to extract it from its seemingly intractable position.
I’m going to riff a bit on something I’ve been thinking about regarding ancient human bones. This isn’t an official stance or anything; I’m just thinking out loud. Let me know what you think in the comment board.
Hypothyroid has been covered to death before. I’m particularly fond of The Healthy Skeptic’s coverage – check out Chris Kresser’s ongoing series (possibly before you read on) for some great information on the thyroid. Carnivorous Danny Roddy did a good piece on it last year as well. As such, I won’t be redoing or rehashing an “intro to thyroid.” Instead, I’ll give a brief overview and then discuss why I think some of us may be looking at thyroid “dysfunction” in the wrong light.
The thyroid is a complicated little bugger wielding a lot of influence over the metabolism, and it seems like just about anything has been fingered as a trigger of its dysfunction. Lack of carbs in the diet, too few calories, too much iodine, too little iodine, too many grains, intermittent fasting, excessive cortisol, and multiple other factors have gotten the blame. Unraveling the multiple potential triggers for its dysfunction can be tough. But is dysfunction always the right way to describe a slight reduction in thyroid hormones? I’m not so sure.
If you’ve been reading recently you know I’ve been on a hormone kick recently. That sexy looking molecule to the right and the hormone du jour: testosterone. Testosterone is the principal anabolic and sex hormone in humans, responsible for sexual desire and function, muscular hypertrophy, densification of bones, and hair growth. Compared to females, males famously produce about ten times the amount of testosterone, but females are far more sensitive to its effects. Though testosterone is largely responsible for those traits and characteristics that are considered “masculine” – physical strength, body hair, dominance, and virility – both sexes require it for proper sexual and physical development. In mammals, males secrete it primarily from the testicles (about 95% of the total amount, in fact) and women secrete it from the ovaries. A modicum is produced in the adrenal glands in both sexes.
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