Science or Nonsense:
Did Humans Evolve into Weaklings?
“Humans Evolved Weak Muscles to Feed Brain’s Growth,” says the National Geographic headline. The idea that humans are the cloistered wimps of the animal kingdom is an old and commonly repeated meme. I was intrigued by the promise of scientific evidence in its favor, so I clicked the above article. What I found was even more interesting but less straightforward.
The NatGeo article is based on a scholarly article from the Max Planck Institute in Germany, with the much less catchy name of Exceptional Evolutionary Divergence of Human Muscle and Brain Metabolomes Parallels Human Cognitive and Physical Uniqueness. The full text is available @ PLOS One, along with an accompanying commentary article. (Yay for open-access publication!)
Based on its coverage in NatGeo as well as other media coverage, you would be forgiven for thinking that the entire research paper was a literal tug-of-war between humans and apes. “All participants had to lift weights by pulling a handle,” states the NatGeo article. It quotes the editorial commentary, “Amazingly, untrained chimps and macaques raised in captivity easily outperformed university-level basketball players and professional mountain climbers.”
Digging Through The Science
Reading between the Press Releases
If you read through the original article you’ll find that pull strength really wasn’t the point. The actual scientific study was an opus of molecular biology, specifically metabolomics. Using a combination of liquid chromatography and mass spectroscopy, they could examine the patterns of metabolites in each tissue in the body. By comparing humans to chimps, macaques and mice they could figure out what metabolic pathways had the most human-specific differences. Not surprisingly, the human brain was very different from animal brains: around 4x as many human-specific changes as the kidneys (their “control” tissue). However, muscles actually managed to outdo brain: they had 8x as many human-specific changes!
The authors were intrigued by this large difference in human muscles, so they embarked on additional studies. First, they performed a genomic (mRNA) analysis to confirm that their metabolomics weren’t totally crazy. They found that the gene expression analysis matched very closely with their metabolomic analyses. Then, they did a less scientific but more newsworthy confirmatory study: the “pulling strength” experiment.
This metaphoric human versus ape tug-of-war accounted for a single paragraph in a very long paper. If you asked any of the authors what experiment they were most proud of, I’m pretty sure none of them would say “Pulling Strength!” Unlike the great detail they gave of their biochemical and statisticcal analyses, the authors gave very little detail on the physical setup of their pulling strength experiment. All we know is that humans, chimps and macaques had to pull a handle to get food, the weight on the machine was progressively increased until the subject couldn’t pull it, and the heaviest weight pulled was recorded as a datapoint.
The final conclusion of “apes are stronger than humans”, came from the endpoint of “pull strength per kilogram of body weight”. This metric is inherently biased toward the smaller animals, and with chimps weighing in at 40kgs (88#) they certainly had an advantage. A similar analysis done on a smaller weight machine would have proven the supernatural strength and agility of spiders.
The authors and the editor all made comments about how they did not control for biomechanics. Yes, biomechanics is a valid criticism – everyone knows that the same amount of weight can feel a lot heavier or lighter depending on which weight machine it’s on. Maybe the geometry of their experimental weight machine was bad for humans. However, biomechanics is really the least of “pull strength”s problems.
The problem with a human-ape tug-of-war should have been much more obvious: Humans are bipedal. Apes and monkeys are not. A chimpanzee walks on its knuckles and hangs from tree branches. Ape shoulders are angled toward its head, and ape scapulae are narrow and elongated. This gives apes a more solid muscle attachment for knuckle-walking, hanging, and swinging motions, at the cost of restricted range of motion. For example, apes cannot scratch their own backs, one of many reasons that they spend a lot of time grooming each other. Ape arms are significantly longer than their legs, as is necessary for their walking posture.
On the other hands, human arms are not meant for locomotion, they are meant for manipulation. Our arms are 30% shorter than our legs. Our hands are much smaller than ape hands, trading raw grip strength for dexterity and opposable thumbs. We can cross our arms behind our backs, something that apes cannot do. While our arms are strong enough to hang from monkey bars, it takes us an awful lot of effort to do so. (much as a chimp can walk upright with effort) And humans really can’t knuckle-walk; our arms are too short and our knuckles too small.
Given that ape locomotion uses a lot of pulling motions and human locomotion doesn’t, the fact that apes can out-pull humans shouldn’t surprise anyone. As long as we use a tug-of-war to judge strength, humans just don’t stand a chance. Change the strength test to throwing speed and now the ape seems much weaker.
The idea that humans are the pathetic weaklings of the animal kingdom flies in the face of everything we know about primitive humans. Human biology evolved hundreds of thousands of years before effective weapons like spear-throwing slings or bows. Armed with rocks and sharp sticks, there’s no way we could have survived if our muscles were 2-3x weaker than any other animal.
It is quite likely that human muscles are weaker in short bursts but better at prolonged exertion when compared to other animals. The practice of persistence hunting, primitive tribesmen catching prey by running at it until it collapses from exhaustion, gave a spark to the barefoot running movement. If our muscles aren’t able to produce as much peak pulling force, it’s only because they are optimized for endurance and heat tolerance instead.
So, Are Our Muscles Different?
Get your hands off me!
So if I don’t buy the “human muscles are useless” theory, then why are there so many biochemical differences in human muscle compared to our kidneys and brains? The answer of course, is that we don’t know. I’m sure there are plenty of researchers trying to figure this out – every good study needs follow-up studies!
That said, you could make a few guesses based on simple metabolic facts:
The Real Paleo Diet: The great apes are omnivores, but they eat meat very rarely. It’s estimated that wild chimpanzees get ~3% of their calories from meat. The overwhelming majority of their calories come from fruits; there’s a reason why monkeys and apes are portrayed with bananas! On the other hand, ancient humans ate a lot of meat. Variations of the Paleo Diet tell you to get 50-70% of your calories from meat and seafood. While the Paleo Diet is of questionable prehistoric accuracy, humans definitely eat way more meat than the great apes that we evolved from. After all, our bipedal gait and heat tolerance were really good for running down prey!
On a broad scale, human metabolism can be described as two different modes: a glucose metabolism and a ketone metabolism. In well-fed humans with an unrestricted diet, glucose is the basic energy source. Glucose produces a small amount of energy through anaerobic glycolysis, then it feeds into the citric acid cycle which produces a large amount of aerobic energy. During a meal, we digest carbohydrates into very large amounts of glucose. We produce insulin in order to allow our cells to uptake the glucose and use it for energy. Excess glucose is stored as glycogen in liver and muscle. When fasting, we digest the glycogen back into glucose, keeping our blood sugar stable. Glycogen is such a good energy source, athletes often practice “carbohydrate loading” to increase their own glycogen stores.
When we become carbohydrate-deficient, either due to starvation or due to a low-carb diet, the human body switches track completely. The liver breaks down fatty acids into ketone bodies, which become the primary energy source for the body. Ketone bodies enter the citric acid cycle as acetyl-CoA, producing aerobic energy in the same fashion as glucose. However, insulin-mediated glucose transport and glycolysis are completely left out of the picture. Low-carb diet advocates claim numerous benefits of ketosis, some of them more plausible than others.
So what’s my point? The human diet is sufficiently different from that of monkeys and apes that it’s likely that our metabolism has changed in response to diet. A chimp never has to worry about ketosis; its diet is way too high-carb. And since the majority of our carbohydrates are stored as glycogen in the muscles, it only makes sense that our muscle metabolome would change more than any other organ.
Well, that’s my theory at least.