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November 26 - December 2, 2021
jaws and teeth have independent evolutionary origins. Recognizing the evolutionary independence of jaws and teeth is critical to answering the “why don’t our teeth fit?” question. When one of the structures underwent a notable change (like the jaw becoming smaller, for example), the teeth did not automatically follow suit. A mutation in a gene that controls the size of the jaw cannot magically spur a mutation in a gene controlling the size of the teeth.
Heterodont dentition, meaning having different types of teeth, is a hallmark of mammals—it allows mammals to eat a diet unparalleled in its diversity.
dietary versatility has allowed mammals to conquer all corners of the globe because no matter where we go, we can usually find something to eat.
The original formula for mammalian permanent tooth dentition consisted of 11 teeth in each quadrant for a total of 44. Eventually, as mammals spread out to fill nearly all the diverse habitats on earth, the number and shape of teeth began to vary based on the dental needs in those different environments.
Researchers continue to debate the date of the last common ancestor of humans and chimpanzees, with estimates ranging from 5 to 13 million years ago.
Recent fossil evidence suggests human ancestors bailed on the arboreal life somewhere around 3 to 4 million years ago.
The dietary transition from carrion to fresh meat would not have been trivial for our ancestors. Most predators rely on a combination of guile and speed to bring down their prey. Early hominins had guile in spades, but they didn’t have the quickness of most quadrupedal hunters. Bipedalism is highly advantageous when traveling long distances, but over short distances it is a slow way of getting around.
Hunting as a soft and slow human demands intelligence. The smartest hominins were the ones who could work rocks into sharp spears and cooperate to use tools to bring down big game. Killing large prey would also have required significant social interaction, another trait typically associated with increased cranial capacity.
As members of hominin societies grew smarter and smarter, they would have, in turn, become increasingly successful hunters. Our ancestors would have entered a positive feedback loop where the smarter they got, the more successfully they could hunt. The more successfully they could hunt, the smarter they got by feeding a growing brain in demand of energy.
Paleoanthropologists have hotly debated the origins of hominin-controlled fire for years. Some place the date as far back as a million years,12 others put it at more like 350,000 years ago,13 and still others argue this crucial milestone occurred as recently as 12,000 years ago.
The brain accounts for about 2% of human body mass but uses up to 20% of our caloric intake. By unlocking the true nutritive potential in meat via roasting, early hominins were able to feed their growing brains.
The most convincing evidence suggests humans first made pots around 20,000 years ago in modern-day China.18 Having a pot meant food could be cooked for hours until it was so soft that chewing was hardly necessary.
The combination of hunting, cooking, and farming created a diet that greatly deemphasized the need for a large jaw and left humans with a mouth full of large teeth that are total overkill for their modern job. In spite of their large brains and relatively small jaws, however, the teeth of hunter-gatherers were still a good fit in their mouths as recently as 10,000 to 15,000 years ago.
Vertebrate eyes originally evolved to view objects underwater,
When vertebrates started living out of the water 375 million years ago, they already had eyes that had been around for 100 million years.
one of the most important features of evolution. Lousy function trumps no function.
The eyes are basically extensions of the brain out at the very edge of the skull. The optic nerves connecting the eyes to the brain are, in fact, pieces of central nervous system tissue, like the brain itself.
Hagfish have fantastically ancient eyes. They consist of small sets of photoreceptive cells (basically very simple retinas) sitting beneath layers of skin thin enough to let light through.
The earliest eyes did not form an image. They simply detected the presence or absence of light.
We blink, on average, about 15 times a minute when awake. Running the math for a 16-hour waking day comes out to more than 14,000 blinks per day. When we are awake, we spend up to 10% of our time with our eyes temporarily shut,
People do not blink at random times. For example, when reading, people are more likely to blink when they reach the end of a sentence. When having a conversation, people subconsciously time their blinking to occur during a pause or a lapse. The thought is that blinking somehow helps the brain reset its focus.
When bathed in excessive light, rod cells become washed out and unusable. When exposed to copious amounts of light, the photosensitive pigment made by the rods (rhodopsin) breaks down at a faster rate than it is put back together. Upon plunging into darkness, rods take some time to recharge, which explains why it takes a few minutes upon entering a dark room for our night vision to kick in.
Color blindness is particularly common in men with northern European ancestry, affecting nearly 10% of males with that particular background.
The most common cause of a blockage is some type of food. Hot dogs make a tight seal on the airway and are the most common cause of choking in kids in the United States.
plenty of kids die from asphyxiation because of nonfood items. Again, the most susceptible are the littlest children. The most common cause in these cases, you ask? Balloons. Like a hot dog, a latex balloon sticks perfectly within the airway.
The average tracheal diameter in a 20-year-old adult male is only 1.75 centimeters, which is still smaller than the diameter of most commercially available grapes.
The generally accepted time for the appearance of Homo sapiens is approximately 200,000 years ago, though some researchers in the field suggest early Homo sapiens may have transitioned away from other hominins by 300,000 years ago.
years ago strongly suggest humans from around that time were still not generating anything even close to modern human vocalizations.15 This means that for at least 50% (and probably closer to 75%) of the time Homo sapiens have roamed the earth, they have been incapable of producing the full range of human speech.
The neck had to elongate to allow the larynx to descend, creating the necessary 1:1 horizontal-to-vertical ratio. Even with a shorter jaw, without elongation of the neck, the larynx had nowhere to descend without causing a suite of other problems. Based on fossilized cervical vertebrae of early humans, the neck does not appear to have elongated to anywhere near its current length until around 50,000 years ago.
comparison of our sense of smell with basically any other mammal. Flatten out the olfactory epithelium from a human and it makes a piece of tissue about the size of a postage stamp. Do the same thing for a dog and the piece of tissue is more like the size of two smartphone screens.
When human ancestors became bipedal, the argument goes, it started a gradual shift away from the sense of smell as the dominant sense. With the new upright position, hominins became more dependent on their sense of vision. Being able to see potential trouble, rather than smell potential trouble,
difficult. Printed out single-sided on regular copy paper in Times New Roman 12-point font and with one-inch margins, all the letters of the human story, or genome, would take up 1,206,980 sheets of paper.*
If you control for the number of individuals participating, females are anywhere between two and eight times more likely than males to tear their ACLs.
focusing on soccer players, for instance, the female-to-male injury ratio is 2.67:1. The ratio for basketball players is 3.5:1. What this means is a woman playing basketball is 3.5 times more likely to tear her ACL than a man playing basketball. Each year, approximately 5% of female college athletes playing year-round basketball or soccer tear their ACLs.
The most authoritative published list of mammals includes a total of 5,416 species.7 The only ones who menstruate are the 19 apes (including us), 132 old-world monkeys, half a dozen or so new-world monkeys (there are still gaps in the menstruation data for new-world monkeys), four species of bats, a rodent called the spiny mouse, and elephant shrews.
Before the industrial revolution, a young woman could expect to have her first period sometime deep into her teenage years. The average age of menarche in 1840 was 16.5 years.10 These days it occurs, at least in industrialized societies, at more like 12 to 13 years of age, and it is not unheard of for a girl to have her first period before her 10th birthday.
Some 30% of successfully implanted embryos never make it to six weeks of gestation.
Infertility strikes each gender roughly equally. In one-third of infertile couples, the anatomy or physiology of the man is the cause of infertility. One-third of the time the woman is infertile, and one-third of the time the root causes cannot be identified or both partners have fertility complications.
The generally agreed-upon definition of infertility reserves the word for couples who are unable to become pregnant after one year of regular unprotected sex. Somewhere between 10 and 15% of couples meet that definition.
Up to two-thirds of fertilized human eggs fail to develop somewhere along the path between conception and birth.
An aversion to meat is the most common type of aversion experienced by pregnant women, and the aversion is strongest during the first trimester.
The fetus subconsciously plays the role of puppeteer, secreting manipulative hormones to try to get the mother to act exclusively in the best interests of the unborn child.
a woman with gestational diabetes is more likely to need a C-section. The diversion of sugar and energy to the fetus leads to a bigger baby, which is, naturally, more difficult to birth.
Roll the clock back a few thousand years and any European women prone to gestational diabetes would have been in trouble. Without access to modern medical care, on their carb-heavy diets, many of them would have died during pregnancy or childbirth of complications related to gestational diabetes. Genes predisposing women to the condition were weeded out of the population, which is why those genes are seen less frequently in modern women of European descent.
By the third trimester, when the brain is growing dramatically with each passing day, some 60% of the energy the baby gets from the mother is diverted to the brain.
A study of more than 1,600 women from Nordic countries kept track of labor lengths and demonstrated averages of 14 hours for first-time moms and 7.25 hours for mothers giving birth for at least the second time.
According to data from the CDC, black and Native American women are two to three times more likely to die during childbirth than white women.
C-section rates have skyrocketed in the United States, with 31.9% of kids now born via Cesarean delivery.*
multiple studies have shown that children born vaginally are less likely to develop asthma, allergies, and some autoimmune disorders like celiac disease and type 1 diabetes.
We started in the introduction with a question: why is the human body so uniquely prone to aches and pains? In some instances, the answers were logical and more obvious, like in the case of back pain. You cannot take a horizontal spine and make it vertical and not expect a few problems. In other cases, the answers were more complex and confusing. For example, we still do not have a complete picture of the historical underpinnings of human fertility issues.
