This merest smattering of the everyday miracles of the anatomy and physiology of human beings inspires awe at the power of evolution’s forces.
Only Humans Have Opposable Thumbs
One specialization that differentiates humans from other animals is the opposable thumb, which is a thumb that can touch each finger on the same hand. (Go ahead — try it now.) Along with that, the human thumb is prehensile, meaning capable of grasping. This anatomy underlies the development of manual dexterity and fine motor skills in humans. The prehensile, opposable thumb makes possible tool making, hunting and gathering, textile and metal crafts, art, writing, cooking, and possibly the very existence of human culture.
Human Milk Is the Best Milk
The best nutrition for a human baby is human milk. Human milk is a complex mixture of more than 200 different components, and no other substance produced in another animal, or yet in a laboratory, matches its ability to meet the needs of a human infant.
Like all foods, the core components of milk are carbohydrates, proteins, and fats. Human milk also contains many other substances that affect nutrition and development in different ways. Milk and its precursor, colostrum, essentially lend the baby part of the mother’s immune system until it can make its own: B cells, T cells, neutrophils, macrophages, and antibodies. Milk also has human hormones and growth factors that are believed by some to be required to optimize the development of the brain and other organs.
You're (Surprisingly) as Hairy as a Gorilla
Among the great apes, the genus Homo is distinguished by an apparent lack of hair. Evolutionary theorists suggest that the ape forebears of Homo were about as hairy as gorillas, who put their hair to many uses, including mechanical protection, UV protection, thermoregulation, sexual selection, social signaling, and waterproofing.
Anatomists note that humans haven’t “lost” their hair — the skin is covered with hair follicles at about the same density as other apes. But the hair itself is different. Most of it is short and fine. Head hair is longer and coarser than body hair. Head hair and body hair may be curly. (No other ape has curly hair anywhere.) It may be lightly pigmented or apigmented. How does Homo escape a predator or thermoregulate under that? Maybe he runs away on his long, hairless legs, cooled by a steady stream of water from newly evolved glands on his hairless chest and arms. Homo evolved evaporative cooling for a hunter’s life on the hot, dry, equatorial savannah.
You Can Process Fear and Emotion
The amygdalae are paired structures in the middle brain almost exactly the size and shape of almonds, which is where they get their name. The amygdalae have been associated clinically with a range of mental and emotional conditions, including religious rapture, depression, autism, and even “normalcy.”
Physicians have widely and publicly discussed one case in particular — a woman whose amygdalae are partly nonfunctional. This patient is incapable of experiencing the emotion of fear. The doctors have tried everything, not just for research purposes but because a total lack of fear is a maladaptive trait; it threatens her well-being and survival. This patient has been injured and victimized in situations that normal, healthy fear would have kept her far away from.
You Smell Like You See
As with other mammals, the human olfactory structures are located at the interface of the brain and the airway. Specialized neurons called olfactory neurons, actually protuberances from the brain, sit right on the border of the nasal passages, behind and slightly above the nostrils. An olfactory neuron bears olfactory receptors on its plasma membrane. An olfactory receptor recognizes a certain chemical feature of an odor molecule, but that feature is present in numerous kinds of odor molecules. The receptor can bind any odor molecules that have that feature. Thus, humans don’t have a single receptor for “coffee” or “lavender” or “wet dog.” They have many receptors for many kinds of molecules released into the air and drawn into the nose. The brain assembles its olfactory perception of the environment by aggregating the signals from the various receptors. The process is similar to that of vision. Odor recognition is like object recognition based on aggregating many different impulses from the retina.
Your Mouth Is More Efficient Than a Chimp's
Compared with other apes, the human buccal apparatus (mouth) is puny. The capacity of the male adult human mouth is about the same as that of the male adult chimp, who is only about two-thirds the man’s size. The chimp can open twice as wide, showing teeth about twice as big. The man has nothing like the chimp’s enormous, muscular lips for expressing juice from fibrous foods. The human temporalis and masseter muscles are small and weak compared with the chimp’s powerful jaw muscles. Yet, with all this extra power, the chimp spends about six hours each day masticating (chewing food), whereas the human hunter-gatherer spends less than one hour. Yes, the puny mouth of a human chews food much more effectively than the mighty mouth of a chimp.
Millions of Microbes Owe You Their Lives
For thousands of millions of tiny creatures, your gut is the only universe they know. They live and die in that warm, moist, nutrient-rich, immune-protected environment. They work almost every minute of their lives, providing a service to their community and their universe. These good citizens of the gut are adapted specifically to that environment, in the way of symbiotic organisms, and can survive nowhere else.
The internal tissues — blood, bone, muscle, and the others — are normally free of microbes. But the surface tissues — the skin, the digestive and respiratory tracts, and the female urogenital tract — have distinctive colonies of symbiotic microorganisms. The term symbiosis (adjectival form, symbiotic) describes a more or less cooperative and reciprocal relationship between organisms and species.
Oxygen Can be Dangerous
It was the biggest environmental disaster ever on the planet. After life forms had gotten along for a billion years or so, obtaining energy from chemosynthetic processes, some bacterial forms evolved the ability to capture the energy of light itself. This was a huge advantage for them, but unfortunately for all life forms then in existence, photosynthesis has a toxic byproduct: molecular oxygen. Slowly, as photosynthesizing bacteria prospered, overly reactive molecular oxygen replaced moderately reactive methane in the earth’s atmosphere.
We can only speculate about what happened next, and some cell biologists have speculated very imaginatively. They started by noticing the strong similarities between certain organelles and some bacteria (between mitochondria and rod-shaped bacteria, for example). Then came the stunning discovery that mitochondria and chloroplasts (the photosynthesizing organelles of plant cells) had their own DNA, completely separate from the genomic DNA in the nucleus, and that this DNA was bacterial in character. Gradually, the view has come to be accepted that eukaryotic cells began as symbiotic communities of different types of bacteria sheltering from the damaging effects of atmospheric oxygen inside a bubble of phospholipid bilayer.
You Can Control Your Breathing but Fido Can't
You don’t have to think about breathing. The steady in-and-out continues while you sleep and go about your daily business. The depth and rhythm adjust to your level of effort. Just climb those stairs; breathing will take care of itself. Many of us would have died young if breathing required constant attention.
Humans are capable of controlling their breath, though. Cetaceans (whales and dolphins) can, too; must, in fact, and some also use breath control to sing. Other animals can’t — or at least don’t appear to. Canines in chorus aren’t really controlling their breath.
Humans use breath control to generate speech. Humans can make finely controlled exhalation pass over the vocal cords while the length and thickness of the cords is changing to generate different frequencies of sound.
Hemoglobin Dominates Your Red Blood Cells
Hemoglobin is the predominant protein in the red blood cells (RBCs). Specialized for the transport of the blood gases, hemoglobin has the capacity to transport molecular oxygen (attached to the heme group) and carbon dioxide (attached to the globin portion) simultaneously. In its oxygenated state, it’s called oxyhemoglobin and is bright red. In the reduced state, it’s called deoxyhemoglobin and is dark red. Thus the colors of oxygenated arterial and deoxy- genated venous blood. (Venous blood isn’t blue as it looks through the skin, especially fair skin.)
A typical RBC contains almost nothing but hemoglobin molecules floating in cytoplasm. Hemoglobin makes up about 97 percent of the dry weight of RBCs, and around 35 percent of the total content, including water. The hemoglobin molecules are put together in RBCs as they mature, before the nucleus and other organelles die off. The heme subunits mostly stay together during the four- month life span of the fully differentiated RBC. When the cell dies, the complex is released and broken up in the liver. The iron is salvaged and recycled in new hemoglobin molecules. The rest of the hemoglobin becomes a chemical called bilirubin, which is secreted through the bile and into the large intestine, where it gives feces their characteristic yellow-brown color.