Why Humans Can Throw
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Try a simple experiment. Reach behind you and feel how high on your back you can touch your spine. The fact that you can touch your back at all with your forelimbs should astonish you: except for monkeys and apes, humans are the only species that can do that. The evolution of that ability created an important part of the evolution of Humanity.
We assume that our remotest ancestors walked on all fours as most other mammals do. Imagine walking through an African forest some sixty million years ago. At some point on your walk you may see a creature that resembles a squirrel climbing a tree. Like a squirrel, it walks on all fours on the ground and climbs with all fours, using its sharp claws to dig into the bark of the tree as it climbs. But it’s not a squirrel; it’s a prosimian, the ancestor of monkeys, apes, and humans.
Some five million years after the event that extinguished the dinosaurs, mammals, the little rat-like insect eaters that were among the lucky survivors of the Chicxulub event were radiating into the ecological niches that the great reptiles left empty. One of those niches was the arboreal, life lived in the trees. In the Paleocene Epoch of the Tertiary Period of the Cenozoic Era the mammals that found food and security in the trees became the prosimians. In the Eocene Epoch (55.8 Mya – 33.9 Mya) the prosimians’ arboreal lifestyle promoted some truly strange mutations.
The creatures’ paws changed. The adapt-or-die logic of natural selection promoted those mutations that made the phalanges longer and made the innermost phalange make a right angle with the others. Generation after generation the mutations accumulated and spread throughout the population. At the same time the phalanges became broader, the claws flattened out into nails, and the pads at the ends of the phalanges became more sensitive to touch. Thus the dance of mutation and selection transformed the paws of the prosimian ancestor into hands and feet suitable for grasping branches.
Life in the trees also promoted mutations that changed the prosimians’ eyes. The eyes themselves became larger, to cope with low light levels in the forest, and developed color vision when the prosimians began to eat fruit (color indicates when the fruit is ripe enough to eat). Also the creatures’ skulls began to change, shifting the eye sockets from facing sideways (as in most mammals) to facing forward, thereby giving the prosimians the predatory stare of a carnivore. The resulting stereoscopic vision gave the prosimians depth perception, a useful faculty for a creature living in the three-dimensional maze of the forest canopy.
Those changes promoted other changes in the prosimians’ skulls to accommodate alterations in the brain and the olfactory sense.
Certainly the prosimian brain had to become larger due to the evolution of the neural circuits necessary to give the animal full color vision. But the brain also had to grow to accommodate more complex neural circuits, especially in the neocortex. The more complex the neural circuit, the more precisely it will pulsate at a given frequency; thus, the larger brain refines the timing between the animal’s intent and the response of its muscles. That refinement has definite survival value for a creature that leaps from branch to branch in the forest canopy.
As the prosimians became more dependent upon their full-color, three-dimensional vision, they became less dependent on their sense of smell; consequently, their snouts became shorter. That shortening did not happen because of any selective pressure that gave the animal a reproductive advantage over its fellows with longer snouts. Any trait that’s not useful to an animal eventually fades away (think of the forearms of the Tyrannosaurus rex). Mutations that coded for a shorter snout simply were not eliminated by natural selection, but, then, neither were any mutations that coded for a longer snout. On the principle that non-construction takes less in the way of resources than does construction, we can see that over a long span of time (hundreds of thousands, perhaps even millions, of generations) the snout-shortening mutations would come to dominate over the snout-lengthening mutations and the prosimians evolved flatter faces.
One other change in the prosimians’ skulls reflected a change in their posture. During the Eocene Epoch the foramen magnum, the opening through which the spinal cord enters the skull to connect to the brain, shifted from the rear of the skull toward the center of the skull’s underside. That shift reflects a change in the animals’ behavior, from holding their spines horizontal (as most mammals do) to holding them vertical. We expect such a change in behavior from animals that hang from, stand and walk on, or sit on the branches of trees.
At the end of the Eocene or the beginning of the Oligocene Epoch (33.9 Mya – 23 Mya) one group of prosimians evolved into monkeys. Their tails became longer, less fluffy, and more muscular, able to serve as a fifth appendage by which the creatures could hang from branches. More importantly, the creatures’ shoulders changed, enabling the animals to use an overhanded grasp to hold onto branches and enabling them to brachiate, swinging from branch to branch, hand over hand, as they propelled themselves through the forest canopy. Monkeys and the apes that evolved from some of them can rotate their arms through most of the four-pi steradians the constitute the solid angle of a complete sphere.
That ability to rotate the arm through a wide variety of angles enables primates to throw. Of course, monkeys and apes don’t throw well. From a range of twenty feet a chimpanzee can’t hit the broad side of a barn. But in one species of ape the ability to throw accurately provided a life-or-death challenge that promoted the growth of the brain.
As noted in the essay "The Calvin Throwing Hypothesis", William H. Calvin (1939 Apr 30 – ?) in the 1980's proposed the evolution of the human brain through the throwing of what he called killer frisbees, stones that had a point.
I have one of those killer frisbees, which I found in the Negev Desert near Beersheva in February 1972. It is simply a rounded cobble that has had three pieces whacked off to give it a point. Some texts that I consulted called it a Stone Age knife, but it seems remarkably unsuitable for cutting anything. Its edges are just too dull to rip through the skin and meat of an animal. It does, however, feel eminently throwable. It fits neatly into my hand and it has about the weight of a baseball.
Archeologists typically find relatively large numbers of these stones around waterholes in Africa, perfect locations for ape-men to ambush herds of herbivores. In the ambush the ape-men threw their stones at a high angle (to give them maximum range certainly), thereby ensuring that at least some of the stones would come down point first, dig into an animal’s back, and tug on the skin. That tugging triggers a reflex that makes the animal crouch down and an animal that crouches in a stampede gets trampled. The hunters then swooped in and used their spears to finish of the wounded animal.
Certainly the more accurately the hunters could throw their stones, the more meat they got for their families and, thus, the greater the probability that they would survive to reproduce their kind. That selective pressure promoted any mutations that made the ape-men’s throwing more accurate; in particular, it promoted mutations that gave the ape-men bigger brains, especially brains with a bigger neocortex. But before they could engage that process of mutation and selection for brain growth our ancestors had to inherit two traits from their ancestors – an adaptation for long-distance running and an innovation in behavior.
As climatic change transformed the forests of East Africa into savannah (beginning 6 – 7 Mya) and the proto-chimpanzees living in that area evolved into australopithecines in response, the creatures’ hips evolved to transform the bowlegged waddle of a chimpanzee into the straightforward stride of a human. At the same time the legs grew proportionately longer relative to the size of the rest of the body, giving the early hominids a longer stride. And the shape of the foot changed: the toes became shorter, the big toe shifted its orientation to lie aligned parallel to the other toes, and the part that we would call the palm of the foot lengthened to almost as long as an outstretched hand. Those traits gave the hominids the support and balance necessary for long-distance running, a survival-enhancing activity that promotes the necessary mutations; but those traits also gave hominids the support and balance necessary for accurate throwing.
The innovation in behavior apparently began about 1.8 million years ago as a result of something hominids began doing at least 2.5 million years ago. As indicated by cut marks made on bones from that period, by 2.5 million years ago at least one species of Australopithecus was using stone tools to strip meat from carcasses abandoned by other predators. Adding meat and bone marrow to their diet gave the hominids more nutrition than they got from a purely vegetarian diet: they may have noticed that eating meat and marrow made them feel more energetic.
About 1.9 to 1.8 million years ago one species of Australopithecus evolved into Homo erectus, the first humans. The environmental factor that promoted the mutations that transformed Australopithecus into Homo seems to be that innovation in behavior: by 1.8 million years ago hominids were cooking their food. According to an hypothesis devised by Richard Wrangham, an anthropologist at Harvard University, the mastery of fire and the cooking of food enabled australopithecines to become fully human.
It likely began in the aftermath of a wildfire. A family of hominids came across the carcass of an animal that had been burned to death. As the family fed on the carcass someone discovered that the meat closest to the severely burned areas was softer, easier to chew into the paste-like bolus that they then swallowed (try chewing down a small piece of uncooked, unground beef and you will understand what a wonderful discovery that was). Using their mastery of the art of making fire, to provide warmth and to repel predators, the hominids began to put meat into the fire to soften it. Eventually they discovered that putting the roots and tubers that made up much of the vegetable component of their diet into the fire also softened them.
Switching from raw food to cooked food may, on first impression, seem a minor advance in human evolution. But consider the fact that chimpanzees spend roughly half their waking hours chewing their food while we humans spend an hour per day or less chewing our food. The time that our ancestors gained by eating cooked food enabled them to do more foraging, more hunting, or to create culture (think of hominids building huts to protect them from the weather and thereby inventing the village).
But cooked food gave our ancestors another advantage. Rachel N. Carmody of Harvard University has done with mice experiments that demonstrate the fact that cooking releases more of the nutrients in food than are available from the food in its raw state. Pound for pound cooked food provides more chemical energy than raw food does. That fact enabled the evolution of the humans’ large brains.
Although the human brain makes up less than three percent of the body’s weight, it uses sixteen percent of the energy that the body acquires. In the absence of an abundant source of energy that fact demotes any mutations that code for a larger brain. A large-brained hominid who ate only raw food would have to spend too much time eating or waste away. The eating of cooked food gave hominids the abundant source of energy that they needed (though they were not aware of it as such): although cooked food in itself did not promote the mutations that gave hominids bigger brains, it allowed the process that did. Once they began eating cooked food about two million years ago proto-humans engaged the evolutionary process that William Calvin described, increasing the size of their descendants’ brains, and eventually evolved them into Homo sapiens.
Appendix: Another Piece of Ape Anatomy
Another unique feature of the human anatomy also seems to have evolved during our ancestors’ arboreal phase. The palatine uvula (more commonly just the uvula) is a cone-shaped extension that protrudes from the middle of the rear edge of the soft palate. It appears to have evolved to help deflect solids and liquids into the esophagus, presumably through the Coanda effect. Along with the soft palate, it also helps prevent nasal regurgitation by closing the nasopharynx during swallowing.
In the Letters column of the Dec 2011 issue of Science Illustrated Scott McCleve of Douglas, Arizona added the following datum:
"In the July/August issue, you had a tiny Ask Us item about the function of the uvula. My experience may speak to this issue. A few years ago, mine was removed because of a small wart. Since then, I have had problems with food or drink going down the bronchial path instead of the esophageal path. I think that the uvula functions to deflect in a downward direction food bits and sips of liquid that are aspirated. With mine gone, these food bits seem to fly straight back and enter the bronchial path."
McCleve appears to have made a minor error in the relative placement of the trachea and the esophagus; in fact, in primates the trachea is located in front of the esophagus. Thus, food and drink must pass over the entrance to the trachea on their way to the esophagus. When we swallow, the epiglottis, the fleshy flap under the rear of the tongue, closes the entrance to the trachea. But sometimes the epiglottis doesn’t close completely or in time – such as when we are startled – so some material gets drawn into the trachea and we choke. The uvula evolved to help prevent such occurrences by drawing the liquid or semi-liquid material coming from the mouth away from the trachea through the Coanda effect, which effect makes fluids adhere to surfaces across which they flow. That evolution happened because of the upright posture of primates.
In other animals the combination airway and food passage lies oriented horizontally, so food and drink must be lifted into the esophagus. The coordination of the muscles required for that act minimizes the possibility of the contents of the mouth going into the trachea and choking the animal. But in monkeys and apes the upright stance promoted by the arboreal lifeway reorients those passages to the vertical. Food and drink can simply fall into the esophagus and, thus, can also fall into the trachea and choke the animal. Any mutation that helps to minimize the possibility of choking will be promoted by natural selection; thus, we have the uvula.
The merging of the nasal cavity and the oral cavity at the back of the mouth, where they then split into the trachea and the esophagus gives us a perfect example of truly unintelligent design. Indeed, the fact that we are able to choke on food or drink makes Intelligent Design a hideous act of blasphemy, one that accuses the Designer of either being malicious or stupid. In actuality, it reflects the fact that we evolved from fish. In fish the relative placement of the esophagus and the tube that would become the trachea in land animals is irrelevant to the animal’s survival and reproductive success, so natural selection doesn’t affect it. By the time vertebrates emerged onto land the pattern was set and now it can’t be changed. When that or any other feature of a living being becomes troublesome, evolution produces a kludge. That’s how evolution works and it seems to work rather well because that’s how evolution created all of the life we see all around us.
Appendix: Immunity of the Long-Distance Runner
In "Long Live the Humans" in the October 2013 issue of Scientific American Heather Pringle reports on research that revolves around the observation that Homo sapiens is by far the longest lived of the primates. The typical chimpanzee, our closest relative, has a life expectancy of 13 years while the typical American has a life expectancy of almost 80 years. In the absence of physical insults to the body (accidents, violence, environmental stresses such as famine, drought, excessive heat or cold, etc.) the typical human can live for the better part of a century, even without the advantages of modern technology and social organization. The elites of ancient societies, protected from physical damage, lived into their 60's to their 80's. Pharaoh Ramesses II of Egypt lived into his 90's, as did the Greek philosopher Xenophanes. As long as we have been human, apparently, we have been long lived. Why?
In the absence of physical damage the greatest threat to multicellular life is infectious disease. Bacteria or viruses get into a larger creature (plant or animal) and take over its cells to feed themselves or reproduce themselves, thereby causing the damage that debilitates or kills the host. Any traits that can defeat such attacks will thus enlarge the longevity of the creature possessing those traits. We thus infer that humans possess an immune system that far outperforms the immune systems of other primates. Again, why? What factors in the evolution of humans led the interplay between mutation and selection to produce a hyperactive immune system?
It had to be something that allowed infectious agents to enter hominid bodies more easily than they can enter the bodies of other primates. That something is the soft, thin, virtually hairless skin that evolved as one of the adaptations to long-distance running across a hot savannah. Unlike the skin of elephants and rhinoceros, which responded to the sparsity of hair by becoming thick, leather-like hides, the skin of humans had to remain soft and thin in order to accommodate the waterworks that issue the sweat that keeps our bodies from overheating.
Hair originally evolved as armor, likely resembling the spines of an echidna at first. In the early Triassic Period, when mammal-like reptiles evolved into true mammals, the placodes that produced the reptiles’ scales suffered mutations that transformed them into follicles that produce hair. Covered with hair instead of scales, mammals are effectively protected from the cuts and abrasions that let infection into their bodies.
As the forests of East Africa slowly thinned out into savannah starting six or seven million years ago, the proto-chimpanzees that lived in them evolved into hominids, the australopithecines. The need to cross ever larger distances across open grassland promoted those mutations that made the chimpanzee’s upright walking less clumsy and more efficient and, more importantly, enabled upright running.
Observations of Native Americans in the Nineteenth and Twentieth Centuries indicate that a healthy, well-conditioned human adult can run one hundred miles (160 kilometers) or more in a day at an average speed of about six miles (ten kilometers) per hour. That kind of effort compels the body to generate large amounts of heat and that fact challenged evolution to promote the mutations that enabled the hominid and, later, the human to run long distances without overheating. The genes that code for more or less hair on the body and those that code for more or fewer sweat glands in the skin provide obvious targets for the working of mutation and selection to create an efficient cooling system for running hominids: the evaporation of perspiration draws heat out of the skin and dissipates it and a lack of hair allows air to flow freely over the skin to enhance the rate of evaporation.
But the loss of body hair exposed the hominid to an increased chance of infection. That fact promoted the mutations that enhanced the chimpanzee immune system that the hominids inherited and made it more robust and aggressive. Thus we can see how an adaptation to long-distance running brought about the enhanced immune system that gives humans their longevity.
Of course, that hypothesis gives us little more than a just-so-story. To transform hypothesis into theory requires evidence. In this case the evidence consists solely of fossilized bones. From an examination of ancient skeletons paleontologists can determine when the traits associated with upright running – reshaped hips, longer legs, and altered feet – evolved. More subtle clues, such as the amount of wear on the teeth, can enable an estimate of the individual’s age at death. If data from enough skeletons indicate that longevity evolved along with the adaptations for long-distance running, then we may consider the hypothesis proven and verified. The research is ongoing.
Finally, we may note that the evolution of longevity gave Humanity a wonderful gift – grandparents. As elders of the tribe, grandparents were repositories of the tribe’s knowledge and wisdom. With their long lives they could accumulate knowledge that came slowly, such as knowledge of astronomical cycles, which is necessary for the invention of agriculture. Thus the need of early hominids to run across a hot savannah led ultimately to the evolution of civilization.
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