A Possible Scenario of Human Evolution

September 22, 2012 12:18 pm

Where to start?

While human history is primarily concerned with cultural and technological evolution, we need to understand a possible scenario for the evolution of the biological characteristics that have served as the basis for the human species’ other advances. Maybe a good starting point would be some 75,000,000 years ago. This is a mere drop in the bucket of time, but we have a long way to go before reaching anything closely resembling humans. We pick up our story with the lowly tree shrew.
The tree shrew, which appears quite similar to a mouse, hardly looks like anything we would like to call our ancestor. Yet scientists think this little creature was our connecting link with the lower forms of mammals. Converting this animal into a human would tax the skills of the most imaginative artist. It lacked binocular and color vision, upright posture, hands with opposable thumbs, a larger better-developed brain, and speech. In other words, it had none of the five characteristics that distinguish humans as a species. It also had to lose its tail, fur, and long snout.
The first critical step was moving into the trees away from intense competition on the ground. Life in the trees was more three-dimensional, involving accurately judging distances from branch to branch or else taking some nasty falls. This helped the development of binocular vision. Life in the trees also required hanging on to things to keep from falling. As a result, a primitive grasping hand started to evolve. Also, the more three-dimensional world of the trees required more awareness of things in all directions. This stimulated brain size and development.
Some 25,000,000 years later some tree shrews have evolved into the prosimians. These included the tarsier and ring-tailed lemur, which are often seen at the zoo and mistaken for monkeys. The prosimians resembled humans much more than the tree shrew, having binocular vision, shorter snouts, hands of a sort, and bigger brains. However, they still lacked erect posture and speech, while their brains, hands, and eyes fell far short of human standards. Some 40,000,000 years ago monkeys evolved from the prosimians. Although showing no obvious new developments toward human characteristics, they were more intelligent than prosimians and had better developed hands and eyes.
Next, we come to the apes, our closest cousins. Apes practiced one activity, tree swinging, that helped lead to human evolution in several ways. First of all, since tree swinging put the ape in an upright position, its head had to switch its position in order to see where it was going. A quadrupedal (four-legged) animal’s head connects to the spine at the back of the skull. If we were to stand a dog on its hind legs, its head’s normal position would have it looking straight up. The same was true for the still quadrupedal ape when it started tree swinging, making it more prone to crash into trees. Therefore, the ape’s normal head position moved to connect to the spine at the base of the skull in order to adapt to this new tree swinging posture. This also paved the way for the later adaptation of erect posture that would free the hands for tool use. Speaking of the hands, tree swinging also led to more use and development of the hands giving apes better hand dexterity. The fairly rapid speeds at which apes swung also meant a lot of things came at them quickly and forced them to react quickly, thus leading to further brain development.
If apes had so much going for them, why did they not all evolve into humans? In general, one can say that evolution and natural selection are conservative and do not favor changes unless forced to by circumstances. This was especially the case with chimps, who had an easy niche in nature and felt no need to evolve. It was also true of gorillas whose great size let them stay pretty much the same. Timing was also important. Gibbons and orangutans were swinging in the trees for so long that their arms became over specialized for tree swinging and could not adapt well to life on the ground where our ancestors evolved. On the other hand, baboons came out of the trees too early and had not swung long enough to develop their upright posture. Thus they remained quadrupedal.

Out of the trees

Still, some three to five million years ago some apes did emerge from the trees into the African Savannah (grassland), and the question once again is why? The most likely answer is for food, and this is supported by the most plentiful and durable evidence we have from then: their teeth and jawbones. About this time their molars and jawbones got much bigger, suggesting they were eating lots of seeds and grains, which required massive jaws and molars to grind them up. This also meant that the canine teeth, their main defensive weapon in the harsh and dangerous Savannah, got in the way of chewing. Choosing between defense and eating, nature decided eating was more important and the canines were lost.
This of course created the problem of defense against predators. The solution seems to have been some sort of weapon. It was certainly nothing more than a stick, bone, or rock, but it apparently was effective. If it had not been effective we would not be here to talk about it. The importance of all this is that for the first time in the history of life on the planet, an animal was using a form of technology to extend its power dramatically and increase its chances of survival. The dawn of humans, or more properly, hominids had arrived.
The term hominids refers to modern humans (i.e., ourselves), our most direct ancestors, and collateral branches of our family tree that came to a dead end, such as the Neanderthals. . The earliest of these hominids, known as Australopithecines, lived from one to five million years ago. They were somewhat human in that they had better developed eyes, posture, hands, and brains than the apes. However, scientists do not generally call them humans because their brains were still much smaller than ours (about 450cc compared to around 1400cc for modern man). Their hands also had little or no precision grip, and they probably could not speak. Many see Australopithecines as the missing link between apes and humans.
There were several varieties of Australopithecines. The earliest, Australopithecus Afarensis, provided us with one of the most amazing discoveries in archaeology: forty percent of one skeleton. That may not sound like much, but it was unheard of to find that much of such an old skeleton intact. The scientist who found it, Donald Johansen, was so struck by this find that he even gave it the name Lucy after the Beatles song “Lucy in the Sky with Diamonds.”
Australopithecus Afarensis was the likely ancestor of two other branches of Australopithecines. One branch, the larger in size, was vegetarian. The other branch ate both meat and plants. The importance of this is that hunting for meat required more inventiveness than did collecting vegetation. As a result, the meat eaters developed tools (possibly including containers for better gathering) and weapons much more than the vegetarians did.
Eventually the meat eating Australopithecines evolved into what many scientists call the first true humans, Homo Habilis (“handy man”) with a brain capacity of 650cc. They used and made very crude tools, although they still could not speak. For that reason, other scientists reserve the honor of the first humans for people known as Homo Erectus who had a brain capacity of some 750cc., which gave them the ability to speak.

Technological and cultural developments since the Australopithecines

A good deal of controversy surrounds the evolution of humans and their family tree. However, our evolution over the last million years has revolved increasingly around our technological and cultural innovations rather than biological changes. This is largely because on the one hand, biological changes are purely random, thus making evolution quite slow. However, technological and cultural changes are the products of more conscious and focused efforts to solve problems or create something. Therefore, such innovations happen at a much faster pace and accelerate the pace of change since they build upon previous efforts.
There were two main types of technological development our prehistoric ancestors came up with early on: flint tools and fire. Flint is unique among rocks because, when hit in the right way, it shatters, leaving very thin and razor-sharp pieces that can be worked into blades. Over time, as people spread to areas with little available flint or used up once plentiful supplies, they had to make more efficient use of this precious resource. At first, people were somewhat wasteful of it, maybe making only one hand ax out of a block of flint. It is estimated they got only 2-8 inches of blade for every pound of flint they used. Early Ice Age peoples came up with a method of knocking chips off of a piece of flint and using each chip for an ax or spearhead. As a result, they were able to get up to forty inches of blade per pound of flint. Their descendants would further refine this to get forty feet of blade per pound of flint.

Fire.

Of all the things that our ancestors invented or mastered to protect themselves from the harshness of the physical environment, none was more important than fire. As the ancient Greek playwright Aeschylus wrote, it was the “brightness of fire that devises all” To the Greeks, it was the source of their crafts and civilization itself. It was what distinguished humans from the rest of the animal kingdom and gave them so much power; too much power as far as Zeus, king of the Greek gods was concerned.
The first people who mastered fire could use it, but probably not make it. As a result, they depended on natural sources such as a volcanoes or forest fires caused by lightning for their fire. Considering animals’ natural fear of fire, we must admire the courage of that first individual who dared to pick up a burning ember and take it home. Once our ancestors had harnessed fire and found a way to keep it burning, they discovered some important uses for it.
The first use was probably for hunting and defense against wild animals, since it was obvious that animals feared fire. A common hunting technique would be to start a brush fire and use it to drive game toward other hunters or over a cliff. The value of fire for light and warmth soon became apparent, especially after our ancestors migrated out of Africa into the cooler climates of Europe and Asia. Fire could also harden sharpened sticks into better weapons. Finally, fire was useful for cooking food with several important results.
Cooked meat in particular held several advantages. The heat caused a chemical reaction that created proteins out of the amino acids in meat, thus making it more nutritious and leading to a healthier population. Fire also killed microbes in the meat, making it safer to eat. Finally, fire softened meat, making it possible for the very young and sick to chew it and thus be nourished. Altogether, cooking led to a healthier population that could grow and spread across the globe. We today are so concerned with overpopulation that we lose sight of how important and difficult it was to maintain a stable or growing population until very recently. Back then the average life expectancy was probably no more than twenty years, and half of all children died before the age of five. Thus extinction was a very real possibility. Cooking removed that possibility a bit.

The Ice Ages

Around 200,000 years ago, the planet started turning much colder. The cause of the ice ages is still unknown and subject of several theories including variations in the tilt of the earth’s axis and its orbital path, continental drift, and clouds of cosmic dust blocking some of the sun’s radiation. Whatever the cause or causes, glacial sheets of ice moved south, covering much of the Northern Hemisphere. Summertime temperatures in England probably reached no more than 50 degrees Fahrenheit. By the same token, winters were horribly cold.
Such harsh conditions forced important changes in our ancestors and the various other life forms then. Keep in mind that physical adaptations were not planned or conscious. Rather, natural selection just accelerated the process whereby genetic mutations would be favored. What emerged was a whole new array of animals: giant cave bears, saber toothed cats, and woolly mammoths and rhinos to name a few. Our ancestors also went through some changes as well. Homo Erectus, as our prehistoric ancestors from then are called, had moved into cooler climates in search of game and living space. However, when the glaciers came, they were forced to adapt. What had been a fairly stagnant culture and species in stable conditions now changed at a relatively rapid rate. Even more rapid than their physical evolution was the evolution of their technology and culture.

Accelerated technological development

At this point, we see a cycle of technological development emerge to accelerate our evolution. Tool use stimulated brain development, which helped lead to more successful hunting and gathering. The improved diet and resulting brain development stimulated more tool development, better hunting, and so on. This basic feedback set in motion by hunting and tool use continued to repeat itself through the ages and is still at work today. Each new invention we come up with extends our power and also stimulates us to come up with more new inventions. This was a process that had started long before with the Australopithecines and continues now.

Speech

One of the effects of a bigger brain was the evolution of speech. This allowed both closer cooperation and more efficient sharing of information in such ventures as hunting. Therefore, each generation could easily learn the skills its ancestors had developed and perfected over the years instead of spending most of its time re-discovering them. This stimulated more brain development and ability to speak, encouraging more cooperation and sharing of knowledge, and so on. This feedback also fed back into and further accelerated the previous cycle of technological development, stimulating more sophisticated speech, etc.
However, there were severe limits to early humans’ speech. For one thing, their pharynx, or voice box, did not drop enough to allow the full range of sounds we are accustomed to making now. As a result, their physical ability to speak was only about one-tenth of ours in terms of the sounds they could make and the speed at which they could make them. Their mental ability to speak was also severely limited. It takes a brain capacity of about 750cc to reach the ability to speak. Babies today reach that threshold between one and two years of age. Many prehistoric humans may never have reached that capacity. Or if they did reach the threshold of speech, they probably reached it much later in life than children today do. Combining that with their short life spans, prehistoric peoples had little time to develop anything profound to say, greatly impeding cultural and technological progress for a million years or so.
Better hunting and gathering technology. The Ice Ages also reduced the amount of vegetation available for gathering, thus increasing our ancestors’ reliance on hunting and develop more powerful weapons. When a better ability to speak combined with the process of each invention stimulating ideas for even more new inventions, a dramatic leap in technology and culture also took place. By 10,000 B.C.E., our ancestors, known as Cro Magnon but essentially Homo Sapiens Sapiens (i.e., ourselves) in a primitive setting, had learned to use other materials, notably wood, bone, and antler, in combination with flint, thus vastly expanding their range of tools and weapons compared to the crude and limited tool kit of the earliest hominids:

  1. the use of bone, antler, and ivory for making tools that flint was unsuited for;
  2. the sewing needle that led to warmer, better fitting clothes;
  3. the spear which both extended the range and power of the hunter as a throwing weapon while maintaining a safe distance from dangerous animals when used as a hand held weapon;
  4. barbed and grooved spearheads, which, being more deadly, led to better hunting;
  5. the bolo for tripping up game;
  6. the ability to make fire, giving them a stable source of warmth;
  7. grooved air channels under the fire which led to hotter fires (which would lead to fired ceramics, which led to pottery and the kiln, and eventually to the furnace for smelting metals with all their contributions to civilization;
  8. flint sickles, with bone or wood handles, which led to better gathering and a healthier population;
  9. the burin, the first tool used for making other tools;
  10. woven baskets, which also led to better gathering and more food;
  11. fishing with spears, nets, and gorges (a type of hook), which led to a more stable food supply; and
  12. crude shelters, built at first as wind breaks in the entrances of caves, and later as free-standing structures

Looking at all these inventions, Cro Magnons seem to tower over their ancestors, much as we see ourselves towering above them. This is deceptive, however, because we are building on what our ancestors built. Without the accomplishments of Cro Magnon and those who went before them, our own civilization could never have evolved.
All these new advances had profound implications for the future. For one thing, our ancestors’ larger brains would help lead to the development of the human family. Secondly, increasingly efficient hunting, gathering, and fishing made possible a more settled lifestyle, giving people time and opportunities to notice certain things around them, in particular the way seeds grow into plants. This revelation was the basis for the next great step in human evolution, the food producing revolution, or agriculture. Finally, better brain development and technology inspired and made possible new activities and behaviors that make the Cro Magnons seem much more modern to us.
Our ancestors’ behavior over the last 100,000 years or so also shows a much higher degree of intelligence than ever before. For example, they seem to have first realized the inevitability of death and created a religion to prepare for it. We have found people buried facing east and west, and also with the pollen of flowers in their graves. Our ancestors apparently worshipped the spirits of cave bears with whom they competed for living space. One Neanderthal cave has the skulls of some eighty bears arranged around it.
Prehistoric people also seem to have cared for their sick and infirm as evidenced by the skeleton of one man who lived to about forty years of age (old for back then) with the use of only one arm. They also apparently practiced female infanticide (killing female babies) as a form of population control. This is a comment not so much on our ancestors’ brutal nature as on the brutal conditions they had to deal with in order to survive. Not practicing such a measure might have meant extinction for the whole tribe or species.
Cro Magnons seem more modern to us culturally as well, especially in their art. In southern France and Spain they left a number of cave paintings that are amazing for their artistic touch and sensitivity. These paintings depict the various animals people then hunted. Their function may have been some sort of sympathetic magic in which portraying a successful hunt would cause a successful hunt. Whatever their purpose, these paintings are striking in the way they depict these animals in motion. They also can make us feel much more akin to these people we call our ancestors.

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