HOMO

Two evolutionary trends

As noted above, Australopithecus sediba, Homo habilis and Homo rudolfensis all became extinct as the climate of eastern Africa became very dry.  (Even the Paranthropines, who had adapted to low-quality foods by chewing on their food all day became extinct.)  But some members of the Homo clade survived by either moving to southern Africa or moving back to Eurasia.  Among these surviving members of the Homo clade there was a continuation of two evolutionary trends that had started much earlier:  a switch from mostly gestural languages to mostly vocal languages and increasing brain sizes.

The switch to vocal languages

After the first glacial maximum had passed, European hominins, our ancestors among them, gradually increased in size, making them less vulnerable to predation.  This allowed them, over the course of the next million years, to gradually combine their gestural language with vocal elements.

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Gestures can more easily convey emotional responses, such as surprise, as well as spatial relationships and plans of action, but vocalization can alert an entire group of hominins to something that is happening.  This is undoubtedly one reason why hominins began to combine gestures with vocalization.

Vocalization ensured that everyone was fully aware of what was going on.  Consequently, vocalized language began to be used in parallel with gestures; but then another advantage of vocalization began to assert itself:  Vocal language is more accessible to infants and young children, while their brains are still maturing.

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The switch to vocal languages was not a simple one, because vocal languages are structured linearly -- words are built out of linear strings of phonemes, and sentences are built out of linear strings of words.  The cerebellum (the central portion of the brain that connects diretly to the brain stem) had mediated in the gestural languages by which our ape ancestors had built their own gestural communication systems, and it continued to play that role in our own gestural and vocal communication.

In the course of evolution the cerebellum expanded greatly in size as it took on the task of interpreting the linear strings of vocal language into meaning for the listening infants.  Then, as the infants matured the cerebellum would be tasked with controlling the muscles of the lips, tongue and vocal cords to produce vocal language.

But the operations of the cerebellum are similar to to those of artificial intelligence, and they are unconscious.  This meant that the meaning of a string of vocal signals had to be transferred to the conscious parts of the brain, and conversely an intention to create a meaningful string of sounds had to be conveyed to the cerebellum.

The conscious parts of the brain seem to lie on various surfaces of the cerebrum, which is the rest of the brain, exclusive of the cerebellum.  When the cerebellum determines the meaning of a string of vocalizations and distributes aspects of this meaning, through intermediate brain cells, to some of the conscious parts of the brain.

Conversely, when the conscious parts of the brain arrive at an intention to produce a linguistic vocalization, this intention needs to be sent to Broca’s area, which is a patch of brain tissue located on the left side of the motor cortex near the front of the brain.  This patch of brain tissue then sends the message to deeper layers of the brain, and ultimately the message is sent to the cerebellum, which then converts the intention into strings of phonemes.  The cerebellum then causes the muscles of the lips, tongue, vocal cords and diaphragm to produce the string of phonemes as vocalized speech, or into babble in the case of an infant.

By observing imprints left by the brain on the inner surfaces of fossilized skulls, scientists have been able to determine that hominins who lived about 1.8 million years ago were still using gestural languages --  their neocortices (outer surfaces of the brain) did not have the enlargement at Broca's area that resulted in the extra fold that later resulted from expansion of that patch of brain tissue near the front-left side of the cerebral cortex (the outer surface of the brain).

This patch of tissue needed to expand in order to execute its part in the complex procedure that is outlined above for the vocalization of speech, and this expansion pushed one more crease into the surface of the cerebral cortex, which left an imprint on the inner surface of the cranium of later members of the Homo clade

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In the course of the next million years or so, the imprint of this extra fold gradually shows up on the inner surface of the cranium, but this evidence of vocal speech is contradicted by another piece of evidence:  the dimensions of fossilized inner-ear structures were such that the hominins could not have heard consonants.  Consequently, vocal speech must have been based almost entirely on vowel and tonal sequences and stops -- until later in our evolution when our more recent ancestors perfected a purely vocal form of communication that could quickly convey complex information.  (Homo neanderthalensis could hear consonants, but not as easily as Homo sapiens.)

Brain size

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The remarkable increase in the brain sizes that has occurred among most members of our genus in the course of the last two million years or so is something that needs to be explained somewhere in the story of human evolution, and it is becoming clear that two factors working together jumpstarted the process, starting more than two million years ago.

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The first and earliest factor was bipedalism.  Obviously, bipedalism did not by itself bring about any great brain expansion, because our ancestors have been bipedal for at least eleven million years; but about five million years ago, when our ancestors became fully terrestrial, and their hips narrowed, so as to enable them to walk more efficiently.

This adjustment narrowed the opening through which the birth canal had to pass, and since it was difficult or impossible for the head of a fully mature fetus to fit through the narrowed opening, birth had to occur while the infant was immature and still helpless.  This forced changes in family structure; but still, brain sizes did not increase greatly at that time.

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The second factor seems to have made its appearance between two and three million years ago when gestural language developed to the point where the hunting skills of our ancestors began to match those of other predatory animals; -- not only in the case in which two-legged semi-aquatic hominins drowned competitors and prey alike but also in the case where slow-moving but smart hominins constructed weapons and devised plans of action that would enable them to match the prowess of predatory animals whose skills had been honed by evolution.

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Our ancestors could not afford to wait millions of years for evolution to give them the brain power to accomplish this feat; they needed a quick fix.  That quick fix was to discard many of the controls on proliferation of neurons in the brain and let neuron proliferation run rampant.  In their small, highly inbred groups, a single member with just the right kind of neuron proliferation could give the group a hunting or communication advantage.  With luck, this new skill would spread throughout the group, and a new species would appear on the scene in the course of a few generations.  This rapid evolution would eventually enable us to master the entire globe, for better or for worse.

Return of ​Homo to Eurasia

Our ancestors moved into Africa prior to the fist great glaciation of the Ice Ages, which reached its climax about 2.58 million years ago.  But some of them quickly moved back thereafter as shown by crude stone tools in northern Jordan − flakes that our likely ancestors had struck off from a rather large rock about 2.5 million years ago.

By the time they returned to Eurasia about 300,000 years had elapsed since they had abandoned Eurasia for the tropics, and they had developed new cognitive and social skills that better suited them for trapping medium-sized animals such as deer and for claiming carcasses from large carnivores such as leopards.  Still, some of them returned to their old habit of wandering about in search of bleached bones, because this allowed them to remain in compact groups, where they were less vulnerable to predation.  (Hunting other animals required them to spread out as they sought to trap their own prey.)  Others looked for the best hunting grounds, and they found them at the base of the Tibetan Plateau, where water constantly trickled down from the high plateau to the much lower Loess Plateau.  Erosion from water movement cut deep canyons where animals such as deer and antelope could be easily trapped.  Stone tools found along the southern edge of the Loess Plateau indicate that tool-making hominins occupied this region from about 2.4 million years ago to 1.26 million years ago.

No hominin fossils were found on the Loess Plateau, but three thousand miles to the west, in the Republic of Georgia, hominin fossils, crude stone tools and butchered bones have been found at a site in the Republic of Georgia, while similar fossils of hominins with large braincases have been found at a site about an equal distance to the east, in central China.  Moreover, similarly dated stone tools (but of higher quality) have been found further south, just south of the Sea of Galilee.  These fossils and higher-quality tools found in the Rift Valley might have been left there by a new wave of immigrants coming north along the Rift Valley from Africa.

The fossils and tools found in Georgia and in the Levant have been dated to about 1.8 million years ago, while those found in central China, known as the Yunxian skulls, have now been dated to about 1.77 million years ago.

The fossils found in China and at least some of those found in Georgia belong to the Homo clade.  Interestingly, the teeth of some of those found at the site in Georgia exhibit characteristics of Australopithecines, and these hominins (with a puzzling combination of physical traits) are now known as Homo georgicus, whereas the others (whose fossils were found at the same site and dated to exactly the same time) definitely belong to the Homo clade, and are now known as Homo caucasi.

The bodily proportions (relative arm and leg lengths) and shoulder construction of the two species are almost identical but Homo georgicus had much smaller heads, in addition to having teeth that are more like those of Australopithecines.

The coincidence of the coexistence of these two hominins at the same time and place indicates that they likely relied on different resources for their sustenance.  For instance, Homo georgicus might have relied mostly on water sources for their food, whereas Homo caucasi hunted or trapped medium-sized animals.  Moreover, there may have been a symbiotic relationship between the two species:  Homo caucasi may have relied on Homo georgicus for emergency food -- especially as Homo caucasi was most likely predatory and Homo georgicus might have had a generous layer of fat around his middle section to conserve heat while immersed in water.

The physical similarity between the two species leads to a suspicion that they might have interbred on occasion, although the offspring would most likely be less viable because they would not be well adapted to the lifestyle of either species.

These fossils of Homo caucasi were 630,000 years younger that the oldest stone tools found near the base of the Tibetan Plateau, so members of the clade seems to have occupied a wide swath of Eurasia for hundreds of thousands of years.

Homo caucasi were most likely not our direct ancestors, but they may have been close to our direct line of descent.  Our ancestors eventually wound up in Africa, but that was still far in the future.

Incidentally, Homo caucasi were somewhat smaller in size than their contemporaries in southern Africa, possibly because African hominins were challenged by more dangerous large carnivores.  Alternatively, Homo caucasi may have been smaller because of periodic food shortages whereas at least some of their African contemporaries were already keeping fires burning for long periods, perhaps for years at a time, and therefore able to preserve meat by smoking it.  This could have allowed them to evolve to have larger body and brain sizes.

The bones of Homo caucasi and Homo georgicus show signs of food shortages early in life.  Such shortages might be related to seasonal changes in the food supply or might instead be related to sociosexual dynamics of the hominin societies.

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The fashioning of garments?

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Homo caucasi may have already begun to practice the art of tailoring. but they also needed to learn to wear garments, and to take care of them.  Inclinations to do all of these things are observable in the behavior of modern humans but are seldom observed in the behavior of other animals.

For instance, the notion of personal property -- as opposed to real estate -- is seldom observed in other animals, aside from possession of food and a mother's sense of ownership of her offspring.  Other animals often feel that they own territory, or a dwelling, but seldom feel that they own something that they carry on their back.

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The maternal concern that prompts modern human mothers to make or buy clothing for their children is like the maternal concern that prompted ancient human mothers to make good garments tor their offspring.  Moreover, cultural norms prompt us to wear clothing or to wear tattoos or other body ornaments that define us.  The pride and sense of identity that we feel when we display fine clothing or jewelry on our bodies are like the behaviors that prompted our early ancestors to take care not to lose their garments, regardless of whether or not body coverings were immediately needed in order to keep warm.

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The behavioral changes that prompted humans to make and wear garments did not come easily to our ancestors because those behaviors had not been part of the behavioral repertoire of their own ancestors − the behavioral and the ideational changes that enabled our ancestors to make and wear well-fitting garments were acquired through genetic as well as cultural evolution.

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In any event, it is doubtful that these ancient Eurasian hominins could have produced well-fitting garments without better tools, and even more doubtful that they would have been able to resist the temptation to discard their garments the minute the weather improved.

Control and use of fire?

Likewise, the value of learning to create and control fire was something that dawned slowly on our ancestors.  Once they recognized the value of fire, a long search then ensued for ways to create fire, and this search continued for hundreds of thousands of years before the invention of modern matches and cigarette lighters.

The first use of fire was the opportunistic exploitation of the bounty that followed in the path of natural fires.  The remains of animals that had been caught in wildfires furnished lots of meals, so one of the first uses that humans made of fire was to intentionally set wildfires.

Preservation and use of fire

But wildfires could not be set unless a burning fire had been preserved from a previous wildfire.  This required preserving a fire for at least a year, long enough to allow vegetation to regrow and then die.  This was generally difficult, because fire could only be preserved long-term in a cave environment, where it would be well protected from wind and especially from rain.

Nevertheless, detailed examination of a cave located in southern Africa has revealed that ancient members of the Homo clade did indeed maintain fires almost 1.8 million years ago in a cave located in southern Africa.

Apparently owls nested in the cave, and as they regurgitated the remains of rodents that they had swallowed whole, deposits of the regurgitated pellets built up on the floor of the cave.  Early  who also lived in the cave used the pellets to maintain fires in the cave, and this may have enabled them to start yearly wildfires in the region.  This is the earliest evidence of intentional use of fire by humans.

Another early use of fire, and one that would serve as a motivation for keeping a fire going throughout the years, must have been to preserve meat by smoking it.  Then, if hunting expeditions failed to result in fresh kills, the hunters might not go hungry.  This use of fire prevailed for hundreds of thousands of years, along with the intentional setting of wildfires.

Homo erectus sensu lato

Homo caucasi was the earliest named representative of a series of closely related species or subspecies who are known collectively as Homo erectus sensu lato, and Homo erectus sensu lato were most likely responsible for keeping the fires going for long periods of time in a cave in southern Africa.  Remains of many of these species or subspecies have been found in Africa and elsewhere.  They moved along the coasts and along the river valleys of Africa, South Asia and Southeast Asia between two million and one million years ago.  Like Homo caucasi they had little incentive to fashion and wear garments and never were able to tolerate cold temperatures, probably because they wore no garments.  However, others who are also classified as Homo erectus sensu lato did acquire an ability to survive in colder climates.  Among these may have been the owner of a skull that was found in central China and recently redated to 1.77 million years ago.  This skull, known as the Yunxian skull, was essentially contemporary with those of Homo caucasi, indicating that Homo erectus sensu lato were likely spread throughout an immense territory in Eurasia as early or earlier than two million years ago.

The more seasonal climate of Eurasia must have prompted hominins living there to start covering their bodies with animal hides, while those living In Southeast Asia readapted to finding much of their food by diving into the sea, and may have re-acquired an ability to expel large amounts of salt from their bodies through their kidneys, sweat glands and tear ducts.  This may have helped some of them to reach Luzon in the Philippines and helped others to reach Flores Island (now known by another name) in Oceania.  These island dwellers evolved to be very small -- a condition known as island dwarfism which also affected elephants that reached Flores Island.