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Chapter II. Evolution of the Human Mind >> Introduction: A Quick Guide to Paleoanthropology

In Chapter I, we identified a unique ability of the human mind. Only humans are able to synthesize elements from memory into a new mental image that represents a never-before-seen object in their mind. We named this ability mental synthesis. To an outside observer mental synthesis exhibits itself in the complexity of the human language. We wanted to differentiate between the complex human language and the language of animals. Since human language is intended for synthesis of new objects in the mind of a listener, we named this language a synthesizing language. Since animal language is not intended for synthesis of new objects in the mind of a listener, we named it a non-synthesizing language.

Now we are ready to ask questions about human evolution. What forces shaped the development of the human mind? At what time point in evolution was mental synthesis acquired? At what time point was a synthesizing language acquired? Since we cannot test hominids in a cognitive psychology lab, we will rely on skeletal evidence and on artifacts left by the hominids. Note that in this book the term “hominid” is used to describe modern humans and their extinct relatives. The more scientifically correct term would be “hominan” - a member of the Hominina tribe.

Introduction: A Quick Guide to Paleoanthropology

Bipedal walking in Australopithecus afarensis

All living primates are related to a common evolutionary ancestor. The closest living human relatives include chimpanzees, bonobos, gorillas, and orangutans (the group referred to as great apes). Genetic studies indicate that the last common ancestor of humans and orangutans lived about 12 million years ago. At that time orangutans diverged from the line that includes humans, chimpanzees, bonobos, and gorillas. For 12 million years orangutans developed independently, their genes did not intermix with the rest of the great apes. Gorillas diverged from humans, chimpanzees, and bonobos about seven million years ago. Humans diverged from chimpanzees and bonobos about five to six million years ago. Since then hominids followed their own evolutionary path.

The archeological record supports the genetic studies: the first “chimpanzee-like” hominid specimens appear in the fossil record around six million years ago. The first hominids evolved in the savannas of Africa. The most famous skeleton of an Australopithecus afarensis (from Latin australis "of the south" and Greek pithekos "ape"), named Lucy, was found in 1974 under the direction of French geologist Maurice Taieb. Lucy was dated to be 3.2 million years old. Lucy was only 1.1 meters (3.6 feet) tall and estimated to weigh about 30 kilograms (66 pounds). A number of similar skeletons were found later.

All living primates are related to a common evolutionary ancestor. Apes diverged from monkeys about 25 million years ago. The great apes then parted ways with the lesser apes, a group that includes modern gibbons, about 19 million years ago. Dozens of species of great apes lived during the Miocene, but now only humans, chimpanzees, bonobos, gorillas and orangutans remain. Gorillas diverged from humans, chimpanzees, and bonobos about seven million years ago. Humans diverged from chimpanzees and bonobos about five to six million years ago. Since then hominids followed their own evolutionary path, their genes did not intermix with the rest of the great apes. Bonobos diverged from chimpanzees approximately 2.5 million years ago. Key features of the primates include forward facing eyes which provide good stereoscopic vision with depth perception, relatively large brain, and long childhood.

This illustration represents the timeline and distribution of hominid species. It is likely that all or most hominid species evolved in Africa. The most successful species were able to migrate from Africa to colonize other continents (e.g. Homo erectus who settled in Asia). The most successful was, of course, Homo sapiens who colonized all continents.

In the year 2000, the most complete skeletal remains of a three-year-old Australopithecus afarensis female were discovered by Zeresenay Alemseged. It was named Selam and is often nicknamed “Lucy's baby”. The remains have been dated to 3.3 million years old.

Lucy and other Australopithecus afarensis had longer arms than modern humans, but shorter than chimpanzees: the length ratio of upper arm bone to upper leg bone was 85% compared to 72% for modern humans and 98% for chimpanzees.

Zeresenay Alemseged noted that the shoulder socket of the Selam skeleton was upward-facing and that it was characteristically apelike (modern humans have laterally facing shoulder sockets). The upward-facing shoulder socket may have facilitated australopithecines in climbing by allowing them to raise their hands above their heads.

Full reconstruction of Lucy (Australopithecus afarensis, dated to 3.2 million years old) on display at Museum of Man, San Diego, California (See also Website 2.1).

Lucy and other Australopithecus afarensis hominids could have spent some time in the trees, but they moved by walking upright on the ground as indicated by changes in the knee portion of several skeletons as well as the human-like ankle joint. In addition, the famed 1978 discovery by Mary Leakey and others of the Laetoli footprints in Tanzania 45 km (28 miles) south of Olduvai Gorge provides the strongest evidence for bipedal walking (Video 2.1 describes the process of footprint formation and explains the difference between chimpanzee footprints and Laetoli footprints).

Two lines of hominid footprints were discovered by a team headed by Mary Leakey in 1978 at the Laetoli site in Tanzania, 45 km (28 miles) south of Olduvai gorge. (Altogether the team found over 18,000 individual prints from a variety of animals from rhinoceros to ostrich.) The hominid footprints were preserved in a layer of volcanic ash deposited by an eruption of a nearby volcano.  The rain falling onto the ash created something like wet cement. Animals and hominids crossing the muddy surface left footprints. The chemicals in the ash caused it to harden preserving the details of the footprints. In time, additional deposits of ash and mud covered the footprints. The Laetoli footprints were dated to 3.6 million years ago. There was absolutely no doubt that the hominids who made the tracks were walking upright. These parallel tracks were produced by two individuals walking alongside one another, one larger than the other. The tracks were in step and very close together. These observations were interpreted as indicating that a male and female australopithecine were walking arm in arm. Photo by John Reader. (Video 2.1 describes the process of footprint formation and explains the difference between chimpanzee footprints and Laetoli footprints).

The Laetoli footprints reveal that Australopithecus afarensis foot structure was very similar to that of a modern human. The Laetoli footprints did not have the mobile big toe that we see in the chimpanzee. Rather the big toe is in line with the rest of the toes. They also have an arch typical of that of modern humans.

The Laetoli footprints demonstrate no knuckle impressions. The feet did not have the mobile big toe of apes, rather the big toe was in line with the rest of the toes. The footsteps also had an arch typical of that of modern humans. In humans the arch serves to absorb energy produced by the body-weight, especially important when walking over a hard surface. The Laetoli footprints are so similar to modern human footprints that some scientists have had a hard time believing that the footprints were made by Australopithecus afarensis, the only human ancestor known to have lived at the time. The foot structure demonstrates that Australopithecus afarensis had a foot adapted for supporting the body during bipedal walking. Most scientists now agree that Australopithecus afarensis walked upright when moving in savannas. Based on a computer simulation of the spacing between the footprints, Australopithecus afarensis moved at a leisurely stroll of about 1.0 m/s.

Bipedalism may have benefitted australopithecines by allowing them to stand above the grass when moving on the ground, observing the savanna in an effort to better protect themselves from being hunted by predators such as big cats, hyenas, and now extinct saber-toothed cats, and to hunt or scavenge themselves. Scanning the savanna for prey and predators put a lot of evolutionary pressure on the development of the visual system - australopithecines who could not identify a predator from far away were eaten by predators (as discussed below). In addition, bipedalism freed the hands to do something other than to support the body during walking. When a modern human is attacked by a stray dog, he or she tends to pick up a long stick or a stone for protection. Early hominids could have carried sticks or stones as well. With time they must have noticed that sharp stones worked better than blunt ones. The australopithecines may have even used split cobbles and broken bones as tools; however the oldest stone tools that were undeniably manufactured by hominids are only 2.4 million years old. These tools mark the arrival of a new genus: Homo.

Stone tool manufacturing

The earliest tools were most likely simply split cobbles. Precisely when hominids started to manufacture tools is difficult to determine, because the more primitive these tools are the more difficult it is to decide whether they are natural objects or human artifacts. Therefore, we will start this introduction with the first stone tools that carry clear marks of human hands: Oldowan tools.

Each tool culture is associated with one or several groups of hominids. We will not attempt to cover all species of hominids, nor will we describe all morphological features of the particular hominid species. Rather we will concentrate on features important to our discussion: brain size and ability to generate sounds. Keep in mind that soft tissues normally disintegrate quickly and the only surviving record consists of bones. Whenever DNA is available it provides more insights into hominid abilities, although at this time it is often difficult to associate the DNA sequence with particular features or abilities.

The first manufactured tools: Oldowan tools

Oldowan tools are the first tools that were clearly manufactured by hominids. Their use began about 2.4 million years ago during Lower Paleolithic period. Oldowan tools take their name from Olduvai Gorge, a steep-sided ravine in the Great Rift Valley in northern Tanzania, where many Oldowan tools have been discovered. Oldowan tools were manufactured from hard cobblestones that can be found in riverbeds and beaches. Any material that was hard enough to produce sharp edges was used including basalt, obsidian, quartz, flint, and chert. The cobblestone was repeatedly struck by hammerstone on the edge to remove flakes and to produce sharp edges.

Oldowan tools take their name from Olduvai Gorge, a steep-sided ravine in the Great Rift Valley in northern Tanzania, where many Oldowan tools have been discovered. Two million years ago, the site was that of a large lake. The waters of the lake attracted both animals and hominids. Several active volcanoes often deposited volcanic ash that covered and preserved bones and artifacts. The site was systematically excavated and both fossil bones and stone tools were found. The layered stratification of the site allows accurate dating of the artifacts.

An Oldowan chopper. Museum of Man, San Diego, California.

An Oldowan chopper (side and front views) that comes from the pleistocene site of Dmanisi (Georgia). Its age is estimated at 1.9 million years old.

An Oldowan chopper (front, side, and back views) from an archeological site in the Valladolid province (Spain), in the Douro valley. The horizontal bar is 5 centimeters (2 inches).

The first hominids who clearly handcrafted stone tools were Homo habilis ("handy man", "skillful person"), who appeared in East Africa during the Lower Paleolithic period about 2.4 million years ago. In appearance and morphology, Homo habilis was short and had disproportionately long arms compared to modern humans; however, this hominid had a reduction of the protrusion of its face. Homo habilis had a cranial capacity slightly less than half the size of modern humans (about 630cm³). Despite the ape-like morphology of the bodies, Homo habilis remains are often accompanied by primitive stone tools (e.g. Olduvai Gorge in northern Tanzania, and Lake Turkana, Kenya). We refer to the stone tools of this period as Oldowan tools.

Scientific reconstruction of a Homo habilis. Source: Museum für Archäologie, Herne.

More complex stone tools: Acheulean Tools

Acheulean tools are more complex, pear-shaped handaxes that have been found over a wide area across Africa and much of West Asia and Europe. The primary innovation associated with Acheulean handaxes is that the stone was worked symmetrically and on both sides indicating greater care in the production of the final tool.

The earliest Acheulean tools are dated to 1.65 million years, during the Lower Paleolithic era. The Acheulean tools are named after the site of Saint Acheul in northern France, where some of the first examples were identified in the nineteenth century, although some of the best examples were discovered in the 1.2 million year old layer in Olduvai Gorge.

An Acheulean handaxe found at Saint Acheul in northern France. The stone tool is 20 centimeters (8 inches) high. Drawing by A. Mortillet, end of 19^th century.

An Acheulean handaxe found at Omo Kibish in Ethiopia.

Un-reworked edge of a shape found in both Oldowan and Acheulean tools (front, side, back, and top views). This tool is Oldowan. The horizontal bar is 5 centimeters (2 inches).

Roughly reworked edge; a scalloped effect is produced (front, side, back, and top views). This tool is Acheulean. The vertical bar is 5 centimeters (2 inches).

Acheulean tools are typically found with the remains of Homo ergaster and Homo erectus. Homo ergaster lived throughout eastern and southern Africa between 2 and 1.4 million years ago. The species name originates from the Greek word “ergaster” meaning "workman". Homo ergaster was the first hominid to have the same body proportions (longer legs and shorter arms) as modern humans. The most complete Homo ergaster skeleton nicknamed "Turkana Boy" (alternatively called “Nariokotome Boy”) was discovered at Lake Turkana, Kenya in 1984. The Turkana Boy skeleton provides evidence that despite human-like anatomy, Homo ergaster was not capable of producing sounds of a complexity comparable to modern speech. However, the Turkana' Boy’s larynx was placed in a remarkably similar position to that of a modern human child, suggesting that the speech production of Homo ergaster was superior to that of chimpanzees.

Reconstruction of a Homo ergaster. American Museum of Natural History.

Homo erectus migrated from Africa around 1.8 million years ago, and dispersed throughout most of Africa, Europe and Western Asia. Homo erectus (from the Latin for "upright man") bears a striking resemblance to modern humans. Its brain size expanded over time from 850 cm³ to 1225 cm³ (somewhat smaller than that of modern humans). Homo erectus hominids were tall, at about 1.8 m (5.9 ft) on average. They were also stronger than modern humans.

Reconstruction of a Homo erectus. Source: Museum für Archäologie, Hern.

Advanced stone tools: Mousterian tools

Mousterian tools include handaxes, spearheads, and racloirs (large scrapers created from flint flakes). Handaxes are often manufactured from flint using a more sophisticated Levallois or “prepared- core” technique, which provides much greater control over the size and shape of the final tool. The Levallois technique involves percussion with a soft hammer, made of bone or wood, instead of a hard stone hammer. A stone is elaborately shaped in such a way that a single blow would detach what was an effectively finished handaxe. Mousterian tools were developed around 300 thousand years ago during the Middle Paleolithic period. They were named after the site of Le Moustier, a rock shelter in the Dordogne region in central France.

Examples of Mousterian tools. The horizontal bar is 5 centimeters (2 inches). Source: Dan David Laboratory, Tel Aviv University.

Mousterian tools have been found all over Europe and also in the Near East and North Africa and are associated with Neanderthals (Homo neanderthalensis). The Neanderthals inhabited Europe and parts of western and central Asia from about 400 thousand years ago to about 30 thousand years ago. Neanderthals were similar in height (160cm or 5'3") but were more muscular than modern humans and had stronger bones with thicker walls. Their brain size was slightly larger (from 1200 cm³ to 1700 cm³) and their faces were bigger. They had lower foreheads and bonier brow ridges compared to modern humans. Some Neanderthals had red hair and had skin color that was pale.

Reconstruction of Neanderthal at Neanderthal Museum, Mettmann, Germany.

Neanderthals hunted large animals such as mammoth, using stone-tipped wooden spears for hunting. These spears were made of long wooden shafts with spearheads firmly attached. However, these were likely thrusting spears. Projectile spears are more commonly associated with Homo sapiens. The high frequency of bone fractures, especially in ribs, femur, fibulae, spine, and skulls found in skeletons, suggests that Neanderthals may have been hunting by thrusting themselves onto an animal in an attempt to stab the beast.

It is likely that Neanderthals had a developed vocal apparatus. The Neanderthal hyoid bone found at the Kebara Cave in Israel is virtually identical to that of modern humans. However, the position of the hyoid bone in the vocal tract could not be correctly determined. Position of the hyoid bone, which connects the musculature of the tongue and the larynx, is an important factor when assessing the ability for complex speech.

The hyoid bone connects the musculature of the tongue and the larynx. In the human neonate, the hyoid bone is positioned as high as in other mammals. In both humans and chimpanzees the larynx descends during postnatal development. However, the human larynx descends significantly lower than the larynx of chimpanzees. This allows humans to produce a wide range of sounds that other animals cannot produce.

Further support for a well-developed speech apparatus in Neanderthals is provided by DNA analysis. The FOXP2 gene is the first identified gene that, when mutated, causes a specific language deficit in humans. Affected people have mostly normal motor coordination and cognitive abilities. However, they are unable to correctly pronounce words, form words into grammatically correct sentences, and have trouble understanding complex language. For example, many members of the English family known as KE (abbreviation is used for privacy) with a single mutation in FOXP2 exhibit great difficulties in controlling their facial movements, as well as with reading, writing, grammar, and understanding others. (The protein encoded by the FOXP2 gene is a transcription factor. It regulates genes involved in the production of many different proteins. Therefore it is not surprising that KE-family exhibit several abnormalities.)

The FOXP2 protein sequence is highly conserved. Human FOXP2 protein differs from the zebra finch version by only seven amino acids (humans and zebra finch diverged about 150 million years ago), mouse version by only three amino acids (humans and mice diverged about 75 million year ago), and the chimp version by only two amino acids (humans and chimps diverged 5 million years ago). Recent DNA research indicated that in Neanderthals these two amino acids had already changed to a modern human version (Krause J, 2007) [4]. Thus, modern humans and Neanderthals have an identical FOXP2 protein sequence! This indicates that there was strong evolutionary pressure on development of the speech apparatus before Homo sapiens diverged from Neanderthals (about 350,000 years ago). It also implies that the common ancestor of Homo sapiens and Neanderthals already had a FOXP2 protein sequence that was identical to that of modern humans. This conclusion, taken together with the finding that Turkana Boy’s (Homo ergaster skeleton) larynx was positioned in a remarkably similar position to that of a modern human child, suggests that speech was possible in Homo erectus/Homo ergaster.

Concluding remarks in Paleoanthropology

In summary, the first tools were manufactured about 2.4 million years ago by Homo habilis (see the Hominid species and associated tool culture table). Since then, hominid evolution was closely associated with stone tool improvement. The most striking observation from the stone tool record is that the species capable of manufacturing finer, more symmetrical and sharper stone tools were also more successful evolutionarily: Acheulean tools are sharper and more symmetrical than the Oldowan tools and Homo erectus, a manufacturer of Acheulean tools, succeeded over Homo habilis, a manufacturer of Oldowan tools. Neanderthal (Mousterian) stone tools are even finer than Acheulean tools. In other words, the species that was capable of attending to smaller details in the final product was succeeding evolutionarily.

The second observation is that the use of each particular type of stone tool is associated with a particular hominid: simple Oldowan choppers with Homo habilis, Acheulean handaxes with Homo ergaster and Homo erectus, and Mousterian advanced handaxes and spearheads with the Neanderthal. Each hominid species started at a higher level than the previous one, but once that phase had started, further development was impossible. Homo habilis had over 500,000 years to stumble upon a better stone tools manufacturing technique. But this did not occur: what was easy for Homo erectus (Acheulean tools) was impossible for Homo habilis. Homo erectus did not much improve its tools over two million years. Mousterian handaxes and spearheads easily manufactured by Neanderthals were beyond the reach of Homo erectus. It appears that, within each species, hominids were not smart enough to make stone tools of better quality. In other words, each species of hominids produced the best tools they were able to manufacture. A major evolutionary shift that created a new species (that was different neurologically) was required to improve the tools.

Modern humans improve tools with every generation. What prevented hominids from making better tools over hundreds of thousands of years? Clearly it was not the lack of transmission of knowledge between generations, since all primates are able to teach complicated skills to friends and relatives. Therefore it must have been the limitations of the hominid’s mind that did not allow them to manufacture tools of a better design. In other words, it was the change in the brain structure that enabled each new species to manufacture better tools. Thus, we need to answer the following question: what quality of mind developed stepwise from one Homo species to another that allowed the new species to manufacture better stone tools? And a related question: what new quality of mind allowed Homo habilis to make the first stone tools 2.4 million years ago? Why did chimpanzees never make sharp-edged stones for use in hunting despite their affection for meat?