Geology 143

Lecture #24

April 22, 2002

More on Fossil Hominids

A major episode of global cooling (and drying) then took place about 2.8 million years ago (mid- to-late Pliocene). Global sea level fell dramatically, indicating more water was stored as ice at the poles. Hence there was less moisture available to fall as rain, and the climate in East Africa became much drier. Woodland began giving way to grasslands with smaller, more isolated pockets of trees. This brought about changes in feeding strategies and morphological changes within two distinct lineages of hominines produced two new genera from ancestral australopithecines. One branch evolved to become the genus Paranthropus with adaptations to heavy grinding and chewing. The face became flattened, accented by broad cheeks for the attachment of chewing muscles (of the masseter complex), and in individuals identified as males, a prominent sagittal crest was present for attachment of temporalis muscles. The molars are very large and circular in outline, and were worn flat over the lifetime of the individual, apparently from a diet of coarse seeds, nuts, and tubers. Premolars were somewhat molarized, and the incisors and canines were greatly reduced (smaller than in even modern Homo sapiens). The Paranthropus lineage persisted until just over 1 million years ago. Difficulties caused by a low-quality abrasive diet were apparently offset by a reduction in competition for food resources with other hominines.

The second new genus to appear shortly after the 2.8 m.y. climatic deterioration was Homo, which took a very different approach in dealing with the scarcity of fruit. Homo habilis (and its descendants) developed smaller molars, a larger brain, and the ability to produce tools to allow scavenging of carcasses on the grasslands. The first stone tools (appearing 2.6 million years ago) include scrapers for removing meat from bone and choppers for extracting marrow from bones and processing vegetable matter. Tooth wear in Homo habilis suggests that scavenging was an occasional phenomenon and that vegetation still comprised the majority of its dietary intake. Homo itself first appears 2.5 million years ago (as a jaw fragment that has not been formally assigned to a species). Most, if not all, manufactured stone tools were the product of our genus Homo, given the supeior preservation potential of these stones (in comparison to human bone), it isn't surprising that the first tools would appear in the record before the first tool makers. Unlike Australopithecus afarensis, the metacarpal bone of the pollex (thumb) is broad, allowing for the attachment of muscles to provide the precision grip characteristic of our species, and the possibility of manufacturing stone tools.

Homo erectus, which first appeared 1.9 million years ago in Africa, was the first species to attain modern human height and had a larger brain case than H. habilis. The post-cranial skeleton is effectively modern with a strongly adducted knee (femur directed more fully inward than in Lucy), a spine with modern human curvature in the lumbar (abdominal) region, and long hind limbs. The tall, lanky build of the species, with a long tibia unlike that seen in australopithecines, would have been effective in shedding heat (large ratio of surface area to volume), and also would have brought increased walking and running speed. Homo erectus was the first hominid to be found outside of Africa (they persisted in Asia longer than in Africa). Tooth wear indicates that meat was a primary component of their diet. They were cooperative hunters, not merely scavengers, and may have left the continent in the pursuit of prey.

Homo heidelbergensis evolved from African H. erectus populations about 800,000 years ago. This species possessed a yet larger brain than H. erectus, equal to the lower end of variation in modern Homo sapiens, but the skeleton was more robust (especially the massive lower jaw). H. heidelbergensis, formerly referred to as "archaic Homo sapiens," was the ancestor of both our own species (H. sapiens in the stricter, modern sense, which first appears about 130,000 years ago, also in Africa) and the neanderthals (H. neanderthalensis, first appearing 230,000 years ago).

The neanderthals, which evolved from European populations of H. heidelbergensis, were restricted to Europe and the Middle East. They had brains that were larger, on average, than our own (though the shape of the braincase was longer front-to-back and not as tall in the frontal region). They were short and stocky with a protruded snout and a broad nose bearing additional bony scrolls (termed nasal turbinates) for the attachement of extra mucous tissues to warm inhaled air. Aside from the increase in braincase, these morphological changes are adaptations to cold climate. They are also reversals of previous trends in human evolution, which were earlier influenced by the warm, sunny climate of the African plains. Thus neanderthals appear "less human" to our eyes than they are phylogenetically (they are our sister species, sharing a common direct ancestor). They became extinct 30,000 years ago, soon after western Europe became inhabited by H. sapiens (10,000 years prior).

Our own species, Homo sapiens, has morphological hallmarks (distinguishing us from our ancestor, Homo heidelbergensis) in the increasingly tall frontal portion of the brain, reduction in the robustness of the lower jaw, and formation of a chin. The chin serves as an attachment site for jaw muscles, partially compensating for the weak bite produced by our reduced snout and jawline. Chimpanzees and gorillas, along with early hominines, possess a "simian shelf" that performs the same function (with greater effect in these species with a much stronger bite).

Development of Language in Hominids

This material will not be on Exam 3, nor the final, but is here if you are interested

Language, like tool making, is thought to have been an important factor in the development of large brains within Homo. {Note: brain tissue is metabolically "expensive;" it takes more energy to feed the electro-chemical reactions of the brain than it does to feed an equivalent amount of muscle tissue. Thus, the brain will not increase unless it produces a significant adaptive advantage to counter its energy demands.} Although the larynx (a mostly cartilaginous funnel sitting atop the trachea; its muscular folds are the familiar "vocal cords") and tongue do not fossilize, nevertheless skeletal evidence can be used to assess vocalization potential in fossil hominines.

Four primary skeletal differences between modern humans and chimpanzees have been assessed, and support a model of successive increase in the potential for articulate speech within our lineage.

1) The upright posture of all hominines (primates more closely related to us than to chimpanzees) puts less stress on the vocal cords.

2) The reduced snout and jaw (seen in Homo [and Paranthropus], but not Australopithecus, a trend that was reversed in the cold-adapted Homo neanderthalensis) aids in the formation of consonant sounds.

3) Basicranial flexion (again seen in Homo, but not Australopithecus) is a term used to describe the shape of the basicranial wall of the skull (just ventral to the foramen magnum, on the underside of the skull). This region is flat in other mammals and in humans younger than two years old. Later in human development, the basicranium becomes arched or "flexed" as the larynx (and associated epiglottis) descends deeper within the throat. Unlike all other mammals, in humans the epiglottis cannot form a seal with the soft palate (which descends from the roof of the mouth). In other mammals, food from the mouth takes a path that is consistently lower than the top of the larynx. Food migrates around the sides of the larynx and into the esophagus. In humans, the descent of the larynx provides extra room, a chamber of air, above the larynx that allows humans to produce a greater variety of vowel sounds than is possible in other mammals. However, the descent of the larynx also produces a greater risk of choking in humans, as the food and air pathways are crossed within the throat. Breathing or speaking while simultaneously swallowing produces a great risk of food becoming lodged in the trachea. The adaptive benefits of complex speech must have been great to overcome this hazard.

4) The hypoglossal foramen, a passage through the base of the skull for the hypoglossal nerve, which services the tongue, became larger starting approximately 400,000 years ago within Homo heidelbergensis. The large hypoglossal foramen is a trait passed down to both descendent species, Homo sapiens and Homo neanderthalensis. This is presumed to signal an increase in the hypoglossal nerve itself, and perhaps a more limber tongue.

One the hallmarks of Homo sapiens, the tall frontal portion of the skull, may also be related to speech competence, as this is where Broca's area (one the most important parts of the brain with regard to the localization of speech) resides. Perhaps "fully human" speech was within the grasp of later representatives of Homo heidelbergensis or perhaps it was restricted to our species. Neanderthals, with their extruded snouts, were probably not quite as articulate and this may have been a factor in our outcompeting our extinct sister species.