Homo habilis

DATES: 2.5 to 1.6 million years ago

SITES: Olduvai, Tanzania; Koobi Fora, Kenya; Omo, Kenya; Sterkfontein, South Africa; Uraha,; Longuppo, China? (Culotta, 1995; Larick, 1996; Wanpo, 1996)

CHARACTERISTICS:

     Homo habilis differs from Australopithecus at the base of the skull.  The foramen magnum (the opening for the spinal chord) is closer to the middle of the skull and the skull base is reduced in length but increased in width.  The face decreased in width and the nasal opening was more sharply defined.  The postcanine teeth were smaller than in Australopithecus.

     Its cranial capacity was 500 to 800 cc. and values increase from the earliest specimens to latest ones.  This range overlaps with Australopithecus at the low end and Homo erectus at the high end. It can be debated (indeed, there has been a debate several decades long) on whether early H. habilis should be classified as Australopithecus and late H. habilis should be called Homo erectus.  H. habilis stood approximately 5 foot tall and weighed 100 pounds with females being smaller than males (Leakey, 1973b; Wood, 1987; Leakey, 1971b; Hughes, 1977; Johansen, 1987; Bilsburough, 1988; Tobias, 1972).  Early Homo populations coexisted with australopithecines (Johanson, 1976).

     In one brain endocast, there is a bulge corresponding to Broca’s area (an important speech center) in modern human brains.  Two aspects of wrist bones (the scaphoid tubercle and the articular surface of the trapezium) were chimp-like.  The thumb was similar to humans in the carpo-metacarpal joint and the flattened metacarpal surface.  The foot was less flexible than in chimps and its degree of possible abduction was limited.  Some characteristics of the lower leg were primitive and others were advanced, not found in any ape (Susman, 1982; Skelton, 1986).  

.

Although humans are not speedy sprinters compared to quadrupeds and running is metabolically twice as expensive as it is for many other animals, humans are the only primates and one of the few mammals which practices endurance running. (Dogs, hyenas, wildebeest, and horses are also capable of endurance running.) It is thought that endurance running evolved with the genus Homo given that some of the specializations which allow endurance running (such as short toes, an adducted big toe, a more balanced head, neck ligaments, and an enlarged calcaneus) evolved in Homo habilis, other features (such as enlarged semicircular canals, shorter forearms, and larger hip, leg, and back muscle attachment sites, longer legs, and a stronger sacroiliac joint) evolved in Homo erectus, and several other features (a head which was more independent from the shoulder, a narrow pelvis, an arched foot, and a long calcaneal tendon) evolved early in the Homo lineage although the precise point is not known (Bramble, 2004).

The following image compares a Homo habilis skull to that of a female bonobo chimp.

Homo erectus

DATES: 1.8 million years ago to as late as10 to 27 thousand years ago (have been dated with K/Ar, Ar/Ar, U series, ESR, and faunal dating methods)

SITES: Kenya, Tanzania, Algeria, Morocco, Georgia, the Middle East, China, Indonesia, and Java

CHARACTERISTICS:

      The cranial capacities range from 750 to 1225 cc; early forms average 900 cc; later forms average 1100 cc.  The depth and robusticity of the zygomatic bone differs from modern humans.  H. erectus is similar to H. habilis in its protruding jaws, large molars, lack of a prominent chin, thick brow, and the long, low skull.  H. erectus had human limb proportions and human body shape (it was the first hominid to have this). Homo erectus increased the length of its thighs and the hips became more human-like (McHenry, 2006). Its skeleton more robust than modern humans, suggesting considerable strength (Rightmire, 1981; Brown, 1985; Leakey, 1973; Walker, 1981; Day, 1971; Clark, 1976; Leakey, 1976; Wolpoff, 1984.  There was some sexual dimorphism in the skeleton and one specimen shows signs of hypervitaminosis A (Walker, 1982).

     The assemblages of tools found with Homo erectus are more sophisticated than earlier tools and 2 sites are associated with fire.  In Asia, Homo erectus coexisted with Gigantopithecus.  In Africa, Homo erectus coexisted with Australopithecus (Leakey, 1971, Gowlett, 1981; Wanpo, 1995; Leakey, 1976b; Jacob, 1978; Weiner, 1998; Tianyuan, 1992).

Fossil specimens dating from about 1.75 million years ago discovered at Dmanisi, in the Republic of Georgia have been tentatively identified as the most primitive known Homo erectus/Homo ergaster. Although the most primitive skull whose classification as a member of Homo is not in dispute, it resembles Homo habilis. It appears that early hominids similar to Homo habilis migrated from Africa prior to the evolution of the more typical Homo erectus form (Vekua, 2002).

     Homo erectus is considered to be the first hominid to travel out of Africa (although a find from China and others in Central Asia may belong to H. habilis rather than H. erectus).  Homo erectus is known from Asia (Java, China, Pakistan, and Georgia) as of 1.7 million years ago and the Middle East as of 1.5 million years ago (Gabunia, 1995; Klein, 1973; Larick, 1996; Howells, 1980)    

     Homo erectus is known in Java as early as 1.8 million years ago and as late as 50,000 years ago.  The combination of very thick crania and evidence of healed skull fractures and head trauma have caused some to propose that male Homo erectus fought with each other using clubs (Boaz, 2004).  The last Homo erectus populations existed in Java between 27 and 53 thousand years ago dates which overlap with the existence of Homo sapiens sapiens in the region (Gibbons, 1996; Thorne, 1972; Swisher, 1996).  The earliest African forms of Homo erectus have smaller and more projecting noses.  Some researchers classify these specimens as Homo ergaster and feel that Homo ergaster or some pre-erectus population migrated from Africa before H. erectus proper evolved.  One find of Homo from 1 million years ago from Afar (near the Red Sea) is similar to H. ergaster but also has characteristics of Homo sapiens.  Some anthropologists feel some skeletal characteristics (the frontal keel, torus mandibularis, os epactale, and incisor shape) of Asian erectus populations indicate that there was interbreeding between these populations and later H. sapiens emigrants from Africa.  The transitional nature of the fossil material makes it difficult to assert which populations should be regarded as separate species. (Dean, 1995; Lewin, 1989; Wolpoff, 1984; Turner, 1989; Gibbons, 1998; Abatte, 1998). 

A skull similar to the Dmanisi skull (considered an early Homo erectus) has been found representing a toothless individual. This suggests that the diet of these hominids included soft food items and perhaps that the social structure of this hominid allowed for support from other members of the group (Lordkipanidze, 2005).

The skeleton of an infant Homo erectus indicates that the endocranial volume was within variations for Homo sapiens and distinct from the chimpanzee range (Leigh, 2006).

Homo fossils are known in the Caucasus since about 1.8 million years ago, Spain and Italy from 800,000 years ago and northern Europe from 700,000 years ago (Parfitt, 2005).

Homo floriensis inhabited Indonesia at least for the period from 95-74 to 12 thousand years ago. Heights of two individuals have been estimated at 1.06 and 1.09 meters. The structure of the brain is most similar to Homo erectus but the size of brain relative to the body is comparable to australopithecines and chimps. It lacked a chin. While facial and dental features justify its classification in the genus Homo, it may not be a species of Homo erectus that evolved a shorter stature (Morwood, 2005). Flores humans had a brain the size of a grapefruit, more similar in size to extinct hominids and modern apes than to ours.  Despite this small size, Flores humans apparently used both tools and fire (Associated Press, 2004). Although Flores humans possessed brains comparable in size to those of chimpanzees, they apparently made use of advanced stone tools. Brain endocasts indicate that their frontal lobes were large and highly folded and that their temporal lobes were enlarged as well (Small but Smart?, 2005).The overall brain size (440 cubic centimeters) and the brain size/body size ration of Homo floriensis is similar to that of australopithecines. The shape of its brain, however, is more advanced and bears similarities to that of Homo erectus, although it is not simply a smaller version of a Homo erectus brain. Enlarged frontal lobes (Brodmann region 10) and temporal lobes, indications of the use of fire, stone tools, apparent predation on large mammals, and the fact that these hominids somehow traveled to the Indonesian island of Flores all indicate advanced intelligence (Falk, 2005). The oldest Homo floriensis fossils date from 38,000 years ago and the youngest from 18,000 years ago. It is presumed that they reached Indonesia by boat (Morwood, 2004). Brain analyses support the classification of Homo floresiensis as a separate species rather than a microcephalic modern human (Falk, 2007).

ARCHAIC Homo sapiens/ Homo heidelbergensis

DATES: 500 thousand years ago to 200 thousand years ago

CHARACTERISTICS:

     The earliest specimens classified as archaic Homo sapiens or Homo heidelbergensis  have features of both H. erectus and H. sapiens sapiens with robust skeletons and teeth.  Some specimens have brow ridges, receding foreheads, and receding chins.  Some specimens already display some neanderthal facial characteristics (morphology of the eye sockets, cheekbones, and nasal bones) suggesting that neanderthals may have diverged from other lineages quite early.  There is an advanced expansion of the cranial vault and the average cranial capacity was 1200 cc (Brauer, 1993; Howell, 1952; Trinkaus, 1997; Stringer, 1988; Stringer 1993; Arsuaga, 1997).

       One significant change that occurred between the archaic and modern humans was a shortening of the sphenoid bone in the braincase.  Externally, this change resulted in jaws, which did not protrude to the same degree.  Internally, the change could have had a much more profound effect: it would have affected the dimensions of the oropharynx and the variety of sounds that humans could have made in speech.  (Chimps and gorillas are capable of large vocabularies when using sign language and their use of signs suggests a degree of complex thought which they were previously thought to be incapable of.  Apes do not speak because they do not have the anatomy for it.  Thus changes in the vocal tract could have had a much larger impact on language than increases in brain size.) (Hublin, 1996; Rightmire, 1997; Rightmire, 1976; Holden, 1998). The development of human speech required a movement of the splanchnocranium under the neurocranium, a movement of the tonge posteriorly, and the development of a pharynx which was longer and narrower (Davidson, 2005).

Homo antecessor displays a mosaic of traits, such as primitive dental features and advanced aspects of the midfacial region, which indicate that it could represent the common ancestor of Neanderthals and modern humans (De Castro, 1997). Some have suggested that Homo erectus was not directly ancestral to modern humans and hominids such as Homo antecessor represent the common ancestor of Neanderthals and modern humans (De Castro, 1997). Homo antecessor possessed a styloid process (De Castro, 1997).

Homo neanderthalis

DATES: 230 to 30 thousand years ago

SITES: throughout Europe east to Israel and Uzbekistan

CHARACTERISTICS:

     Although some neanderthal characteristics are found in modern humans or earlier Homo species, the combination of these characteristics in neanderthals is unique.  These characteristics increased in frequency as later neanderthals evolved.  As a result, there are differences between the robust “classic” neanderthals of Western Europe and the populations of the Middle East.  

     Neanderthals had a long trunk, short legs, and a robust skeleton.  The size of muscle attachment sites imply considerable muscle power.  A diastema (gap) exists between the jaw and lower wisdom teeth.  The skull and face were long, the forehead low, there were protruding brow ridges, the rear of the braincase was round (not pentagonal as in modern humans), there was a large nasal cavity, the floor of the eye socket was flat or even receding laterally, there was a strong mandible lacking a chin, a protruding midface, the cheekbones were weak and oriented obliquely, the occipital bone bulges posteriorly (the occipital bun, a trait which very rarely can still occur in modern humans), and the occipital bone has a conspicuous depression known as the sura-iniac fossa.  Hand bones indicate that Neanderthals may not have been as dextrous as modern humans (Musgrave, 1971; Trinkhaus, 1982).  Although neanderthal faces seem abnormally long, they are actually similar to earlier specimens of Homo.  Modern human faces are much shorter, including the region of the pharynx which affects the ability to produce the sounds of human speech (Trinkhaus, 2003).

     Males averaged 5’6” in height but because of their robustness, they may have averaged 30% larger than modern humans.  The average brain capacity was 1450 cc (a little larger than that of moderns) and one young male specimen had a capacity of 1750 cc (Kappelman, 1997).  While some concluded that neanderthal nasal specializations make them unique among hominids, others have refuted this (Schwartz, 1996; Laitman, 1996; Franciscus, 1999)..  Some neanderthal hand characteristics are unique.  Neanderthal hip morphology suggests that they were more active as children.  Some analyze this as a potential culture difference between neanderthals and the modern humans that replaced them (how active/mobile the young members of a group were).

     Between 300 and 127 thousand years ago there were two periods of glacial advance separated by a cool interglacial period and many features of the Neanderthal body are adaptations to cold temperatures.  Neanderthals have provided the first evidence of burying the dead 100 thousand years ago and later burials include animal bones and flint.  One 47,600-year-old cave has an impressive 4-meter x 5-meter structure built of stalagtites and stalagmites. Neanderthals arrived in Israel after modern humans and the two groups coexisted (with apparently no exchange of technologies) for 60 thousand years (Bar-Yosef, 1993; Hovers, 1996).  Neanderthals used fire and used more tools and weapons than Homo erectus.  The types of tools used change very gradually and the style of neanderthal tools are referred to as Mousterian.  At some sites, human and ungulate bones are smashed to similar degrees, suggesting cannibalism (Defleur, 1999; Jelinek, 1982). Neanderthal populations decreased at about the same time the European climate was becoming colder and drier about 35 to 40 thousand years ago (Mellars, 2006).

.  In Southern Spain, Neanderthals survive 5-10,000 years after modern Homo sapiens appear in the north of Spain but they don’t acquire any new technologies.  In other parts of Europe, Neanderthals do acquire new technologies just before their disappearance, possibly as a result of interaction with modern Homo sapiens (Balter, 1996;). 

The oldest remains of Homo sapiens sapiens are known from Ethiopia and have been dated at 195,000 years old (Wilford, 2005). East Africa underwent severe drought during the period of 135 to 75 thousand years ago. Lake Malawi, for example, seems to have been reduced to 95% of its original extent. This may have been a factor in the migration of some humans out of Africa (Scholz, 2007). Modern humans became lighter and more gracile.  (Gibbons, 1997).  The sphenoid bone shortened, meaning that the face did not project quite as far.  The skull became higher and rounder in contrast to the low, long skulls of Homo erectus and Neanderthals (Lieberman, 1998).  How did modern humans evolve?  There are two different models.

--Replacement Model:

     In the replacement model, it is thought that somewhere between 1 million and 100,000 years ago, modern humans left Africa, migrated throughout the continents, and completely replaced all other hominids (such as Homo erectus, Homo neanderthalis) so that they alone were ancestral to all modern human groups.  Proponents of this model vary on the timing, often due to discipline; paleontologists favor older dates, geneticists favor younger dates.  One strong piece of evidence that is frequently observed is that the greatest genetic difference in human populations occur between African populations.  The evidence for some of the oldest branches of humanity exist in Africa suggest that humanity evolved in Africa (Zischler, 1995; Waddle, 1994; Vigilant, 1991; Wood, 1997). Advanced tools (referred to as Acheulan; they included items such as hand axes) first appear in Africa and are known outside Africa as of 500,000 years ago (Lewin, 1987).

--Regional Continuity Model:

     Although modern humans did leave Africa over 100,000 years ago, they interbred with the hominid populations which were already present on other continents (Homo erectus in Asia, Homo neanderthalis in Europe) to produce many of the regional differences in modern humans.  The discovery of apparently ancient DNA sequences in Asian and Australian populations which are not found anywhere else in the world supports the model of regional continuity (Harding, 1997; Holden 2001).  Some anthropologists conclude that a number of fossil finds of Homo erectus and Homo neanderthalis have physical traits which correspond to the modern humans from those regions from much later dates (Wolpoff, 1993; Thorne, 1992; Thorne, 1981) . A Homo erectus skull from India supports a relationship to modern Homo sapiens in India (Sonakia, 2006). Asian fossils combine features of early modern humans with those of archaic human lineages, in support of the regional continuity model (Shang, 2007).European fossils which combine the features of modern humans and Neanderthals support the regional continuity model (Trinkhaus, 2007).

     In comparing the microsatellite DNA in the human genome, the most divergent patterns are observed in some African populations, suggesting an African origin of humanity (Goldstein, 1995).  Comparisons can be made between mitochondrial genomes (which are primarily passed from women to their children) and Y-chromosome sequences (which are passed from men to their sons).    These comparisons tentatively indicate that women have actually been more mobile than men (in that they did not necessarily stay in the areas in which they were raised) and thus were greater agents of gene dispersal (Pennisi, 2001).The gene microcephalin (MCPH1) and its surrounding genes can exist in various forms, known as haplotypes. One form, known as haplotype D, is found in 70% modern humans. Although the haplotype seems to have spread through human populations several tens of thousands of years ago, the degree of nucleotide divergence between members of the same haplotype is far greater than what would be expected. One explanation is that interbreeding between archaic humans and modern Homo sapiens resulted in a beneficial set of alleles which were then positively selected for (Evans, 2006).

      Modern humans arrived in Australia by 40-60,000 years ago (there are sites in East Timor which are slightly younger at 35,000 years old).  The Clovis culture of big game hunters is known in the New World as of 12,000 years ago.  In recent years, there have been a number of pre-Clovis sites found in both North America (ranging from 12-19,000 years old) and even in Chile (12,500 years old).  The ancestry of the first Americans may be more complex than what was first thought—some skeletal traits of fossils and genetic analyses indicate that native Americans are more similar to Europeans than Asians (Marshall, 2001; Pena, 1999; Gibbons, 2001; Holden, 1996; Morell, 1998).

      Molecular analyses point to three groups of aboriginal Americans: Amer-Ind, Na-Dene, and Eskimo.  The Na-Dene and Eskimo might not represent separate migrations but rather early offshoots of the Amer-Ind group which were isolated in northern regions by changing climates (Bonatto, 1997).