DIVERSITY OF LIFE

HOMININ EVOLUTION

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INTRODUCTION TO THE HOMININAE

Evolution of the primates (Figure 1), and especially the apes, the Hominidae, has been of general interest since Darwin's publication of The Origin (Darwin 1859).  Even when he wrote The Descent of Man (Darwin 1871), the fossil evidence was only a partial Neanderthal skeleton. He did predict, though, that the fossil evidence would likely be found on the African continent where our closest living relatives now reside.  The picture of hominid evolution is far from complete, but the number of taxa within our line, the Homininae (hominins), has grown steadily since the 19th century.  In addition, topology of the hominin branch of the primate tree has changed from a linear sequence of taxa to one of many branches pruned by local extinctions.

The earliest ape-like primate seems to have been a tailless transitional catarrhine called Proconsul (Figure 2).  Their skeletal and dental remains have been found in Kenya and Uganda from strata ranging in age from 23-15 million years old (the first half of the Miocene; see Stringer and Andrews 2005).  Three species have been recognized and vary in size from small forms (11-36 kg) to large animals (~76 kg). Furthermore, they seem to have exhibited sexual dimorphism (males ~1.3X larger than females). Aside from lacking a tail, Proconsul had a mix of ape and old world monkey characters.  The one complete skull that has been found shows obvious monkey-like characters like a narrow nose, short face, and lack of brow ridges.  The post cranial skeletons show that the animals had a pronograde stance (4-legged) and long, supple spinal column.  However, they were unable to reach over their heads and hang by their forearms like current apes can.  Nonetheless, these animals were well at home in the trees scampering in the branches as monkeys do today.  They occurred in closed forest and forest edge habitats.  Likely, this animal was a sister to the Hominoidea (the apes) rather than a member of the group.

 

FIGURE 1. Relationships between the major primate groups.  Modified after Benton (2005) and consistent with Goodman et al. (2005).  The transition between the Old World Monkeys and Apes (collectively called the catarrhines) occurred with Proconsul.  The Dryopithecines likely are among the earliest of the Ape or Anthropoidea line.

FIGURE  2. Images of Proconsul.  Left: Reconstruction of a skeleton on display at the University of Zurich.  Photo by Guerin Nicolas.  Right: Restoration of Proconsul by Nobu Tamura.  Both images published under the Creative Commons license.

 

FIGURE 3.  Left: The earth as it appeared at the beginning of the Miocene (~20 mya).  Image used according to the Creative Commons license, created by 36ophiuchi. Middle: Sahelanthropus.  Right: A reconstruction of Dryopithecus shown walking on the ground.  Though it likely could have moved on the ground, its home probably was high in the trees.

ORIGIN OF THE APES

The earth during the Miocene epoch (23-5 mya, Figure 3 Left) was warm and saw the proliferation of forested lands across the African-Eurasian supercontinent (Potts 1996).  The climate experienced a maximum around 17 mya and then a gradual cooling trend began to occur.  The movement of the African plate into Europe began the uplift of mountains across southern Europe and into Anatolia (present-day Turkey).  Near the end of the Miocene, the Mediterranean Sea was blocked multiple times with an ensuing dehydration of the basin.  This period, called the Messinian salinity crisis (Garcia-Castellanos and Villasenor 2011), must have caused powerful environmental changes in the lands that border the Mediterranean basin.  In addition, the upper half of the Miocene saw a gradual cooling with the concomitant growth of continental ice on Antarctica.  The Miocene also saw the appearance and radiation of early apes (>100 ape species) throughout the open woodlands of Africa-Eurasia. 

Dryopithecus (Figure 3 Right) was one of the wide-ranging genera that appeared in the second half of the Miocene following the contact of Africa with Eurasia, and its remains have been found from France to Hungary.  The animal was about 0.6 meters long and weighed approximately 14 kg.  Superficially, Dryopithecus resembled a small chimpanzee and seems to have been adapted to open woodlands.  It was a generalist and had mobility of its shoulder joint that allowed it to hang, but it could not knuckle-walk.  The suspensory lifestyle of Dryopithecus made it similar both to the Asian and African apes.  It has been allied to both and may be close to the common ancestor of the living great apes (Pongidae + Hominidae).  In its body size and life style, Dryopithecus resembled most the gibbons of south-eastern Asia.  Such wide-ranging and successful groups as the dryopithecines may well have given rise to the Asian and African apes.  This supports a "Back-to-Africa" scenario in which the Hominoidea originated in Africa and then dispersed through suitable habitats in Eurasia.  The dryopithecines, the basal members of the Hominidae according to Moya-Sola et al. (2009), radiated within Eurasia and then dispersed back into Africa giving rise to the African apes.  The analysis of Moya-Sola et al. (2009) also suggests that the Hylobatidae are not members of the Hominoidea.  We are working with the hypothesis that the dryopithecines are members of the Hominoidea and sisters to the major taxa of living apes as defined by Wilson and Reeder (2005; see Table 1).  This taxonomy also is consistent with the cladogram of Figure 1.  The accepted system of Hominoidea has two families and 20 species.

The most likely living sister of our species is the line of chimpanzees (common and bonobo).  These have been sequenced and compared with Homo sapiens.  Molecular analyses suggest that the genetic similarities between Homo and Pan are so great that all chimpanzees should be placed in the genus, Homo (Page and Goodman 2001; Wildman et al. 2003).  Curiously, Linnaeus also named the Common Chimpanzee Homo trogylodites, in recognition of the similarities.  Most commonly, Homo and Pan are placed in the same tribe, Hominini, and all taxa within the tribe are called hominins.  The separation of Homo and Pan occurred near the end of the Miocene and beginning of the Pliocene epochs (~5-6 mya; Page and Goodman 2001).  Indeed, The Pliocene and Pleistocene epochs encompass the period of hominin radiation 

The penchant for splitting, particularly at the generic and species levels has produced a confusing and chaotic stream of names.  For example, the robust australopithecine Paranthropus boisei might be known as Zinjanthropus boisei or Australopithecus boisei as synonyms, any of which might be found in the literature.  If the proposal of immersing Pan (the genus of chimpanzees) is immersed within Homo, then all of the generic diversity disappears.  All hominin taxa would be in the genus, Homo.  Unfortunately, that proposal has not been accepted generally among biologists and physical anthropologists.  Table 2 is a comprehensive list of bipedal hominin species which also appear in Figure 4.  Those taxa that have an asterisk are recognized as distinct. 

 
TABLE 1.  The system of the extant Hominoidea, the apes, after Wilson and Reeder (2005).  The number in parentheses following the generic name is the number of species within that genus. TABLE 2.  Bipedal hominins arranged according to grades following the separation from the line leading to chimpanzees.  The species are designated according to an extreme splitting taxonomy (all of the species are separate and distinct) and an extreme lumping taxonomy (taxa with an asterisk).  We have underlined the binomials that we describe in the text.  The table is a modification of Table 1 in Wood (2010).  

Superfamily Hominoidea

Family Hylobatidae (gibbons, the lesser apes)

Hylobates (7), Hoolock (2), Symphalangus (1), Nomascus (6)

Family Hominidae (hominids, the great apes)

     Subfamily Ponginae (orangutans)

Pongo (2)

     Subfamily Homininae

Gorilla (2), Homo (1), Pan (2)

GRADE SPECIES INCLUDED IN A SPLITTING TAXONOMY  
POSSIBLE HOMININS

Ardipithecus ramidus*

Orrorin tugensis

Sahelanthropus tchadensis

Ardipithecus kadabba

ARCHAIC HOMININS

Australopithecus africanus*

Australopithecus afarensis*

Australopithecus bahrelgazali

Australopithecus anamensis

Australopithecus garhi

Kenyanthropus platyops

Australopithecus sediba

MEGADONT ARCHAIC HOMININS

Paranthropus robustus*

Paranthropus boisei

Paranthropus aethiopicus

TRANSITIONAL HOMININS

Homo habilis*

Homo rudolfensis

PREMODERN HOMO

Homo erectus*

Homo neanderthalensis

Homo heidelbergensis

Homo ergaster

Homo antecessor

Homo floresiensis

ANATOMICALLY MODERN HOMO Homo sapiens*
* A lumping taxonomy might recognize only these.  Taxa underlined are described below.

 

FIGURE 4.  A composite illustration from Wood (2010) of the bipedal hominin taxa with ranges in the fossil record and color coded according to grade (see Table 2).

 

FIGURE 5. Top Left: Sahelanthropus skull. Photo by Dider Descouns and used under the Creative Commons License.  Top Right:  A reconstruction of Sahelanthropus tchadensis by the Smithsonian Museum of Natural History. Bottom Left: A photograph of the a frontal view of a digital reconstruction of a female Ardipithecus skull.  Image from Smithsonian Museum of Natural History.  Bottom Right: A reconstruction of a female Ardipithecus.  Note the bipedal stance, divergent great toe, and long arms.  Image from White et al. (2009).

 

THE BIPEDAL HOMININS: POSSIBLE PRIMITIVE MEMBERS

Most of the bipedal hominin radiation occurred during the Pliocene (5.3-2.5 mya) and Pleistocene (2.5-0.012 mya) epochs.  The end of the Miocene was relatively warm, but at the end of a gradual cooling trend.  The trend continued through the Pliocene and into the Pleistocene, when the amplitudes of the climatic swings became very great during a time of extremely unsettled and variable climatic shifts.  The last 1.5 million years was especially unsettled with glacial advances and relatively warm interglacial periods.  

The earliest grade of bipedal ape, the possible hominin grade, was represented by Sahelanthropus (~7 mya, Figure 5 top right and top left) and Ardipithecus (~6.0-4.2 mya, Figure 5 bottom left and bottom right) from eastern Africa.  These possible hominins are important in that their fossil remains have been dated near the separation of the human-chimp line.  Sahelanthropus was found in Chad, very far from the Rift Valley and South Africa where almost all other early hominin remains had been found and around the time when the Sahara Desert had just begun to form (Schuster et al. 2006).  Its presence in central Africa suggests that the hominins may have ranged much more widely on the African continent (Brunet et al. 2002). Brunet (2012) describes Sahelanthropus, which is known only from features of the skull, as having a suite of primitive and derived characters that make it unlikely to be a precursor to the chimpanzee line.  Also, it lived in a mosaic of woodland and open spaces, a landscape more like that of the bipedal hominins.  Thus, the postcranial skeleton likely will be somewhat similar to that of Ardipithecus.

Postcranial remains of Ardipithecus ramidus are known for females only.  They were bipedal but had a foot with a divergent toe (Haile-Selassie et al. 2012).  The interpretation was that they must have been arboreal, at least part of the time.  Indeed, their remains were found in association with plants that occur in forests.  The upper body had the proportions of a chimpanzee and the range of movement in the shoulder that suggested aa animal well adapted to climbing and swinging in the trees.  This seemed to support the theory that bipedalism did not evolve in animals adapted to life in the savannahs, but rather by animals that brachiated and walked on limbs very much like orangutans do.  If so, bipedal stance evolved in the trees well before our line began to walk on the ground.  Aside from bipedality, Ardipithecus resembled chimpanzees.  The brain case was about 300-350 cc in volume (females) and they weighed about 50 kg.  Also, females were approximately 120 cm tall.

 

ARCHAIC AND MEGADONT HOMININS

Among the archaic hominin grade animals two are well-represented in the fossil record: Australopithecus afarensis and Australopithecus africanusAu. afarensis had a brain volume that ranged from 375-550 cc and showed significant sexual dimorphism.  Males stood 152 cm tall and weighed up to 42 kg, and females were smaller (~107 cm tall and 29 kg).  They lived between 3.9 and 3.0 mya in eastern Africa.  Unlike Ardipithecus, Au. afarensis had a foot more like ours and a fully bipedal gait (Haile-Selassie et al. 2012), as demonstrated by a famous trackway in Laetoli, Tanzania (see Figure 6).  The upper body still had adaptations for hanging and climbing in trees.  These were animals of variable habitats: forest edges and savannahs.  Australopithecus africanus must have emerged from the Au. afarensis line.  They appeared in the fossil record around 2.8 mya and persisted for about a half million years.  These australopithecines were the same size or slightly smaller than their predecessors (males 140 cm and females 110 cm).  Their brain volume also was comparable.  Au. africanus, seemed to be more dedicated to a terrestrial lifestyle. 

Australopithecus africanus seemed to be in the line to the megadont archaic hominins which persisted from 2.5 to 1.0 mya (Wood 2010).  These are represented by Paranthropus robustus, whose remains have been found through eastern, southern, and western Africa.  Adults ranged in size from 1.1 m (females) to 1.3 m (males) and had a brain volume of about 530 cc.  Most notable are their teeth, which are quite large, especially the molars.  An isotopic examination of their teeth confirmed that their diet was made up of plants, with a large proportion being grasses (Cerling et al. 2011).  This line diverged from the hominins that gave rise to us and died out around 1 mya.

 

FIGURE 6. Australopithecus afarensis. Left: An artist's rendition of a pair of afarensis walking at Laetoli during an ash fall 3.4 mya. Right: A trackway unearthed by Mary Leakey at Laetoli, Tanzania.  The footprints clearly show the bipedal gait of these archaic hominins.  Images licensed through Creative Commons.

 

THE GENUS HOMO

The transitional hominins are represented by Homo habilis (~1.44-2.3 mya) who occupied sites through eastern Africa.  This hominin had a brain volume that exceeded 500 and approached 800 cc.  Considering their small size (females -1m, 32 kg and males 1.4m, 37 kg), the relative brain volume was fairly large.  In addition, their remains have been associated with particular types of stone tools, usually choppers made of lava (Figure 7 Left).

Premodern Homo includes a set of taxa that begin with Homo erectus who stood 145 cm (female) to 185 cm (male, see Figure 8 Left ). Their brain volume also was much larger ranging 750 to 1225 cc.  These were bipedal apes that resembled us and moved like us.  There is evidence for the controlled use of fire and butchered animals with marks on the bones made by a particular tool kit.  They seemed to have ranged through Africa and then moved into suitable habitats through southern Asia and possibly into southern Europe.  They appeared in the fossil record around 1.8 mya and disappeared only around 300,000 years ago.  During the 1.5 million year period they gave rise to the common ancestor of Homo sapiens and Homo neanderthalensis.  This transitional hominin was Homo heidelbergensis (Figure 9 Left) As the name implies, the first identified remains were found in Heidelberg, Germany.  However, fossils assigned to H. heidelbergensis occur into Asia and Africa.  These hominins were tall (males 175 cm and females 157 cm) with brain volumes (1100-1400 cc) that were substantially larger than H. erectus.  They appeared in the fossil record around 600,000 years ago and disappeared 200,000 years ago when our species emerged.

 

FIGURE 7.  Typical tool kits of Homo habilis (left), Homo erectus (center), and Homo neanderthalensis (right).  Images from the Smithsonian Institution Museum of Natural History.

 

FIGURE 8. Left: Homo erectus. Image from the Smithsonian Museum of Natural History. Right: Distributional map of Homo erectus from Athena Review, Vol. 4, No.1: Homo erectus. 

 

FIGURE 9. Reconstructions of select Premodern Homo. Left: Homo heidelbergensis, Right: Homo neanderthalensis. Images from the Smithsonian Museum of Natural History.

 

NEANDERTHALS

Homo neanderthalensis (Neanderthal, Figure 9 Right) appeared around 500,000 years ago either from Homo erectus or Homo heidelbergensis in southern Europe.  Neanderthals were stocky (155 cm, 54 kg -females and 164 cm, 64 kg -males) and heavily muscled.  They lived through southern Europe, the middle east and possibly east toward present-day Georgia.  Animal remains associated with Neanderthals show evidence of butchering as do Neanderthal remains, suggesting ritual cannibalism.  They seem to have been scavengers and also used spears to kill large animal up close.  This was supported by Berger and Trinkhaus (1995, Figure 10) who compared the types of injuries sustained by Neanderthals to typical injuries inflicted on those who work in various risky occupations.  They found that the suite of  injuries as evidenced by broken bones was most similar to trauma sustained by rodeo competitors.  Neanderthals developed a tool kit of stone, wood, bone, tusks, and antlers that was more complex than that of H. erectus and was comparable in sophistication to the tool kits of early modern humans (see Figure 7 Right).

Neanderthals also seem to have had a concept of ritual.  We have mentioned possible ritual cannibalism, but they also may have had rituals associated with burials.  There does not seem to be evidence of neanderthal art or symbolic communication.  Nevertheless, neanderthals and heidelbergensis both had the proper anatomical structures for vocal communication (Barney et al. 2012).  For example, the hyoid bones of all three species are indistinguishable.  The resonating chamber of neanderthals would have made sounds that are more nasal and higher pitch that in modern humans.  Furthermore, Krause et al. (2007) report that Neanderthals had the same derived FOXP2 gene, the so-called language gene, as modern humans.

Homo sapiens and Homo neanderthalensis may have interacted directly.  For example, they both occupied sites in Palestine, but it is unclear as to whether or not they occupied those sites simultaneously.  Finlayson (2009) suggests that the neanderthals formed a barrier to the expansion of modern humans into the Levant from the Nile valley.  There must have been some interaction, though.  Neanderthal DNA markers have been found in the genomes of modern humans, exclusive of subsaharan Africans and native Australians.  

The fate of the Neanderthals has been an enduring question.  Did we displace them or did modern humans move into regions that had been abandoned by the Neanderthals?  Stringer (2011, 2012) says that the differences between Modern Human and Neanderthal group structure may have led to their demise.  That is, Neanderthal groups seemed to be small, about 20-30 individuals.  Human groups, however, were larger than 100.  He suggested that the larger, multigenerational groups of humans were able to maintain cultural knowledge and allow for specialization thus promoting technological innovations.  

Recent proposals include a catastrophic disease brought by modern humans (Wolff and Greenwood 2010), Transmissible Spongiform Encephalopathies (TSE) through ritual cannibalism (Underdown 2008), and an inability to respond to drastic climatic change (d'Errico et al. 2003).  The last scenario does seem more likely.  Despite common belief, Neanderthals may not have been particularly well adapted to cold.  Rae et al. (2011), in a detailed study of the Neanderthal face, conclude that the face was not "likely to be an adaptation to resist cold stress".  Longo et al. (2012) invoke the mechanism of direct competition between modern humans and Neanderthals based on a study of a site in northern Italy which seems to have been occupied by both groups around 35,000 years ago, the time when Neanderthal remains disappear from the fossil record throughout most of Eurasia.  They did persist in a small population on Gibraltar, at the tip of the Iberian peninsula until about 24,000 years ago (Carrion et al. 2008; Lopez-Garcia et al. 2011; Pachero et al. 2012) and are best known from a site known as Gorham's Cave.

Homo sapiens, 24,000 years ago with the demise of Neanderthals became the last bipedal ape in Europe.  This was not the case in south-east Asia.  Finlayson (2009) points to remains of pre-modern Homo in multiple sites of what is now Indonesia.  There, Homo erectus may have persisted and given rise to dwarfed island variants such as Homo floresiensis, which became extinct only 13,000 years ago.  These were small, a little over a meter tall and only about 30 kg.  They used and made tools and had a brain smaller than that of a chimpanzee, which, given their size, provided the same brain: body mass ratio as that of Homo erectus.

 

FIGURE 10.  A comparison of traumatic injury between Neanderthals and Rodeo riders.  Figure modified from Berger and Trinkhaus (1995)

FIGURE 11.  Distributional map of Homo neanderthalensis from the Wikimedia Commons.

 

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By Jack R. Holt.  Last revised: 04/27/2012