To anyone with a rudimentary understanding of paleontology and anthropology, it may not be readily apparent that these disciplines can be in any way related to one another or useful in informing the other’s primary interests. Anthropology, broadly speaking, is concerned with the study of human culture and behavior, with data provided directly by investigations of modern human populations, as well as historical and ethnographic texts and objects. Paleontology, however, is the investigation of the history of fossil flora and fauna and is, as such, allied closely with geological sciences.
Despite what may appear to be two entirely separate and unrelated forms of study, both are united in their multidisciplinary nature, rigorous comparative methodologies, and crucially, an emphasis on context. An object is more meaningful when one grasps the variety of cultural, historical, and geographical contexts of the object itself. Likewise, the context of a fossil discovery can tell us a great deal about how old it is and under what circumstances the organism lived, died, and was deposited. Aside from simply sharing a context-based interpretive framework, where the two disciplines meet, there is a unique branch of biological anthropology known as paleoanthropology. Like the straightforward combination of their individual names, this subdiscipline is a straightforward combination of the essence of both paleontology (fossil life-forms over time) and anthropology (human behavior and culture), resulting in the study of the evolution of the biology, behavior, and culture of humans and our hominin ancestors in the past several million years.
We modern humans, Homo sapiens, are primates. We are the only living member of Hominini (the group to which all human ancestors belong), and the only living member of our genus Homo, but this was not always the case. Like many of the living, or extant, nonhuman primates (e.g., the great apes, Old World monkeys, New World monkeys, etc.), our hominin ancestors frequently lived during the same time periods and sometimes in the same geographical regions as closely related species. The majority of accumulated evidence suggests that our ancestors diverged from chimpanzees between 5 and 7 million years ago (henceforth referred to as mya, and thousands of years ago as kya). The story of how this hominin clade (a group of related species that has the same common ancestor) evolved, subdividing into a number of lineages on the human family tree during the past several million years but now represented solely by H. sapiens, is what paleoanthropology seeks to understand.
Fossil evidence constitutes a major aspect of our quest to make sense of our complex evolutionary history. Prior to the earliest evidence for stone tools—found in an area of Ethiopia known as Gona and dated from approximately 2.5 to 2.6 mya—our prehistoric ancestors leave us no evidence other than their fossilized remains (Semaw et al., 1997). After this time period, stone artifacts become critically important and comprise yet another major component of paleoanthropological investigations. Archaeologists who examine the material culture of hominins rely in particular on their stone tools, in the discipline known as lithics, and on the remains of other mammals living at the same time. We can glean a great deal about hominin behavior from the differences in lithic industries across time and space, and the marks left by teeth and tools on fossilized bones of other mammals provide evidence of hominin dietary adaptations and the ways in which meat resources were acquired and processed. There are many more sources of evidence included in the paleoanthropo-logical repertoire as well, such as investigations of living primate behavior and molecular studies.
This article will focus primarily on the mammalian fossil evidence for hominin evolution. The dental and skeletal features of hominins and nonhominin mammals can tell us a great deal about the evolutionary relationships of the different species as well as how they moved, what they ate, and the types of habitats to which they were best adapted. You will be presented first with background theory in paleontology and ecology focusing on the ways in which fossils can be analyzed and interpreted, and this will be followed by brief summaries of the hominin fossil species known from the major phases in hominin evolution. Emphasis will be placed on Late Miocene through Plio-Pleistocene hominins because at this stage in evolution the fossil evidence is critical as there are fewer archaeological traces compared with more recent stages of evolution where this type of evidence becomes equally important.
Fossils, Depositional Context, and Taphonomic History
Major paleoanthropological fossil finds are rare, and they are often celebrated scientific events. It is not unknown for the discovery of one fossil to radically alter the way in which we understand hominin relationships or the behavior of a particular species. Therefore, it is important to understand what a fossil is, how it is created, and what happens between the death of the organism and its subsequent discovery by field researchers.
A fossil can be simply defined as the remains or traces (such as animal tracks or the imprint of a fern) of a once living organism. In the case of skeletal elements and teeth, which are particularly interesting to paleoanthropologists, they are buried and mineralized in a long process during which minerals present in the sediments replace the hard tissue. It is extremely rare for the remains of soft tissues, such as muscles, skin or hair, to be preserved in the fossil record. For fossilization to occur, skeletal material must be buried quickly. When subjected to a lengthy period of exposure on the land surface, the material may become badly weathered or damaged. Thus, a quick process of deposition, which favors fossilization, increases the likelihood that an element will survive in a more complete form that ultimately preserves more information for scientists analyzing the material.
Because of this, we can say that the fossil record is biased. If only particular conditions favor the fossilization of bone, then there must be many more bones that are not deposited in such conditions and will never make it into the fossil record. In fact, there are a number of factors that influence a bone’s entry into the record; some of these are geological and others biological. Paleoanthropologists must therefore examine the possibility that any one or a combination of these factors influenced the history—and subsequent interpretation—of a particular fossil. The history of a fossil from its deposition to discovery is what is known as taphonomy.
As described above, the depositional contexts that favor the preservation of fossils are ones in which sediments accumulate quickly. Lakes, rivers, and seasonal flood-plains provide excellent conditions for fossilization; mammals frequently gather at water sources, and if their remains are deposited there, they can be quickly buried by silts that accumulate rapidly as compared with the soils that accumulate on dry land surfaces. Lacustrine (i.e., lake) and riverine deposits are common in the fossil record, but while both are formed through the action of water, they are distinctive depositional contexts that can be identified by their unique signatures in the sedimentary record. There is a wide body of taphonomic literature pertaining to the effects of water movement on the transport and damage of bones in modern environments (e.g., Behrensmeyer & Dechant Boaz, 1990), which can be useful analogues when we interpret the depositional context of fossil sites. Bones that have been moved by the flow of water are often abraded, and indeed, we can see this type of damage in a number of assemblages. The speed of the water is also a factor; rapidly moving water will transport bones differentially according to size, moving light bones and those with a large surface area a greater distance than denser elements. Low-energy water contexts, such as lakes, will result in different patterns of accumulation.
Volcanic sediments also preserve fossil remains. Ash is particularly good at preserving the tracks of mammals such as the deposits from 3.7 mya at Laetoli, Tanzania, where hominin footprints were discovered (Leakey & Hay, 1979), and both ash and lava rapidly bury living organisms, capturing a picture of the living community in a fairly short period of time in one geological stratum. In addition, volcanic deposits can not only be traced back to the source volcano but also often be dated with a high degree of precision. The issue of dating is a critical one in paleoanthropology because without sound dates associated with particular fossil finds, it is difficult to interpret the evolutionary relationships of the different species represented in the record. Finally, cave deposits also preserve fossil remains. Bones may accumulate through the activity of carnivores dragging in their prey, mammals simply falling in and becoming trapped, or even water washing remains inside. The stratigraphy of cave deposits is often quite complicated, and the deposits themselves, often hard, cement-like breccias, can be difficult to excavate.
The depositional contexts described above have a significant impact on the fossil assemblage that is eventually uncovered. However, these geological factors are not the only taphonomic forces at work. We must also take into account certain ecological and biological conditions. Consider what the entire mammal community, or living assemblage, of an African bushland or European forest would look like. There would be many different species present, and they would represent different body sizes from the smallest of rodents to the largest quadrupedal mammals. They would live in all manner of physical spaces present in their habitat including underground, in the trees, on rocky outcrops, or simply on the land surface. They would also consume a variety of dietary resources from herbs, grasses, leaves, and fruit to insects and other animals.
The relationships of the different species with each other and to their ecological niches will also have an effect on what the death assemblage, or the bones of dead organisms in a natural environment, looks like. Carnivorous mammals, reptiles, and birds prey on other animals. As a general rule of thumb, smaller carnivores prey on smaller species, and the remains of their meals look quite different from the remains of the meals of larger carnivores. Small-prey remains might accumulate in dens or other sheltered places as well as being found in the fecal matter of their predators such as in hyena droppings or in regurgitated owl pellets. Larger carnivores, like bears or leopards, may also retreat into caves or trees before consuming their prey. Once the primary, large predator has had its fill of a carcass, other carnivores will often attempt to displace the original predators or wait until the original predators have left the carcass alone so that they may consume the remaining meaty portions of the skeleton. Some carnivores, like hyenas, are specifically adapted to crushing and breaking open bones where marrow may be accessed. Several carnivore species may contribute to the destruction of one carcass, and at some point in the past, hominins entered this process when they left their purely vegetarian-based diet behind them.
In addition to the effects of meat consumption by various predators, if the bones of a dead animal are not transported or buried rapidly, they are subjected to the general effects of exposure, insect activity, damage caused by growing vegetation, or trampling by other mammals. All factors considered, only a small fraction of the living community survives the process of deposition and fossilization, and an even smaller proportion survives without significant breakage or damage. Of course, fossils are only of any use to us when they have been naturally exposed and discovered or intentionally excavated by scientists capable of identifying and analyzing them. These remains, after a long journey from living animal to deposited bone to fossil specimen, constitute the final fossil assemblage, the remit of paleontologists.
Fossil Mammals: A Useful Paleoanthropological Tool
The fossil record of interest to paleoanthropologists is not limited to hominins, and in fact, many other families of mammals far outnumber hominin remains. Nonhominin remains are informative: They can tell us about the nature and overall composition of the once-living community, the relationship between hominins and contemporaneous species and the dominant habitat where the community lived, and where hominins were evolving.
There are many ways to examine and interpret the non-hominin mammalian community. In particular, mammals can provide clues as to the characteristics of the habitats where hominins were known to live. It can be as simple as noting the presence or absence of a particular species that is believed to indicate specific conditions. For instance, the presence of a hippopotamus species, such as the extinct Hippopotamus aethiopicus, would signify that a permanent water source was in the area in the past. However, these so-called indicator species are not always such reliable sources of information. We know that the fossil record can be biased, so in fact, the absence of a particular species in the fossil record does not necessarily mean that it was not present in the past. In addition, we cannot assume that the behavior that we observe in modern mammals is the same behavior that they or their ancestors exhibited in the past.
A much more useful analytical approach to mammalian fauna is an assessment of the functional morphology of the skeletal and dental remains, which is an investigation of the relationship between specific morphologies and the functions for which they are best adapted. This approach is neatly grounded in Darwinian evolutionary and ecological theory. Animals have evolved specific adaptations that allow them to efficiently exploit—and thus survive in—their natural habitats. Within any environment there are many niches that the animals can occupy. Living spaces can be under- or aboveground, in the trees, and so on, and this is known as the spatial niche. Long bones, such as a femur (upper leg) or humerus (upper arm), will tell us about the kind of environment that an animal was adapted for moving around in or what form of locomotion it used. The trophic niche refers to food resources that are exploited. Fossil dentition will tell us primarily about the diet of an individual because there are certain types of teeth that are best for consuming the meaty flesh of prey animals, crushing insects, shearing leaves, and so on.
Recall the earlier examples of an African bushland or European forest. Both habitats provide a diversity of niches. However, the types of niches will be dictated by the specific habitat. Arboreal animals that live in the tree canopy and those that eat leaves and fruits are abundant in forested habitats where there are plenty of trees. Bushland will support a different array of animals, ones that rely less on trees and more on shrubs, bushes, and grass cover. So by analyzing how an entire fossil community occupied available ecological niches, we can develop a picture of what the overall habitat was like. This is known as an ecological diversity analysis (i.e., Andrews, Lord, & Nesbit Evans, 1979), in which each species that is identified at a fossil site is categorized according to its dietary and locomotor adaptations on the basis of its skeletal and dental functional morphologies. This technique has been successfully applied to a number of important sites where hominins were present (Andrews & Humphrey, 1999; Kovarovic, Andrews, & Aiello, 2002; Reed, 1997), and despite taphonomic biases in the fossil record that inevitably result in the differential loss of particular species, it is possible to identify sites that were heavily wooded, those that were more open, and those that possessed sources of water.
One way to address the taphonomic underrepresentation of certain species in a fossil community is to focus on just one family of mammals that might be less biased in the fossil record or where the bias is better understood. For instance, small species may be transported great distances and/or damaged before deposition and, in the past, were often passed over in favor of collection strategies that focused on large-bodied mammals, which not only are abundant in the fossil record but are now well represented in paleontological collections. Primates, suids (pigs and related species), and bovids (antelopes and related species) are all examples of diverse, larger-bodied families that have been subject to functional morphological analyses attempting to reconstruct past habitats. When we consider functional morphologies specifically in relation to the exploitation of particular habitat types, we refer to this as the study of mammalian ecomorphology.
A classic example of a well-known ecomorphology is the head of the femur of bovids (Kappelman, 1988). The femur articulates with the pelvis and comprises the hip joint, which is relevant to locomotion. In bovids inhabiting more forested habitats or habitats where a high level of maneuverability is required in order to negotiate obstacles, the head of the bovid femur is round. Bovids living in open habitats must be able to run far and fast to escape predators, and they possess a more cylindrical-shaped femoral head, the biomechanics of which relate to efficient running. Knowing this fact, we can look in the fossil record and analyze the bovid femora that we find at individual sites to determine if the bovid community was better adapted for running in open or closed habitats, thus providing a picture of what the dominant habitat was likely to have been. Analyses of fossil bovid femora have been successful at reconstructing the habitats at the important sites of Olduvai, Tanzania, and Koobi Fora, Kenya, where early hominins are known. Other skeletal elements are also useful in ecomorphological analyses including the metapodials (Plummer & Bishop, 1994), astragalus (DeGusta & Vrba, 2003), and phalanges (DeGusta & Vrba, 2005); in addition, one can conduct an ecomorphological survey of the entire bovid skeleton rather than rely entirely on one element (Kovarovic & Andrews, 2007).
Fossil mammals are useful in other ways besides habitat reconstruction. In many circumstances, geological strata cannot be absolutely dated; therefore, relative dating techniques must be applied. One approach is to use fossil mammals in constructing a biochronology of a region, sometimes called faunal correlation. Where particular families of mammals are diverse and abundant and we know the dates of each species’s first and last appearance in the fossil record, we can use them as a framework for understanding the time period over which the deposition of a site is likely to have occurred. This is often the first way in which sites are dated, and many families of mammals are considered, especially Suidae (pigs). For example, we know that the fossil species Kolpochoerus limnetes evolved at approximately 3 mya and persisted for one million years. Nyanzachoerus jaegeri, a species that is part of a different suid lineage, is present in the fossil record from approximately 4.8 to 3.6 mya. Therefore, where we find K. limnetes present in a fossil assemblage, we can infer that the site is younger than a site where we find N. jaegeri. If an assemblage has more than one pig present, this can help us pinpoint a more specific time period if we know when these species overlapped.
There are, of course, other ways in which mammalian fossils can be informative, including methods such as isotopic studies of tooth enamel that give us an idea of what sort of vegetation an individual was eating and therefore was present in its natural habitat (e.g., Sponheimer & Lee-Thorp, 2003). Skeletal remains also provide us with more direct evidence of human behavior by preserving on their surface the tooth marks left by carnivores and cut-marks left by lithic tools (e.g., Potts & Shipman, 1981). The placement of these two types of marks on the bones can tell us about the order in which the different carnivores—including hominins—were procuring meat-based resources, and that, in turn, tells us about hominin scavenging and hunting strategies and the context of the evolution of meat-eating. The paleoanthropological value of fossil mammals cannot be overestimated. One should also not forget that hominins, as mammals, may be approached using some of the techniques outlined above.
Distinguishing the Hominin Lineage: Early Fossil Evidence from Africa
Prior to the first hominin dispersals out of Africa around 2 mya, our evolutionary history is confined to this continent. The environmental and geological conditions in eastern and southern Africa favored the preservation of fossils, and this is where we find the majority of relevant sites; in southern Africa, they are primarily cave sites, while in East Africa they are open-air sites, many of which are in the Rift Valley System. The first phase of hominin evolution began in the Late Miocene, a time period that ends approximately 5.2 mya, so to investigate our earliest evolutionary developments, we would look to Late Miocene sediments for evidence of the first species that diverged from the common ancestor with the chimpanzees. Unfortunately, evidence for the earliest stages of these lineages or the probable first hominins is rare. The lack of a good fossil record at this time is compounded by the fact that regardless of the number of fossils available, it is difficult to identify individual species in the past. Zoologists have extra information when they work with populations of modern organisms: Things that do not fossilize, such as fur color, behavior, vocalizations, and DNA, can all be used to distinguish between taxa. Paleoanthropologists have only fossil hominin remains for identifying the different species and determining their evolutionary, or phylogenetic, relatedness. They do this through an assessment of shared morphological features, or characters, and the amount of variation displayed within each character that is considered acceptable within a single species.
Features that are understood to define the hominin lineage include bipedal locomotion, smaller canine teeth, and increased brain size, among many others. While these features may be obvious at later stages of evolution, it is not known how they would be represented by the first hominins. There are three recently announced species that are generally considered viable candidates for the earliest hominin. The oldest is Sahelanthropus tchadensis, which includes a well-preserved skull, fragments of mandible, and teeth, discovered in Chad (Brunet et al., 2002, 2005). Associated mammalian fossils infer that the region was moist with permanent water sources and heavily wooded, and recent research dates the finds from 6.8 to 7.2 mya (Lebatard et al., 2008). In addition to the early age and habitat, its location in north-central Africa, where hominins are infrequently discovered, makes it a unique species. But is it hominin? It possesses a mix of anatomical features that are considered primitive (i.e., ape-like) and derived (unique evolved characters). Its brain size was small like those of apes, and some aspects of the teeth are also considered primitive; however, the canines are reduced in size, and perhaps more importantly, there is evidence that the species could have been bipedal. Its foramen magnum, the hole at the base of the skull through which the spinal column enters, is located directly beneath the skull in a more forward position. This positioning indicates that the head of the individual was placed directly on the top of the body inferring an upright, or bipedal, posture. This is in contrast to apes where the foramen magnum is located toward the back of the skull. The discoverers of S. tchadensis and many other researchers propose that this species is an early hominin situated close to the chimp-hominin split.
The next oldest Late Miocene species, Orrorin tugenensis, is known from only a handful of postcranial and a few dental remains discovered in the Tugen Hills in Kenya. The deposits have been absolutely dated to between 6.2 and 5.6 mya, corroborated by mammalian biochronology (Senut & Pickford, 2001). The material is fragmentary, and there is much controversy over its interpretation. The scientists who uncovered the specimens claim that they clearly represent a bipedal hominin (Galik et al., 2004; Senut et al., 2001) but one that is more directly related to the Homo lineage as opposed to other species within the genus Australopithecus (who you will read about later) that are present in the Pliocene (5.2-1.8 mya). Many critics agree with the overall conclusion regarding its bipedality but disagree with the particular functional morphologies on which they focused their analysis and from which they drew their conclusions regarding its evolutionary relationship to other hominins. A recent and comprehensive analysis shows a reinterpretation of the morphology of the Orrorin femora and a conclusion that the species was certainly bipedal but did in fact resemble later Pliocene species and not Homo (Richmond & Jungers, 2008).
Straddling the Miocene-Pliocene boundary is the genus Ardipithecus. It is considered by many to contain two species separated by approximately 700,000 years. Both are found in the Afar region of Ethiopia, Ardipithecus kad-abba from 5.7 to 5.2 mya and Ardipithecus ramidus from 4.5 to possibly 4 mya. A. ramidus was discovered first and A. kadabbaafter that, but it was initially described as a subspecies of A. ramidus (Haile-Selassie, 2001; Haile-Selassie, Suwa, & White, 2004; White, Suwa, & Asfaw, 1995; WoldeGabriel et al., 1994). Its discoverers eventually decided that the variation observed in the combined material as well as the time between the two major collections warranted separate species distinctions. A. kadabba appears more primitive, with larger, more apelike canines, but both species are, likeSahelanthropus and Orrorin, a mixture of apelike and hominin-like morphologies. The strongest evidence for the hominin status of A. ramidus is the position of its foramen magnum, which is forward placed on the base of the skull. Isotope analyses of fossil nonhominin teeth and the ecological diversity of the mammalian fauna indicate that both species inhabited a mosaic of woodland and open cover, grass-dominated habitats with ample water sources (Levin, Simpson, Quade, Cerling, & Frost, 2008; WoldeGabriel et al., 1994).
Established in Africa: Diversification of the Plio-Pleistocene Hominin Lineages
The number of fossils recovered from African sediments of the Pliocene (5.2-1.8 mya) and Pleistocene (1.8 mya-10,000 years ago) from approximately 4 mya onward can be considered a veritable explosion compared with the Late Miocene material. Some hominin species from this time period are better known than others, and naturally, there are many debates regarding their relationships to one another and whether or not they have a direct evolutionary relationship to the Homo lineage. Although there are numerous species in the Plio-Pleistocene, they can be summarized as (a) Australopithecus, a diverse and long-lived genus comprising possibly five species; (b) Paranthropus, the so-called robust australopith species; and (c) early Homo.
The best-known and longest-lived australopith is Australopithecus afarensis, which is known from remains from East Africa including both the famous “Lucy” skeleton from Hadar, Ethiopia, and the fossilized tracks of footprints at Laetoli, Tanzania, that provide some of the best and earliest direct evidence for bipedal locomotion (Leakey & Hay, 1979). This species emerges at approximately 3.9 mya and is predated only by Australopithecus anamensis from the Lake Turkana region of Kenya (Leakey, Feibel, McDougall, & Walker, 1995). The functional morphologies of the A. anamensis leg and arm bones and the reduced canine size indicate that it is clearly a hominin and is probably the direct ancestor of A. afarensis, which shares a number of anatomical features with it but many derived ones as well, including further reduced canines. A. afarensis is an interesting species not just for its longevity but also for its extensive geographic range across East Africa and its variability, especially in body size. This diversity led some paleoanthropologists to suggest that it should be split into two separate species (e.g., Häusler & Schmid, 1995). Although the discussion periodically resurfaces in the scientific literature, most scientists believe it is a single species displaying a large but acceptable amount of morphological variability and sexual dimorphism, or variation in size between males and females (e.g., Gordon, Green, & Richmond, 2008).
There are potentially two other East African australopiths recognized, but less is known of them. Australopithecus bahrelghazali was named a separate species on the basis of mandibular material discovered in sediments, which were dated by mammalian faunal correlation to between 3.5 and 3 mya (Brunet et al., 1996). However, referring to this taxon as East African is misleading; the remains are known from Bahrel-Ghazal in Chad. Although its location is exciting because it extends the known hominin geographical range further west at this time period, some people argue that the specimen is not unique enough to warrant its own species designation and it simply represents a geographical variant of the East African A. afarensis (e.g., Kimbel, Rak, Johanson, Holloway, & Yuan, 2004). Australopithecus garhi from Ethiopia is one of the youngest australopiths, from approximately 2.5 mya (Asfaw et al., 1999). Cutmarked bovid bones discovered at the same site provide the earliest evidence for hominins making and using stone tools, presumably for the procurement of meat (de Heinzelin et al., 1999).
The final australopith species to be considered is not from East Africa but a long-lived species known from South African cave sites, Australopithecus africanus. The latest date for this species is 2.4 mya, and there is the possibility that it extends as far back as 4 mya, but as we mentioned earlier, cave sites are difficult to date and material of this age is scarce (Partridge, Granger, Caffee, & Clarke, 2003). Mammalian biochronologies are constructed based on what is known of East African material where better dates can be secured, and the majority of A. africanus sediments fall between 3 and 2.4 mya based on this methodology. Overlapping in time with some of these species is also a different genus of hominin, Kenyanthropus platyops, known only from Kenya between 3.5 and 3.2 mya (Leakey et al., 2001). It has an extremely flat face and other derived characteristics that distinguish it from the australopiths and potentially link it to Homo.
Appearing later in time than Australopithecus and Kenyanthropus but overlapping with some of those species at the end of their evolutionary lifespan are three species commonly referred to as the “robust” australopiths. They have evolved a very distinct suite of craniodental characteristics, in particular, wide flat faces and molars with large, broad chewing surfaces. On these grounds, most researchers place them in an entirely different genus, Paranthropus. Functional morphological interpretations were traditionally seen as indicating a tough vegetarian diet that required the teeth and surrounding facial architecture to withstand significant force from grinding (see Wood & Straight, 2004, for review). However, isotopic studies indicate that they may have also consumed a certain amount of animal matter, and a microwear analysis of several tooth surfaces suggest that the diet might not have been as tough as previously assumed (Sillen & Lee-Thorp, 1993; Ungar, Grine, & Teaford, 2008). The only South African species known is Paranthropus robustus, which was found in a number of cave sites dating from between 2 and 1.5 mya. In East Africa, Paranthropus boisei persists for a million years between 2.3 and 1.3 mya and is predated only by Paranthropus aethiopicus, which is known from a limited number of sites in Tanzania, Kenya, and Ethiopia and is dated from between 2.5 and 2.3 mya. A partial mandible lacking teeth and a tibia from Laetoli, Tanzania, are likely to represent P. aethiopicus, pushing their geographical range further south and their date range back slightly further to 2.6 mya (Harrison, 2002). It is not clear how these species are related to each other, but the general consensus is that they were not direct ancestors of Homo.
There are possibly four species of early Homo in Plio-Pleistocene Africa: Homo habilis, Homo rudolfensis, Homo ergaster, and Homo erectus. Species in the Homo genus are placed there on the basis of a number of unique features including an increase in brain size and smaller teeth and jaws. The oldest species, H. habilis (2.4-1.6 mya), is usually separated from another, H. rudolfensis, which possessed a larger brain and dentition, as well as a wider face (Alexeev, 1986). A recent review of the morphological evidence of African Homo infers that these two taxa share more features in common with earlier australopiths than with later Homo, prompting a revised taxonomy that places them in the genus Australopithecus, which is followed by some researchers (Wood & Collard, 1999). This system proposes only two species of Homo in Africa, Homo ergaster and Homo erectus emerging at 1.9 mya. Others argue that all of these later African fossils can be lumped under H. ergaster and believe that H. erectus is a distinct taxon found only in Asia. This later Homo material is interpreted as more “modern,” particularly in the further reduction of the dentition and jaw and longer lower limbs adapted for more efficient bipedalism than earlier hominins. H. ergaster skulls show only a slight increase in brain size from H. rudolfensis, but H. erectus brains are most certainly larger.
Plio-Pleistocene hominins inhabited a variety of habitats; at nearly every site there is evidence for grass-dominated areas as well as more wooded areas. This is in contrast to early work in the field that interpreted most sites as representing quite open, arid habitats, suggesting further that bipedalism evolved in response to this type of ecological setting. However, ecomorphological interpretations of certain aspects of the earlier hominin forelimb anatomy, for instance, the long curved finger bones of Australopithecus, indicate that they retained the ability to locomote arboreally, which would certainly be advantageous in a habitat providing adequate tree cover where both shelter and food can be sought. Newer interpretations of African paleoenvironments indicate that most sites did in fact possess a significant woodland component and open settings did not arise until approximately 2 mya when we also see more modern forms of bipedalism evolve with Homo ergaster/erectus (e.g., Reed, 1997; Spencer, 1997). Despite the diversity of habitats exploited, some distinctions can be drawn regarding hominin habitat preferences. Ecological diversity and ecomorphological analyses indicate that Australopithecus is generally associated with habitats that have a considerable amount of woodland present while Paranthropus occupied similar areas but also ones with a higher proportion of more open woodland and bushland where wetlands were often, but not always, present (e.g., Kovarovic & Andrews, 2007; Reed, 1997).
In addition to these varying habitat preferences, hominins also evolved different diets, which are indicated by their craniodental adaptations. Perhaps the most significant dietary adaptation to emerge during the Plio-Pleistocene was the evolution of meat eating. The development of the first stone tool industry, the Oldawan, around 2.5 mya, and the first evidence of cutmarked mammal bones in Gona, Ethiopia (Dominguez-Rodrigo, Pickering, Semaw, & Rogers, 2005), infers that a shift to a greater amount of meat consumption evolved around this time (note though that earlier evidence of meat eating may not be visible in the fossil record; in other words, hominins may first have consumed or scavenged meat without the use of stone tools). We mentioned above the isotopic evidence for Paranthropus robustus meat consumption, and there is even evidence that this species may have used mammal long bones to forage for termites (Backwell & D’Errico, 2001). A large body of archaeological and paleoanthropological literature pertains to the first stone tool industry and to using mammal fossils to determine how hominins obtained and processed meat, but this will not be reviewed herein (for informative reviews see Blumenschine & Pobiner, 2006; Plummer & Bishop, 1994). Still, it is important to note again at this point the relevance of fossil mammalian remains in our paleoanthropo-logical investigations.
Moving Abroad: Fossil Evidence of Homo in Asia
Determining if the taxonomic distinction between H. ergaster and H. erectus is indeed correct will be aided in the future by analyses of Homo ergaster such as from material from a site called Dmanisi in the Republic of Georgia (Gabunia et al., 2000; Gabunia & Vekua, 1995). The biochronology of mammals found at the site indicates an age of approximately 1.7 to 1.8 mya. This material may provide clues as to which species migrated from Africa and if H. erectus evolved in or en route to Asia. Regardless, the material confirms that Homo had ventured far from its African home some time toward the end of the Pliocene, expanding into less tropical climates and new environs.
The antiquity of Homo in Asia is confirmed by sites in mainland Asia and Indonesia where classic H. erectus material is well documented. Many of these sites cannot be absolutely dated, and biochronological dates are offered for most. Some of the oldest dates previously accepted in Asia were from Javanese sites that indicated an approximate age of 1 mya. However, radiometric dates were determined for two critical sites in Java where absolute dating is possible, and they indicate that the H. erectus occupation of Asia was well established in the islands by 1.6 to 1.8 mya (Swisher et al., 1994). However, there is some doubt as to the exact location where some of the relevant hominin fossils were found at these sites and if the dates derived from these analyses are indeed from the correct strata. The time frame for Homo’s extraordinary migration is not yet clear, but it appears to have started with a rapid exit from Africa shortly after the evolution of H. ergaster since the earliest evidence for this species emerges in the fossil record at roughly the same time that Homo remains emerge in Asia. Some paleoanthropologists also suggest a scenario in which there were migrations back to Africa and others out again, with some of the African material representing H. ergaster and other material representing H. erectus.
Classic Homo erectus features are not displayed by every known specimen, and there is substantial variation in the Asian sample, as well as between H. erectus and H. ergaster. However, some of the basic features that define the species include a thick browridge above the eye sockets, thick skull bones, and an angular shape to the top and back of the skull. In addition, more humanlike limb proportions (i.e., shorter arms in relation to earlier species) are evident in both H. ergaster and H. erectus; it is particularly obvious in the Nariokotome Boy, a 12-year-old male skeleton found in Kenya (Brown, Harris, Leakey, & Walker, 1985).
Until recently, researchers believed that H. erectus persisted in Asia until approximately 30 kya. It was therefore an exciting moment when a team of researchers announced the discovery of the remains of a small hominin, Homo floresiensis, at the site of Liang Bua on the Indonesian island of Flores (Brown et al., 2004). The youngest sediments in which the remains were found are only 17,000 years old. This means that hominins existed as an island population long after H. erectus had disappeared from the mainland. Analyses of its small skull and other elements, including the wrist bones, indicate that these remains represent an entirely different species of Pleistocene hominin (Tocheri et al., 2007). It retains some primitive traits in relation to modern Homo sapiens; this is relevant because some arguments arose claiming that the specimens were modern humans suffering from disease. Although H. floresiensis’ exact evolutionary relationships are not agreed on, its unique characteristics clearly distinguish it from other hominin material.
It is likely that Homo ergaster/erectus consistently consumed meat (although we still do not understand if they were hunting or scavenging), and the lines of evidence for this are various and come from both African and Asian sites and specimens. A reduction in the size of the molars, which are so helpful in grinding tough-plant foods, indicates a greater reliance on the canines and incisors, which would be good for tearing and biting. The force required for this might have been supported by the more robust nature of the skull. In addition, the Acheulean industry in Africa from 1.6 mya onward, which is characterized by teardrop-shaped hand axes, represents a more advanced form of stone tool. They have been demonstrated to be excellent butchery tools, and in fact, many sites have associated tools and mammalian remains bearing cutmarks (e.g., Schick & Toth, 1993). All lines of evidence in Asia indicate that the climate was colder and more temperate than in Africa, which would affect the seasonal availability of vegetation, inferring that there were times of the year when meat was the most plentiful and likely to be an exploited resource. Finally, there is some evidence for the controlled use of fire in both East and South Africa, as well as in Asia (Bellomo, 1994; Brain & Sillen, 1988). Fire would be useful for processing meat, as well as generally allowing for the colonization of colder climates. It is clear that at this point in evolutionary history Homo had developed the technology and know-how to exert a certain amount of control over its environment. Despite this, hominins were still mammals in a diverse landscape of many other well-adapted species. Fossils bearing tooth marks, for instance, the remains of deer at the Zhoukoudian cave site in China where both hyenas and their prey species are found, remind us that they were not the only carnivore species in the community (Boaz, Ciochon, Qinqi, & Jinyi, 2000).
Recent Homo Evolution
The main players on the stage of recent Homo evolution used to be restricted to Homo neanderthalensis and Homo sapiens. However, it is now generally understood that a number of fossils in both Africa and Europe, which were once lumped under the general heading “archaic Homo sapiens,” can be designated a separate species that is evident by approximately 700 kya, Homo heidelbergensis. Other species have been suggested as well, generally with regard to particular geographical populations, but these are contested and not well-known with the exception of Homo antecessor. This taxon relates specifically to material from Atapuerca, Spain, where cave sites have preserved large numbers of fossils, with more than 20 hominin individuals attributed to one site alone (e.g., Bermudez de Castro et al., 1997). Researchers there believe that the material may be ancestral to Neanderthals on the basis of shared characteristics, such as a large brain size and the shape of the middle part of the face. The validity of this species will be more generally accepted if fossils that can be definitively assigned to it are discovered outside of Atapuerca, and until then, some prefer to assign these specimens to H. heidelbergensis (e.g., Rightmire, 1996).
We may eventually discover that H. heidelbergensis merits more than one species distinction as there is considerable variation displayed by the specimens. Generally, they are united by derived features, such as a larger brain size, round cranium, and reduced dentition, that distinguish the species it from H. ergaster and H. erectus, with which it overlapped in time. This species is widespread across Europe and Africa (where the earliest material is known), with some suggestions that it ranged as far as China by 200 kya (see Stringer, 1993, for a summary of Pleistocene sites and dates). One of the major questions is how H. heidelbergensis is related to later Neanderthals and modern humans. It possesses features that seem to indicate a relationship with Neanderthals and is present in Europe before them. This is significant because Neanderthals are only found in Europe, an area of the Middle East known as the Levant, and Central Asia, although we do not know in which region the Neanderthals evolved. H. heidelbergensis also overlaps chronologically with the earlier Neanderthal material, so it is possible that Neanderthals evolved from a population of H. heidelbergensis, which also persisted as a species. Others believe that H. heidelbergensis led only to modern Homo sapiens and H. neanderthalensis evolved from H. erectus. More alternative views exist, often hinging on interpretations of specific fossils, and there is no common viewpoint regarding the relationships of later Pleistocene hominins. It is difficult to interpret the extensive fossil record of these species. This stage in evolution, in contrast to Miocene and Pliocene evolution, presents us with a much greater amount of fossil material, but regional morphological variations and complicated movements across three continents, coupled with the problematic nature of dating different types of sites, make it difficult to construct a clear framework for evolutionary relatedness.
One of the best-known hominins is, of course, Homo neanderthalensis. The wealth of fossil material and great number of sites, in addition to the fact that it was the first hominin material to ever be discovered and identified as different from Homo sapiens, means that it looms large in both the popular consciousness and the scientific literature. It has become increasingly clear that they were a distinct and unique species, and advances in genetics will help us understand when they might have diverged from a common ancestor with modern humans. In addition, DNA analysis will also throw more light on the debate regarding the possibility that Neanderthals contributed to the modern human gene pool. Morphologically speaking, they were robust and large brained with many unique features in the skull and dentition including a distinctive projection in the middle of the face, large nasal aperture, and large incisor teeth. Many of their features have been interpreted as representing adaptations to a cold and seasonal climate, which is exactly what reconstructions of their environmental conditions indicate they had to contend with during their evolutionary lifespan. Fossils of large mammals typical of cold Pleistocene habitats are present at their sites, often in association with stone tools and bearing telltale cutmarks of hunting and butchery.
Neanderthals survive until approximately 30 kya, after which point, other than the relict H. floresiensis population in Indonesia, Homo sapiens is the only remaining hominin (the complicated debate about why Neanderthals did not survive is beyond the scope of this article, but see Hall, 2008, for an easy-to-read review). It is likely that H. sapiens evolved in Africa, as the oldest material of what is sometimes referred to as “anatomically modern humans” is known from this continent. Cranial remains from Ethiopia and South Africa in particular have been pivotal in discussions about the location, timing, and context of the evolution of H. sapiens. The South African material from a site known as Klasies River Mouth was ascribed a date much earlier than people once believed modern humans had evolved from, 120 kya (Deacon & Geleijnse, 1988). Biochronological dating of a modern-looking specimen referred to as the Omo I skull, from Ethiopia, also indicated an age of 120 kya; however, recent radiometric dates for this and another slightly more primitive but still modern skull from the site have proposed an even earlier age of 200 kya (McDougall, Brown, & Fleagle, 2005, 2008). Additional Ethiopian remains from Herto are likely to be from at least 160 kya (White et al., 2003). The likeliest scenario is that H. sapiens evolved in Africa and migrated outward, probably several times. There is extensive evidence for anatomically modern human occupation in the Middle East where Neanderthals are also known, but it is only H. sapiens that survived and colonized the world.
And what makes modern humans modern? Compared with other hominins, including the ones our species once coexisted with, we have many distinct characteristics although expressed to a variable degree. We lack the robustness of earlier hominins in the thickness and form of our cranial bones, limb bones, and facial features; we possess large brains for our body size, straight limb bones, a projecting chin, and smaller dentition. Simply look around at the people who walk past you every day, and you can see how diverse we are as a species even today, let alone in the late Pleistocene.
Major recent discoveries have significantly expanded our understanding of hominin evolution, and the recovery of new fossils is of primary importance. In the past 15 years, we have established the antiquity of hominins in Late Miocene Africa and Pleistocene Asia; furthermore, we have enhanced our awareness of the diversity and longevity of the different lineages. In particular, we await more evidence of the earliest hominins and fossils found in regions not traditionally associated with the evolutionary narrative, such as northern and central Africa. Defining Homo ergaster and Homo erectus as separate species and determining the timing of Homo’s migration out of Africa will also be critical and feeds into another complex issue that must be resolved: the evolution of later Homo and how some of the Pleistocene species (H. antecessor, H. heidelbergensis) are related to modern humans. We also continue to set hominin evolution in the context of the evolution and migration of other mammals in relation to changing climates and shifting habitat availability and to develop better techniques for assessing how geological and biological processes affect fossil assemblages.
This article has focused primarily on the paleontological evidence for hominin evolution, that is, hominin and nonhominin fossils, their geological and environmental contexts, and how paleoanthropologists currently assess the hominin species identified in the fossil record. But this discipline is multidisciplinary in the truest sense; anatomists, geologists, archaeologists, zoologists, geneticists, and primatologists all examine aspects of our evolutionary history from both the biological and cultural perspective. New methods and techniques for approaching the fossil and archaeological record are always under development, and we increasingly rely on modern technologies. The recent announcement of the sequencing of the Neanderthal genome reminds us that what were once unapproachable questions now have answers within our grasp—and the spirit of discovery in the field, in the lab, and elsewhere is very much a part of this unique science (Green et al., 2008).
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