Human Evolution

Jeffrey H Schwartz. 21st Century Anthropology: A Reference Handbook. Editor: H James Birx. Volume 2. Thousand Oaks, CA: Sage Reference, 2010. 

It might seem unnecessary to begin by stating that our species, Homo sapiens, is the sole surviving species of a once diverse group of now extinct relatives. For after all, the media is not shy about promoting the assertions of human paleontologists (paleoanthropologists) who claim to have finally discovered the long sought ancestor of our group or a missing link in the chain of human evolutionary succession. Even the repudiations of evolution by the scientific creationists and intelligent designists cannot derail the paleoanthropologists’ pursuit of human fossils or submerge the widespread public interest in our evolutionary heritage. Yet for all the publicity that follows the discovery of and supposedly scientifically well-founded pronouncements about previously unknown potential human relatives, many of the assumptions on which paleoanthropology is grounded differ significantly from those that inform the rest of paleontology. And these differences have had profound intellectual consequences for the discipline of anthropology at large.

Before Human Evolution

Among the earliest preserved writings from the Western world that relate to considerations of human history are those from the Greeks (for example, Hippocrates and Herodotus) and Romans (for example, Lucretius) that discuss differences between known groups of humans in both physical and psychological attributes. Common to all was a theme we might identify as adaptation. That is, people as well as other animals look and behave as they do because of the effects of their specific local circumstances. Consequently, for example, Herodotus believed that Egyptian soldiers had thick cranial bones because men typically shaved their heads, which were fully exposed to the beating rays of a strong sun. Scythian soldiers, however, had softer skulls because they wore hats and lived in less sunny climes.

Perceptions of the place of humans in nature were also based on attempts to understand something of the biology of our own and other species. As such, the Greek philosopher Aristotle, for instance, sought to organize animal life in anatomically meaningful ways, with the result that he defined a group we would call tetrapods on the basis of its members articulating with the world around them via four “points.” For unambiguously quadrupedal animals, such as dogs or deer, all four limbs communicate with the ground via feet. But Aristotle’s perspective also allowed him to include in this group birds because although not “footlike,” their wings connect them with their surroundings as well as hands connect humans with their surroundings. Aristotle was not, however, immune to error. Thus, for example, he believed that the bipedal or two-legged stance of a bird differed from a human’s because of differences in their knee joints. Not knowing the actual anatomy of the hindlimb, Aristotle thought that the “knee” of a bird, as well as the knee of other quadrupeds, such as a dog or deer, bent backward, whereas a human’s knee bent forward. This confusion was not corrected until the late 15th century when Leonardo da Vinci pointed out that the “knee” of a bird or any other nonhuman animal was actually its ankle; the true knee joint, which was situated higher up along the limb, bent in the same direction as the human knee joint. But the spirit of discovery that guided the Greek and early Roman method of scientific inquiry, particularly with regard to humans and their place in nature, was unfortunately squelched from the 2nd to 3rd CE centuries with the institution of Christianity, which demanded that “science” could be pursued only through acts of revelation and that its “discoveries” were in keeping with scripture.

One consequence of this credo was the concept of a sequence of creation or Great Chain of Being first proposed by Aristotle. Beginning in force in the 15th century, taxonomists sought to demonstrate this sequence of creation through a hierarchical arrangement of creatures from the perceived lowest to the highest—the lowest often being tied to the inorganic and the highest being the white male Western European who was generating his own particular and idiosyncratic classification of life. Although seafarers exploring far-off lands with the intention of establishing outposts and trade routes continually brought back to their countries examples of previously unknown plants and animals (including humans) that taxonomists eagerly inserted into the gaps in their classifications, there remained many missing links in the Great Chain of Being.

Fortunately, in 1669, Danish geologist and anatomist Nicolaus Steno published a treatise in which he demonstrated that not all rock-hard objects that came from the earth were indeed rocks and minerals. Rather, rock-hard objects with the shapes of bones, teeth, and shells characteristic of living vertebrates and invertebrates were actually the fossilized (“petrified”) remains of once-living organisms. Suddenly, there was a new and vast source of missing links that taxonomists could include in their classifications as they strove to flesh out the picture of creation. As practitioners of the nascent discipline of paleontology quickly realized, the picture of past life mirrored that of the present: Not only is there now a diversity of plant and animal species; such taxic diversity had always existed.

But this treasure trove of extinct life went only so far. It did not include humans because according to the book of Genesis, they had been created last, and therefore, their history did not extend into antiquity. For all intents and purposes, Adam and Eve and their immediate descendants barely preceded the Great Flood of the Old Testament, and thus, no specimen of a human could be appreciably antediluvian. Consequently, while the evolutionarily uninformed 18th and early 19th centuries witnessed the development of nonhuman paleontology as a scholarly endeavor that sought to refine its understanding of the diversity of life in the past, even if actual human fossils had been discovered, they would not have been identified as such.

The Feldhofer Grotto Neanderthal: The Test Case of Human Antiquity

In 1857, two years prior to the publication of the first edition of Charles Darwin’s On the Origin of Species, miners quarrying limestone at the Feldhofer Grotto in the Neander Valley (Thal in German) near Düsseldorf, Germany, uncovered a skullcap and various parts of a skeleton of a humanlike individual. They gave the remains to Carl Fuhlrott, a local schoolteacher, who in turn presented them to his country’s most eminent human anatomist, Professor Hermann Schaaffhausen at the University of Bonn. Because of the state of “petrifaction” of this specimen and its obvious morphological differences from humans, Fuhlrott concluded that the Feldhofer Grotto Neanderthal represented an extinct race of a human relative. Schaaffhausen, however, brought all his resources and clout to argue that this was not a specimen of any antiquity; rather, it merely represented a member of a recent savage and barbaric race, not unlike the living Australian Aborigines.

It would not be until 1886, when Julien Fraipont and Max Lohest published incontrovertible evidence from the Belgian site Spy of the contemporaneity of Neanderthal remains with the bones of acknowledged extinct mammals, that it became difficult for dissenters to sustain the notion that humans could not be antediluvian. Nevertheless, before then, a few scholars did accept the antiquity of the Feldhofer Neanderthal. One of them was Thomas Henry Huxley, who maintained this position in “On Some Fossil Remains of Man,” one of three essays published collectively in 1863 in the volume Man’s Place in Nature.

Thomas Henry Huxley and the Stultification of Paleoanthropology

It is an odd twist of fate that while Huxley (1863) unabashedly accepted the Feldhofer Grotto Neanderthal skullcap and postcranial bones as representing the fossilized remains of a being of some antiquity at a time when few scholars did—one of the few, the geologist Charles Lyell, had visited Feldhofer Grotto and concluded that the bones were ancient—Huxley’s interpretation of them could not have been predicted. As a salvationist who believed that novel features and thus species emerge abruptly as a result of a prenatal reorganization of developmental pathways, a comparative anatomist who reached national prominence while still in his twenties, and a systematist who undertook a reclassification of all vertebrates, one could reasonably characterize Huxley as having a critical eye when it came to interpreting morphological differences between organisms. Yet when Huxley turned to the clearly distinctive features of the Feldhofer remains, he abandoned his approach to nonhuman organisms and discussed the Neanderthal from an entirely unexpected perspective.

Similar to Fuhlrott and Schaaffhausen, Huxley (1863) was clearly impressed by the features of the Feldhofer skullcap. It was long, elliptical in outline, flattened both in lateral profile and when viewed from the rear, and of unusually large size. The brow ridges met in the midline of the skullcap, and their extraordinary prominence was accentuated by a depression on the frontal bone behind and another depression below at the root of the nasal bones. In seeking a suitable modern human skull for comparison, Huxley focused on perceived similarities between the Neanderthal and an Australian Aborigine, which he took as the most primitive living human. But prior to embarking on this comparison, he discussed and then dismissed the ways in which other scholars had tried to equate human variation with racial distinctiveness. To the contrary, Huxley argued, upon comparing the “lowest” through the “highest” forms of the human skull, one was confronted with a continuum, not unlike that which he had constructed between the “lower animals” and “man.” And it was on to this continuum that Huxley appended the Feldhofer Neanderthal, stating with assuredness that all it would take to convert the most primitive living human—which Huxley took to be the Australian Aborigine—into the aberrant fossil would be some flattening and lengthening of the skull with an increase in brow ridge size. In turn, Huxley could proclaim that even though the Neanderthal skullcap was the most “pithecoid,” or apelike, of known human skulls, which might at first glance lead one to classify it in its own species and thus apart from living humans, this specimen was actually one end of a series that led gradually from it to the most modern-looking human cranium.

The effect this perspective had on the study of human evolution was, unfortunately, long lasting. For even though he briefly considered that possibility that the Feldhofer Neanderthal represented an “intermediate” between “man” and “apes,” by diminishing the unique and distinctive features of the fossil to the status of racial difference, Huxley conflatedwithin species differences due to individual variation (i.e., differences in the degree of expression of a particular feature—as in robustly versus weakly developed brows) with between species differences that reflect taxic diversity (i.e, differences of kind or configuration of a feature—as in partitioned versus continuous brow). The year after Huxley’s publication, professor of geology William King at Queen’s College, Galway, Ireland, would criticize Huxley’s oversimplification of the differences between the Feldhofer Neanderthal and any Homo sapiens, Australian Aborigines included. King proposed a separate species, H. neanderthalensis, to receive the extinct human. However, Huxley’s perspective has persisted in light of the analysis of more evidence. A version of the racial bias that Huxley had inserted into the interpretation of human evolution would reemerge less than a century later, largely through the efforts of two of the “fathers” of the so-called modern evolutionary synthesis, Theodosius Dobzhansky and Ernst Mayr.

Human Evolution after Huxley and Before the Synthesis

In 1871, Darwin published his thoughts on human origins in the first part of The Descent of Man. But contrary to received wisdom, this was not a treatise on human evolution as much as it was a discourse on the development of “civilized” from “primitive” humans. True, Darwin did suggest that the African apes (chimpanzees and gorillas) were the closest living relatives of “man,” but the basis for this claim was anything but morphological. Guided by his belief that one should find the fossilized remains of the common ancestor of living forms in the same geographical region as its descendants, as well as Huxley’s 1863 essay “On the Relation of Man to the Lower Animals” detailing similarities between humans and the large-bodied apes, Darwin faced a dilemma. While humans were distributed widely across the globe, orangutans were restricted to Southeast Asia and African apes to subSaharan Africa. Darwin chose to resolve this problem by arguing that humans could not have evolved in Southeast Asia because its lush tropical environment would not have afforded the kinds of dangers and thus selection pressures (for example, from predators) that abound in southern Africa. Although African apes are primarily distributed throughout the tropical evergreen forests of central Africa and not in the open savannas of South Africa, this inconsistency did not prevent Darwin from arguing that since Africa was also home to the “most primitive” humans, humans and African apes had shared a common African ancestor. It is perhaps also worth mentioning the obvious: Darwin’s primitive humans and African apes are “black,” whereas the orangutan has reddish hair.

While paleoanthropologists of the 20th century increasingly cited Darwin’s genius in suggesting that human evolution began in Africa, historical accuracy demands recognizing that until the beginning of the concretization of the modern synthesis around Darwinism in 1941, with the publication of the second edition of Dobzhansky’s Genetics and the Origin of Species, most evolutionists—the German anatomist, embryologist, and paleontologist Ernst Haeckel (1876) being the leading exception—were not Darwinians. Consequently, it is not surprising that efforts to discover fossils that might represent links between humans and Neanderthals on the one hand and the apes on the other were concentrated not in Africa but in Asia, which had long been believed to be the seat of human antiquity.

Thus, the Dutch polymath Eugène Dubois, with the sole purpose of finding ancient human relatives, traveled to Southeast Asia where at the site of Trinil on the island of Java, he discovered two isolated teeth, a skullcap, and a number of variously preserved femora. He believed both that these fossils represented a single entity and that it was the missing link between Neanderthals and apes: In cranial shape and size it seemed rather pithecoid, but the humanlike femur suggested that its bearer had walked upright and bipedally. In 1892, Dubois allocated these specimens to a new species, erectus, of the genus Anthropopithecus, which was often used to subsume the chimpanzee. Two years later, Dubois referred his species erectus to a genus Haeckel (1876) had invented in anticipation of the discovery of a then unknown human ancestor: Pithecanthropus (“ape-man”). Thus, Pithecanthropus erectus, or “Java Man,” was born, and the picture of human evolutionary history was changed forever.

By the 1930s, paleontological excavations had resulted not only in the discovery of additional Neanderthal specimens in western and eastern Europe but also in the discovery of specimens of human relatives of different and diverse morphologies from far-flung Old World sites: from Western Europe, the Mauer, or Heidelberg, mandible (the holotype of Homo heidelbergensis); from what is now Zambia, a skull that was given the species name Homo rhodesiensis; from northern China, an isolated molar that served as the basis of the genus and species Sinanthropus pekinensis, to which a number of partial craniums were subsequently added; and from various sites in South Africa (first, Taung, followed closely by Kromdraai, Sterkfontein, and Swartkrans), cranial and mandibular specimens that became the holotypes of various species distributed among three different genera (Australopithecus, Plesianthropus, and Paranthropus). During the 1940s, the discovery of new and often unexpectedly different-looking specimens fueled the practice of erecting new species or even new genus and species names, often for each specimen (e.g., a somewhat crushed mandible with some teeth from the South African site of Swartkrans was named Telanthropus capensis because it was smaller and less robust than specimens from the same site allocated to Paranthropus crassidens). When in 1950 Ernst Mayr, the systematist of the synthesis, turned his sights on the field of paleoanthropology, he was appalled at what he characterized as a “bewildering diversity of names.” Following a path already taken by the geneticist of the synthesis, Theodosius Dobzhansky, Mayr waded into an area about which he knew absolutely nothing: human evolution.

Ernst Mayr and Theodosius Dobzhansky: Where Angels Fear to Tread

In 1944, the year the vertebrate paleontologist George Gaylord Simpson published Tempo and Mode in Evolution, which was the last of the three volumes that became the foundation of the synthesis, Theodosius Dobzhansky began to speculate about the course of human evolution. As a fully converted selectionist by the time he published the second edition of Genetics and the Origin of Species in 1941, Dobzhansky proposed that culture-bearing humans, with their cognitive and musculoskeletal abilities to create and manipulate their own environmental circumstances, were exempt from the Darwinian process of change that resulted from adaptation by means of natural selection. Moreover, any human relative with these neurological and anatomical proclivities was also protected and exempted from the whims of selection. Consequently, since living humans are such a geographically varied but single species, it is likely that this was always the case with hominids.

In 1950, Mayr added his reasons for thinking that human evolution was merely a single, nondiversifying continuum of change. First, Mayr claimed, a “real” systematist would see that regardless of differences, all hominids shared the same adaptation: bipedal locomotion. Consequently, since a real systematist knows that a genus is best defined by its ecological specialization—which bipedalism would constitute—all known hominids should be subsumed in the same genus, in this case, Homo. Second, humans today are an amazingly varied and geographically widespread species, occupying all available econiches. By extension, hominids of the past must have been at least as varied (Mayr even went so far as to proclaim without justification that earlier humans were even more variable than living humans) and thus must also have occupied all econiches available to them. Since, as Mayr had argued in his 1942 opus, Systematics and the Origin of Species, in order for speciation via diversification (rather than via unilinear transformation) to occur, a sub-species had to be able to invade a vacant econiche so that it could fall victim to new selection pressures and subsequent adaptations, it was clear that hominids had never speciated and never would. Consequently, there was always only one hominid species at any point in time, and the entire picture of human evolution could be thought of as a highly variable continuum of transformation through time. The upshot of Mayr’s speculation was translated taxonomically into three time-successive species: Homo transvaalensis (to receive the earliest hominids, which at the time were known from sites in South Africa), Homo erectus (which subsumed all specimens from Asia as well as the Mauer jaw from Germany), and Homo sapiens (which was the wastebasket for everything younger than H. erectus, including Neanderthals).

As if this series of untested (and untestable) assumptions weren’t sufficient to convince paleoanthropologists of the errors of their ways, Mayr (1942, 1950) added another element to his case for a unilinear picture of human evolution, namely, the ugly face of “race” and “racism.” For, he claimed, even though we know that “Congo pygmies” and Watusi are members of the same species, Homo sapiens, it is likely that without this knowledge a systematist confronted with their skeletal remains might allocate them to different species. More subtly, though, the implication of Mayr’s folding morphologically disparate living humans and Neanderthals into the same species or the diverse array of earlier hominids into H. transvaalensis was the following: If very morphologically distinctive hominids are members of the same species, then it is ludicrous to pretend that any perceived differences between groups of living humans are biologically and thus evolutionarily significant. Although in hindsight, one can interpret Mayr’s taxonomic action as a reaction to the racism and ethnic cleansing that fueled the Nazism of the recently ended Second World War, it nonetheless had a long-lasting and intellectually stultifying effect on the field of paleoanthropology.

Ernst Mayr Meets Louis Leakey

For more than a decade thereafter, most paleoanthropologists followed Mayr’s taxonomic revision of hominids. The notable exception was Louis Leakey, who, together with his wife, the prehistoric archaeologist Mary Leakey, had returned from England to the East African country of Tanzania (formerly Tanganyika) to excavate a portion of the Great Rift Valley system known as Olduvai Gorge. Beginning in the late 1950s and continuing into the early 1960s, Louis and Mary discovered specimens of fossil hominids at various sites distributed throughout the uppermost portion of the lowest and thus earliest of the four deposits, or beds, that make up Olduvai. Although the specimens would change the picture of human evolution, the date obtained from the lava flow that capped this stratum, which was identified as Bed I, shocked the world by providing the first evidence of deep antiquity for human relatives. Upper Bed I dated to approximately 1.75 million years ago (mya)—more than three times the length of time previously allotted to human evolution.

At one Upper Bed I locality, which became known as the “Zinj” site, Mary Leakey found a virtually complete cranium, replete with teeth. The incomplete eruption of the last or third molar and the patent sutures of this specimen indicated that the individual had not been fully adult at death. The cheek teeth (premolars and molars) were huge while the anterior teeth (incisors and canines) were so small that all six of them fit easily in the front of the upper jaw. The face was massive, somewhat so across the rather flat midface but especially in the distance from the brows (supraorbital margins) to the teeth; the rounded supraorbital margins themselves, while not anteriorly protrusive, were quite tall, and where they met in the midline above the nasal region, the bone swelled out. The top of the skull bore a midline (sagittal) crest, to which huge masticatory muscles (the temporal muscles) had attached, and markedly expanded mastoid regions, which lie posterior to the ear region and to which the thick bandlike muscles on either side of the neck attach. Louis Leakey coined the genus and species names Zinjanthropus boisei to receive this specimen, which was catalogued as Olduvai Hominid (OH) 5. Being the only skull of its kind then known from East Africa, he had to turn to the South African hominids for comparison.

Although Mayr (1942, 1950) had placed all early hominids from South Africa in the same species—Homo transvaalensis—the obvious morphological differences between groups of hominids were considerable. One type, represented by the first skull discovered at the site of Sterkfontein, had a small and modestly domed cranial vault, thin supraorbital margins that swelled where they became confluent above the nasal region, and slightly expanded mastoid regions. Although the midface of this specimen was relatively flat across, the lower face curved outward somewhat so that the dental margin extended in front of the nasal opening (aperture). In the Zinj cranium, the lower face region was about as flat and vertical as the midface.

When he was first presented with this Sterkfontein specimen, which was catalogued as Sts 5, physician cum paleontologist Robert Broom (1951) thought it might be a species of the hominid genus his younger colleague, the neuroanatomist Raymond Dart, had created 11 years earlier—in 1925 to be exact—for a partial child’s skull that had come from the lime works at the site of Taung, farther to the south and west of the Sterkfontein site. Dart had been impressed by this small specimen’s human rather than apelike features, such as smooth supraorbital margins that flowed into a domed frontal region, a very mildly protrusive lower face, small canines (especially uppers), and judging from a mold of the inside of the right side of the braincase that formed as once-dissolved limestone hardened, a very large brain with what seemed to be an apparently greatly expanded cerebral cortex (the “thinking” part of the brain). As such, and because he believed the Taung child was more ancient than Dubois’s Pithecanthropus erectus, Dart declared his South African specimen to be the missing link between apes and humans and the true ancestor of modern humans for which he created the genus and species Australopithecus africanus (“southern ape” from “Africa”), which he placed in a new family, Homo-simiadae. Although Broom (1951) provided only conjecture that his Sterkfontein specimen was a species of Australopithecus, he nonetheless allocated it to a new species, A. transvaalensis (the species in which Mayr would subsume all South African hominids). In 1946, with coauthor G. W. H. Schepers, and again without justification, Broom declared that Sts 5 actually represented a distinct genus. Thus, Plesianthropus transvaalensis was born. Taxonomy aside, the Leakeys’ Zinj specimen was not even a close match for the Sterkfontein specimen. But it did seem to be relatable to the other type of hominid represented in South African deposits.

Not too far from Sterkfontein in the Transvaal lies the site Kromdraai, which beginning in 1938 had attracted Broom’s attention and at which a partial facial skeleton and palate, some isolated upper teeth, and a partial lower jaw had been discovered by a schoolboy. Although it was obvious that the isolated upper teeth had been purposefully removed from the former specimen, it was a mere assumption that the lower jaw also went with it. Nevertheless, in their 1946 publication, Broom and Schepers referred both the partial face and the mandible to the new genus and species Paranthropus robustus, so-named because while larger and seemingly more robust in its features (and presumed tooth size) than Sts 5, its inferred smaller upper canine was taken as indicating its greater centrality to human evolution.

More specimens attributed to Paranthropus—but to the species P. crassidens—were soon discovered at Swartkrans, which is within view of Sterkfontein and which Broom excavated from 1948 through 1949. Of the various cranial specimens, one (SK 48) was particularly well preserved. It had thin supraorbital margins that swelled out where they merged over the nasal region, a low braincase that bore a midline sagittal crest, a very broad face that was relatively flat and vertical across its midregion and down to the margin of the upper jaw, and an inflated mastoid region. Of the preserved teeth, the upper canine was tiny and the premo-lars and especially molars relatively much larger. Vacant root sockets (alveoli) demonstrated that all four incisors and the pair of canines had sat along the front margin of the upper jaw. The only mandible with all teeth preserved had similar anterior versus posterior tooth proportions.

In spite of Mayr’s (1942, 1950) collapsing this diverse lot of specimens into a single species, those who actually studied them—which Mayr had not—could not avoid recognizing at least some of their differences. And thus it was that Louis Leakey and subsequently other paleoanthropologists found a better comparison between the Zinj specimen and the Swartkrans kind of skull than between Zinj and the Sterkfontein type. Shortly thereafter, paleoanthropologists began to refer to the Sts 5 type as “gracile australopithecine,” the SK 48 type as “robust australopithecine,” and the Zinj type as “hyperrobust australopithecine.”

But Zinj was not the only discovery at Olduvai that revised the picture of human evolution. A few years later, Louis Leakey found at various locales throughout Upper Bed I a partial mandible, fragments of cranial bone, foot and ankle bones, and crudely manufactured stone tools that he, together with anatomists Phillip Tobias and John Napier, published as representing a new and different hominid—one that had a larger brain than any australopithecine, had been a more proficient biped than any australopithecine, and had the capacity to manufacture rather than merely use objects as tools. Following Dart’s suggestion, Leakey, Tobias, and Napier gave their constructed hominid the species name habilis (“handy man”). Since they had succumbed to the general belief that only one humanlike hominid could exist at any point in time—and Zinj with its chunky cranial features and large cheek teeth did not look the part—Leakey et al. felt justified in placing habilis in the genus Homo. Since, however, the definition of Homo had come to include not only toolmaking but also a brain size (cranial capacity) of at least 800 cc (which accounted for the smallest specimen ofHomo erectus as well as the large-brained Neanderthals), Leakey et al. lowered this “cerebral Rubicon” to 600 cc, which was Tobias’s estimate for habilis based on reconstructing the cranial fragments into pieces of what he thought were paired right and left parietals (the largest cranial bones, which meet along the sagittal suture). A variety of specimens from the lower portion of Bed II were also placed in H. habilis, not because they were similar to the original specimens or even similar to one another but because they were not Zinj-like.

Faced with these Olduvai finds and Louis Leakey’s interpretation of them, Mayr (1942, 1950) recanted a bit. He allowed that at least early in human evolution there could have been some diversity in perhaps two or three species of Australopithecus (A. africanus, A. robustus, and A. boisei). However, Mayr remained firm in his belief that there was always only one species of genus Homo extant at any point in time. Thus, H. habilis had evolved into H. erectus, which evolved into H. sapiens.

Paleoanthropology after Mayr

Although discoveries of fossil hominids continued apace—especially of early hominids by Camille Arambourg, Yves Coppens, and F. Clark Howell in southern Ethiopia at Omo and Richard Leakey, Alan Walker, and later also Meave Leakey in northern Kenya at sites on the east and west shores of Lake Turkana into which the Omo River flows—there was no permanent deviation from Mayr’s second assault on paleoanthropology until 1974. In November of that year, Donald C. Johanson (Johanson & Edgar, 2006) discovered the partial skeleton nicknamed “Lucy” at Hadar, eastern Ethiopia, for which with Coppens and his colleague at the time, Tim White, he created the species Australopithecus afarensis. Although Johanson had initially thought that more than one early hominid was represented in the Hadar sample, he ended up following White in allocating all somewhat contemporaneous Hadar hominids to this single species. The matter of one or more species of Hadar hominid aside, the naming of a new hominid species could be accepted because Lucy et al. were older than any other hominid fossils (dating close to 3 mya) and this taxonomic deed did not affect the genus Homo.

Coppens and Howell had coined the genus and species Paraustralopithecus aethiopicus for their Omo specimens, but these fragmentary mandibles were quickly subsumed in A. boisei, and Richard Leakey avoided the issue of classification for years by referring to his hominid fossils either by their catalog numbers or in vague reference to the species in Mayr’s scenario. A proposal in 1975 by Colin Groves and Vratislov Mazák that one of the mandibular specimens from the site of Koobi Fora (east Turkana)— KNM-ER 992—represented not H. erectus but a new species, H. ergaster, lay dormant until the 1990s when Bernard Wood applied this species name to two cranial specimens from Koobi Fora (KNM-ER 3883 and 3733) as well as to a fairly complete skeleton from west Turkana (KNM-WT 15000) that Leakey and Walker were content to place in H. erectus. But a partial skull—KNM-WT 17000—that Leakey and Walker discovered in the 1980s at a locality in west Turkana and initially referred to as Australopithecus boisei would lead to further reconsideration of early hominid systematics.

WT 17000 differed from both Zinj and also specimens from Koobi Fora allocated to Australopithecus boisei in having a smaller braincase that bore a very tall sagittal crest. Further, at least as Walker had reconstructed the pieces, while the midface was very broad and flat, the lower face and massive palate projected far forward. Since at 2.5 mya, WT 17000 predated Zinj and many of the presumed hyperrobust specimens from Koobi Fora by about 0.75 million years, and the South African robust hominids by about 1 million years, a number of paleoanthropologists accepted WT 17000 as the ancestor of both A. boisei and A. robustus. Other paleoanthropologists, however, thought that WT 17000 had given rise to A. boisei, which had been ancestral to A. robustus. In either scenario, WT 17000 was seen as the morphologically most primitive of the lot and thus deserving of its own species. Since the Omo mandible that was the type specimen of Paraustralopithecus aethiopicus was essentially contemporaneous with WT 17000, most paleoanthropologists were comfortable with using that species name for the Turkana specimen in the binomen Australopithecus aethiopicus. This act led to a discussion of the existence of a robust-type australopith clade (evolutionary group) that was very different from the gracile-type australopith, which in turn led to the resuscitation of the genus Paranthropus to accommodate the species robustus, boisei, and aethiopicus; only africanus and afarensis remained in Australopithecus. By this time, the late 1980s, paleoanthropologists were generally amenable to a broad picture of early hominid evolution in which A. afarensis (at approximately 3 mya) could have given rise to A. africanus (possibly as old as 2 mya) and perhaps alsoParanthropus, with A. africanus giving rise to Homo.

It seems that once the fear of identifying taxic diversity was overcome, recognition of new species if not new genera (pl. genus) and species became acceptable. There were even tentative forays into the genus Homo, with Wood’s 1991 allocation of various Turkana specimens from H. erectus to H. ergaster and yet others to a species that V. P. Alexeev had created for the KNM-ER 1470 cranium, H. rudolfensis. Two years later, Friedeman Schrenk allocated a specimen from Malawi to the latter species. In 1994, from 4.4 to 4.5 mya deposits in the Ethiopian Middle Awash, Tim White, Gen Suwa, and Berhane Asfaw published work on various isolated teeth and cranial fragments that together they thought represented a new species of Australopithecus—A. ramidus—that had been ancestral to A. afarensis. The following year, White et al. transferred ramidus to a new genus, Ardipithecus. Although stating that A. ramidus was still a hominid on the basis of presumed nonapelike features of the canine first-premolar complex and an inferred anterior position of the foramen magnum (through which the spinal cord exits the skull into the vertebral column—its anterior position in humans is associated with bipedalism), the absence of thick enamel on the molar teeth represents a major stumbling block for attributing hominid status to it.

Also in 1995, Meave Leakey, Alan Walker, and colleagues announced the discovery of various jaws, isolated teeth, and a few bones from two sites in west Turkana, Kanapoi and Allia Bay. The approximately 4 mya age of these specimens—which put them chronologically in between Ardipithecus and Australopithecus afarensis—led Meave Leakey et al. to argue that they had discovered a species—A. anamensis—that was more primitive in jaw and tooth than A. afarensisand thus ancestral to it. A potential problem here was that, judging from the lower part of a humerus that had previousely been discovered at Kanapoi and a newly discovered upper part of a tibia, the specimens were humanlike, not australopith-like.

In 1999, Asfaw et al. proclaimed that a 2.5 mya upper jaw with teeth from the Ethiopian Middle Awash represented a new species, Australopithecus garhi. Two years later, Johannes Haile-Selassie announced a new 5.8 to 5.2 mya subspecies of Ardipithecus, A. ramidus kadabba, based on a mandible and worn teeth. But two other events in 2001 also affected paleoanthropology. Brigitte Senut and Martin Pickford pushed further the time depth of human evolution with various jaw, tooth, and postcranial specimens they found near Lake Baringo, Kenya, that dated to just under 6 mya and that formed the basis of their new taxon, Orrorin tugenensis. They argued that a reasonably well-preserved upper femur had hominid-like attributes, and unlike Ardipithecus, the molars bore thick enamel. Also in Kenya, Meave Leakey and colleagues made public a 3.5 to 3.2 mya skull from a site in west Turkana that, although severely cracked, was sufficiently preserved to show that the face was quite flat and the ovoid orbits devoid of supraorbital development. They assigned this specimen and two mandibles to a new taxon, Kenyanthropus platyops, to which they also referred the KNM-ER 1470 cranium, which had already gone through the taxonomic ringer, having been interpreted first as Homo habilis, then Pithecanthropus rudolfensis, and then H. rudolfensis.

At this writing, only one other putative hominid remains to be discussed: a rather crushed skull and presumably associated specimens from Chad, probably closer to 6 mya than its claimed 7 mya age, to which the name Sahelanthropus tchadensis was given. Prior to this discovery, which Michel Brunet and collaborators announced in 2002, Brunet had found only one very fragmentary specimen—the front of a lower jaw missing most teeth—which he made the type specimen of a new species, Australopithecus bahralghazali. Brunet’s interpretation of Sahelanthropus as a hominid and ancestral to all other hominids rested on the antiquity of the specimens as well as canine shape and tip wear, molar enamel being intermediate in thickness between chimpanzees and undisputed hominids (and thicker than that of Ardipithecus), and an inferred anterior position of the foramen magnum. If a hominid, then the skull bears the thickest and most prominent brow ridges of any (even more so than gorillas), which would make it the most derived of its putative clade and thus unlikely to have been ancestral to any known hominid.

As this overview demonstrates, in contrast to the 20 years that preceded them, the last 20 have witnessed a flood of newly named hominid taxa. This may give the impression that paleoanthropologists eventually shed the shackles of taxonomic truncation that Mayr (1942, 1950) had imposed, but it should be noted that the recognition of hominid diversity has largely been confined to the older fossils. All things considered, the situation with regard to later hominids is not that different from Mayr’s formulation. Wood’s revival of Homo rudolfensis fell on deaf ears, and even though his effort to recognize H. ergaster as an African counterpart to Asian H. erectus has been embraced by some paleoanthropologists, a survey of the most popular human evolution textbooks reveals that the H. habilis -evolving-into-H. erectus scenario is still going strong. So too with regard to H. sapiens, which by subsuming such morphologically disparate specimens as Cro-Magnon 1, the Feldhofer Grotto Neanderthal, the Kabwe skull, and the Javanese Ngandong skulls, to name but a few, sports a range of variation otherwise unknown in the animal kingdom.

Granted, some paleoanthropologists have tried to deal with our own unwieldy species. For example, over the past two decades, there has been a growing appreciation of just how different Neanderthals and we are, not just in features of the skull, jaws, and teeth but also in most of the bones of the postcranial skeleton. Consequently, while a handful of stalwart multiregionalists hold steadfastly on to the notion that there was always only one middle Pleistocene hominid— H. erectus—that evolved en masse, albeit in regionally different stages, into H. sapiens, William King’s Homo neanderthalensis has enjoyed increasing recognition among paleoanthropologists.

In addition, the name Homo heidelbergensis, which Otto Schoetensack created in 1908 for a 0.5 mya mandible excavated near Heidelberg, Germany, has in recent years been applied to various 600 to 300 kya (thousand year old) skulls, most notably those from Kabwe (Zambia), Petralona (Greece), Arago (France), and Bodo (Ethiopia). Yet although superficially similar in having robust faces and tall brows, these skulls do not present a relatively uniform picture when details, especially of sinuses and the inside of the cranial vault, are considered. In addition, and more important, with the exception of specimens from Arago, which include mandibles, none of the other craniums can be compared with the type specimen of H. heidelbergensis—which makes allocating them to this species impossible. Nevertheless, it is probably better to have some recognition of the potential diversity that exists among the specimens attributed to Homothan not.

But the recognition of new species of Homo has languished. Indeed, although from the 1980s on, Juan-Luis Arsuaga and colleagues (Arsuaga & Martinéz, 2005) have amassed a large collection of specimens of the same hominid dating to about 350 to 320 kya from the site of Sima de los Huesos, which is in the Sierra de Atapuerca in northern Spain, they have inclined toward regarding it as H. heidelbergensis or vaguely as something Neanderthal-like. However, in 1997, José María Bermúdez de Castro and collaborators did claim that they found the remains of the common ancestor of H. neanderthalensis and H. sapiens at the minimally 780 kya site of Gran Dolina in the Sierra de Atapuerca; they called itH. antecessor. Since then, in 2002, only one other species of Homo has been proposed: H. georgicus, based on a large mandible with massive but very heavily worn teeth from the 1.8 mya Georgian site, Dmanisi. The morphologically very diverse craniums from this site have all been referred to H. erectus. More than 50 years after Mayr (1942, 1950) imposed the picture of unilinearity on the human fossil record, his influence is still keenly felt in the taxonomic realm of genus Homo.

Future Directions

The reader has no doubt noticed that most of this review is the iteration of taxonomic names. That this form of presentation is particularly characteristic of human paleontology, in contrast to the broader domain of nonhuman paleontology, is reflective of the odd history of the former discipline—in which taxonomic minimalism and the perception of unbounded interindividual variation within any taxon has dominated. Consequently, the focus when studying hominid or potential hominid fossils has been less on morphological detail and a concern with systematic theory and practice than on constructing scenarios of how, why, when, where, and from whom a sequence of modern human ancestors evolved. Were this not the case, we would not find that so many type specimens—which are the name bearers of their species or genus and species—are either so fragmentary or devoid of preserved anatomical detail that comparison with other specimens, which would seem a necessary requisite to assigning them to a given taxon, is often impossible. Were this not the case, we would not see the oft-repeated practice of lumping specimens from the same site or the same time period into the same taxon, which gives the locale of discovery or the ages of specimens priority over morphology—which is the only meaningful reflection of evolutionary history. Were this not the case, we would not see the widespread resistance to restudying and reinterpreting the fossils that were discovered during the 1800s and much of the 1900s. If virtually every newly discovered fossil can be regarded as representing a new taxon, especially if it is well over 1 mya, then why should not the already known human fossil record provide a wealth of information that was overlooked in the past? The problem with paleoanthropology in general has been that most practitioners have left observation behind, and instead, they have focused on evolutionary explanation. This problem is, however, easy to remedy. One need only begin by using one’s eyes rather than being influenced by either the past or the perceived weight of received wisdom.