The Story of Hominin, Part I: pre-Homo

In a contemplative article, Bernard Wood, the doyen of hominin taxonomy, identified three boundary problems in the business. The first is, of course, where to draw the line between ape and man; more precisely, the boundary between “ape-like” hominid and man-like hominin fossils. [Hominid includes the great apes; hominin does not.] It is not simply a matter of lineage for there are many species (by which we simply mean morphological types or taxa; not the biological definition—the ability to produce fertile offspring—which cannot be determined on the basis of extant evidence) in the fossil record that are neither the ancestors of modern humans nor that of modern chimps. So the question is above all where to place these dead-ends. In practice, the boundary is fuzzy and boils down to the degree to which the species is arboreal (adapted to living in the canopy) or bipedal. Committed bipeds are regarded as closer to modern humans; committed arboreals as closer to apes; with fossils displaying both adaptations somewhere in between.

Source: Wood and Collard (1999).

The second is the boundary of the genus Homo. Drawing this boundary is an equally fraught enterprise for now we are talking about demarcating which species can be regarded as human senso lato (in a loose sense)Committed bipedalism is not enough, it is reasonable to demand that species in our genus approach the human form if not behavior. Human-like behavior, especially tool manufacture, is of course enough to guarantee you a spot in the genus. But things are complicated due to the fact that multiple hominin species coexisted in eastern Africa when the first tools appear around 2.5 Ma (millions of years ago) so that it is impossible in general to attach the first industries to particular species. In practice, paleoanthropologists regard big brains as the ticket to entry with cranial capacity above 600 cubic centimeters (cc) regarded as the conventional boundary. The defining exception or marginal case is H. habilis (“handy man”, 2.5-1.4 Ma) which has definitely been tied to the first lithic technology but sports an average cranial capacity of only 552 cc. (Although Gamble reports an average of 609 cc. Compare figures 1 and 2.) We will presently return to encephalization in human evolution.

Source: Gamble (2013).

The third is the boundary between fully-human, H. sapiens senso stricto (in the strict sense), and near-human hominin. This is perhaps the most fraught boundary of the three. It is also uncomfortably close to the question of the origin of the races, that perennial obsession of high racialism. There is a virtual consensus that what distinguishes humans from other hominins is behavior—we are more sophisticated, behaviorally-plastic, and dynamic than those other guys. But this is very far from being free of problems. First, the appearance of anatomically modern humans considerably predates the evidence for behavioral modernity however defined, so that the origin of our species senso stricto is thereby shrouded in mystery. Second, some but not all late archaic hominins, in particular Neanderthals, are associated with advanced lithic technology (including Levallois tools) indistinguishable from contemporaneous anatomically modern humans (say around 100 Ka).

It would seem that there are two ways of resolving this boundary problem. Either we impose a less stringent criteria for behavioral modernity (say tool manufacture requiring multiple processes in a specific sequence) and therefore be generous with inclusion. Or we impose a more stringent criteria for behavioral modernity (say art, ornaments, burial, colonization of extreme environments, sea-faring, projectile weapons, and so on; in short, culture, versatility and dynamism) and be thereby stingy with inclusion. But both generous and stingy definitions are deeply-problematic.

For if you say advanced tool manufacture (say Acheulean tools c. 1 Ma, in particular, bifacial hand-axes) is sufficient criteria for inclusion then archaic hominin (including our ancestors) in the western Old World would be included but not those in eastern Eurasia for the spread of the Acheulean industry after 1 Ma was confined to west of the infamous but accurate Movius line. The eastern Old World, populated by late archaic hominins, continued to manufacture unsophisticated Oldowan tools developed c. 2.5 Ma for hundreds of thousands of years after Acheulean industry becomes dominant in the western Old World.

Movious Line.png
Source: Lycett and Bae (2010).

If on the other hand you say let’s be stingy and restrict H. sapiens senso stricto to much more recent hominin populations that display the full suite of modern behavior, then that would imply that populations in western Eurasia (Europe and the Near East) and Sahul (Australia, Tasmania, New Guinea) were fully-human by 50-40 Ka, tens of thousands of years before the rest of the world (say 20 Ka). No doubt this is extremely controversial. But the empirical evidence for global polarization in the Late Pleistocene is overwhelming. Basically, apart from some ephemeral early evidence in southern Africa around 90 Ka, the appearance of the full-package of behavioral modernity around 40 Ka is confined to western Eurasia to which the term Upper Paleolithic is properly applicable. To reach Sahul, of course, required sea-faring so that those populations were definitely fully-modern. A different term, Late Stone Age, applies to Africa from 50 Ka on. It is defined in terms of advanced lithic technology and does not sport evidence of the full-package of behavioral modernity. I’m walking on coals here … many Africanist prehistorians would be furious. But my goal is to problematize the modern-premodern dichotomy in prehistory.

My general point is that all three dichotomies are necessarily fuzzy. Any schema involving sharp boundaries in hominin taxonomy is guaranteed to be shot through with contradictions and glaring anomalies. In short, it’s a fool’s errand. In what follows we will not take a position on these boundary questions.

The title of the present dispatch is also a nod at another problem in narrative accounts of the human career. Namely, the temptation to take a teleological approach is very strong in this domain. ‘The Story of Man’ presumes that tracing how we got to where we are is a sufficient account of the hominin career. But that ignores the evolutionary dead-ends; more sympathetically, it ignores hominin forms and strategies different from ours that went out of business. The explosion of taxa is a testament to the extraordinary variation in hominin morphology (and therefore life history and survival strategy) that is not only interesting in itself, but also informs our own story. Just as a liberal order after the failure of Communism is quite different from the counterfactual without the Communist experiment, the story of us that emerges from a full consideration of alternative lifeways pursued by sister species is different and richer than a story that emerges from the tenuous assumption of telos in human evolution. In other words, we must tell the story of the others as well as of us because the story of the others informs the interpretation of our own story. What follows is a sketch of a broad-brush history of our tribe, the Hominini.

The story begins with the Planet of the Apes. During the Miocene, 23-5 Ma, the climate was much warmer and wetter. Siberia and Greenland were not glaciated and rainforests covered much of the Old World. Apes emerged out of Africa and colonized much of the Old World presumably jumping from canopy to canopy.

Source: Gamble (2013).

By the time of our last common ancestor with the chimps c. 7-6 Ma, it was still warm but the temperatures had fallen dramatically. Greenland was glaciated, the rainforests had receded, and apes had become confined again to Africa. This is where the first hominin began, very tentatively, to walk. The emergence of committed bipedalism was an excruciatingly slow process. Although we have scant fossil evidence for the period, it is clear that for millions of years, hominin refused to commit to bipedalism. They retained skeletal features like opposable toes that show that they were still arboreal and only occasional bipeds. Very little is known about early pre-australopith hominins c. 7-4 Ma other than they sport an ape-like morphology. Indeed, the only thing that distinguishes them from apes is that they were occasional bipeds. In fact, some experts suggest that they are too ape-like to be considered hominin. Regardless, one or more of these hominins, most likely from the genus Ardipithecus, evolved into Australopithecines, when things start to get really interesting.

Early Hominin
Source: Liberman (2013).

Towards the end of the Pliocene, 5.3-2.6 Ma, the climate became much cooler. Rainforests disappeared from eastern Africa and Woodland and savannah expanded. A vast number of hominin taxa suddenly explode in the fossil record at this time. Most of them have been placed in the genus Australopithecus c. 4-1 Ma. The most famous australopith, of course, is Lucy (named after the Beatles’ song Lucy in the Sky with Diamonds) who lived in Ethiopia 3.2 Ma and belonged to the taxon A. afarensis. It is clear from her skeleton that Lucy was an obligate biped, eg the absence of opposable toes. But the earliest evidence for obligate bipedalism is from the Laetoli footprints made c. 3.6 Ma that have also been associated with A. afarensis.

There is no consensus on why bipedalism emerged at this time. Some have claimed that bipedalism might have been a postural adaptation with those able to stand upright being able to gather more fruit. Others have suggested that selection of bipedalism was due to the thermodynamic efficiency of bipedal locomotion in the expanding savannah or woodland habitats where sustenance was more sparsely distributed. Still others have emphasized the thermoregulatory advantage of upright walking. It has been suggested that australopiths engaged in midday scavenging when competition from quadruped scavengers (disadvantaged because they expose a much greater surface area to the sun) was absent or less intense. In all cases, the logic leads straight to the question of foraging strategy and therefore diet. We’ll return to this question shortly.

Australopiths, like all early hominin, had small bodies and small brains. Australopith females, for instance, were just 1.1m tall and weighed 28-35kg. Male Australopiths averaged 1.4m in stature and weighed in at 40-50kg. Thus, males were about 40-50 percent larger than females. From figure 2 we see that the index of sexual dimorphism for Australopiths (1.53) is closer to gorillas (1.68) than modern humans (1.16). This suggests that male Australopiths fought each other for access to females.

With an average cranial capacity of just 464 cc, their brains were nearly as small as that of modern chimps. Their brains were small not just in absolute volume but also relative to body mass. Indeed, their encephalization quotient comes to just 2.6, or less than half as much as modern humans who sport an EQ of around 6-7. Given their brain size, Dunbar’s social brain hypothesis suggests a network size of 67 individuals, less than half that of contemporary human population (136). They also had a much faster life history; taking about 12 years to reach adulthood.

life history
Source: Liberman (2013).

While all early hominin were small-bodied and small-brained, they differed markedly in their dietary strategies. Both gracile Australopiths and robust Australopiths (the latter have recently secured their own genus, Paranthropus) ate a wide-variety of fruit, insects, leaves, tubers, roots, and the occasionally scavenged meat as attested by their greater molar size (compared to Ardipithecus). But what distinguishes the two is their masticatory (chewing) apparatus.

Source: Evans et al. (2016).

Taxa in the genus Paranthropus in general, and the taxon P. boisei in particular, were what Wood called megadonts. Their powerful masticatory muscles and very large molars allowed them to crush and grind hard foods such as nuts, seeds, roots, and tubers in the back of the jaw. Since the genus Homo emerged from the gracile Australopiths, the megadonts are the classic dead-end. None of their descendants survived. They vanish from the fossil record after 1.3 Ma. So the temptation is rather strong to see the roots of their doom in dietary specialization in hard-to-digest and poor quality foods. That temptation must be resisted. Microwear evidence from their tooth enamel suggests that their diets were just as varied as the gracile Australopiths. The decisive difference in dietary strategy between the two was in fallback foods, ie what they resorted to eating when their preferred food was unavailable. However, it is clear that their strategy did not generate the sort of feedback loop between foraging strategy, gut morphology, and encephalization that emerged in our lineage. That will be the subject of part II when we examine the career of the genus Homo. Stay tuned.

Source: Aiello and Wheeler (1995).




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