How ‘Human’ were our early hominin ancestors who lived 1.8 million years ago?

Do early hominins which lived 1.8 million years ago (referred to as ‘ma’ from now on) display traits and physiognomies which we would recognise as characteristically ‘human’? These traits and physiognomies will be defined here as those more generally associated with modern humans, Homo sapiens, e.g. the use of a variety of tools for different purposes, a diet high in meat, large brain capacity and complex social organisation. If there is a threshold between being ‘human’ and ‘non-human’, how would it be revealed to us through the available evidence? This 1.8ma margin comes at the boundary between the early Pleistocene ages of the Galasian on to the Calabrian, and it is around this time that we see a change in the palaeoenvironmental record for Africa, as well, most importantly, as in the fossil record.

Various types of evidence suggest themselves: the Oldowan tool assemblages, diet, bipedalism, morphology and cranial capacity. These can be examined separately, although there is significant overlap between them. We can study the material remains of our early ancestors, but to infer meaning and complex behaviour from the archaeological record is not a straightforward task, nor is its interpretation free of debate and controversy. For example, when we talk about ‘apes’ and the behaviours that may or may not have divided early hominins from them, we have to bear in mind these are proto-chimpanzees– i.e. divergent species with homologous traits gained in evolutionary recent time.

Figure 1 — An example of artefacts found from an Oldowan assemblage from Kanjera South in Kenya. Photograph A shows a hammerstone, photo B a core and C a flake.

The Oldowan assemblages

This earliest known stone tool technology dates from 2.5ma to around 1.7ma and is characterised by its hammerstones and chopping tools (Figure 1), as well as a lack of worked handaxes and bifaces in relation to the Acheulean industry that followed (Dominguez-Rodrigo et al 2005). The presumption for many years in palaeoanthropology has been that the Oldowan stone tool technology could be used as a clear marker between ‘human’ behaviour (i.e.: unique to the Homo genus) and primate behaviour (Noble & Davidson 1997: 162). However, artefacts dated to 2.5ma pull the origins of tool use far before the encephalisation (brain expansion) that seems most strongly to characterise the emergence of Homo. Furthermore, studies on modern human relatives, such as the famous study on the bonobo Kanzi, found they were just as capable of working cores and using the stone flakes which characterise the Oldowan (Toth et al 1993).

Other studies have looked at the possibility of Oldowan re-creation amongst our living primate relatives in larger study groups, gauging their ability to replicate the techniques used in knapping and shaping the stone to create the desired object. One such study compared bonobos attempt at Oldowan-style knapping and found they were unable to grasp the concept that only strikes at the edge of the flint core yielded the sharp, usable flakes that seem vital to the early hominins butchery methods (Toth & Shick 2009: 271). Chimpanzee stone tool use in the wild is only seen utilising the anvil and hammer method, so there is no evidence of their intentional production of sharp flints with a specific purpose, like butchery, in mind. The Oldowan cores and flakes were produced at Gona, Ethiopia at around 2.6ma and were found in proximity to the bones of Australopithecus. africanus, Australopithecus garhi, and Paranthropus aethiopicus, although which of these species was the creator of the tools themselves is not clear (Pobiner 2013:1). Toth and Shick’s comparisons, using criteria based on the different skills required, found that the Gona tools were regularly grouped alongside modern human re-creations, rather than with the less skilful techniques of the bonobo study group (Toth & Shick 2009: 271). This would appear to suggest that human-like awareness of an object and possible associated behaviours differentiating the hominins from primates were visible 2.6ma. However, the study also mentions that the understanding of detaching flakes was understood by the bonobos through trial-and-error, eventually being “mastered after several years” (Toth & Shick 2009: 271). The timescale is seemingly irrelevant in evolutionary terms, just the fact that bonobos were able to ‘master’ Oldowan tool construction indicated that it was not beyond their capabilities, as was previously thought.

Figure 2 — A comparison of the molars from a Paranthropus Bosei, nicknamed the ‘nutcracker man’, with the teeth of a modern Homo Sapien.

Diet

It is well known that meat became a primary component of the diets of later Homo, but by studying the craniodental morphology and associated bones from pre 1.8ma hominin sites, inferences can be made as to what degree meat was being consumed. This can then have implications on what effect this high-quality, high-protein intake of food might have for the development of new or innovative behaviours.

Looking at craniodental morphology first will tell us that the flat, thick cheek teeth of Australopithecus and Paranthropus would suggest large muscle attachments for masticating nuts, grasses and fruits (Figure 2) — and not the sharp canines indicative of carnivores (Strait et al 2009: 2124). Furthermore, studying the enamel and dentine of fossil teeth will tell us that both of these genera “had short dental development sequences more similar to chimpanzees and gorillas than to ourselves” (Clement & Hillson 2013: 65) 
 It would therefore appear that there was some transition between the apparently rare consumption of meat among early Austropithecines and the discovery of mass animal butchery, of large game mammals like giraffe, buffalo, hippo and elephant, at sites like FLK Zinj in Tanzania (Bunn 2007: 194). This discussion links to the analysis of palaeoenvironmental evidence later in the essay, but perhaps the plio-pliestocene environment shift is the sudden trigger that gives early Homo greater opportunity for dietary adaptation, laying the foundations toward the characteristic high-meat diets of later species like Homo Neanderthalensis (Ungar 2004: 617).

There has been much dispute among archaeologists concerning whether or not our ancestors were actively hunting prey on the savannah grasslands as a top predator, or whether they were further down the food chain as a secondary scavenger — with some evidence of early Homo being hunted and eaten by top predators (see Isaac 1978a vs Binford 1981). However, what really seems to be crucial is not how it was procured but the actual dietary shift from a species that had, judging by the morphology, been almost entirely herbivorous, towards a species that was supplementing its diet –at least to some extent, with high protein meat sources (Noble & Davidson 1997: 189). However, we must be cautious when drawing conclusions from a limited, and often fragmented archaeological record, as even artefacts that remain will have been subject to extreme weathering, dispersal and scavenging, which can often have negative effects on the clarity of an artefact’s function when it comes to its interpretation. The marks of animal trampling on bones can sometimes resemble the grooves left by Oldowan flaked tools, while the holes left by scavengers like hyena and porcupine gnawing on bone can be mistaken for stone hammer marks (Klein 2009: 268).

Figure 3 -A wild chimpanzee exhibits hunting behaviour through the crafting of sharpened tools for fishing.

An assumption has been that if the Oldowan associated assemblages can be proven to be linked with hunting — rather than merely scavenging — then this must be the threshold where we start to move toward humanity. However, a look at modern fauna will tell us that hunting technique is by no means indicative of a greater intelligence or distinctly ‘human’ behaviour (Figure 3). Studies have found that chimpanzees in the Taï forest coordinated regular hunts in order to gain access to meat, usually small Old World Monkeys or ‘bush babies’ (Boesch & Boesch 1989: 549). In contrast, wild chimpanzees show no interest in carrion, so it could well be this behavioural difference that distinguishes hominins and apes at this 1.8ma boundary (Klein 2009: 269). It could be argued that if early Oldowan-makers were favouring scavenging over direct hunting it actually shows a greater understanding of their environment and surroundings. They were capable of anticipating risk to their group, by deciding when and where it was safe to access a kill, and possibly had the ability to scare off other predators and scavengers.

Bipedalism

A possible threshold dividing our early ancestors 1.8ma from other apes might come down to bipedalism. We know that even the earlier Australopithecines like afarensis were exhibiting some signs of bipedalism as far back as 3.6ma — prior to the development of the first Oldowan tools (Noble & Davidson 1997: 150). So certainly by 1.8ma all early hominin species alive at the time: paranthropus robustus, paranthropus boisei, Homo rudolfensis and Homo habilis — were bipedal (Smithsonian Institute 2015). Unlike the habitual bipedalism behaviour of our modern primate relatives, these early hominins would have spent all of their time on two legs and were capable of walking long distances (Pontzer et al 2014: 64). Perhaps the best known example of bipedalism among our early ancestors is Mary Leakey’s discovery of footprints preserved in volcanic ash at Laetoli in Tanzania. Although much older in their origin (3.6ma), the findings by can still be useful here (Agnew & Demas 1995). Leakey’s discovery was proof that the emergence of bipedalism came a significant amount of time before the period of rapid encephalisation around 1.8ma (Leakey & Hay 1979: 323). Perhaps bipedalism is not as unique to human associated characteristics as once thought. As we have seen, early Australopithecines were constructing stone tools and most likely consuming some proportion of meat in their diet. This would imply that, although studying the morphology of early hominins to identify bipedalism may be a good way of differentiating between hominins and apes, this does not mean that it necessarily indicates larger brain size and the associated ‘human’ behaviours that may come with that.

Figure 3— Table showing the average cranial capacity for a number of early hominin species. The ‘Early Members of genus Homo’ refers to rudolfensis and habilis, while later ergaster/erectus after 1.8ma had rapid rise in cranial capacity from 700–1200cc (Anton 2003:136).

Brain and encephalisation

A study carried out in 1995 compared the expected brain size of human, hominin and nonhuman primates (in relation to body size), with the actual size of the brain — a so-called “Encephalization Quotient” (EQ) (Aeillo & Wheeler 1995: 200) This study found that when the EQ was calculated, all early hominins were much more similar to modern nonhuman primates like capuchin and squirrel monkeys than they were to modern humans (Aeillo & Wheeler 1995: 208). This would seem to lend significant weight to the argument that ‘humanity’ does not start to flourish, in terms of their intellectual capabilities, until the end of the robust austraolopethicines and parathropus, and the emergence of the Homo genus.

A further study compared the cranial casts of Homo habilis (2.4–1.4ma) from Oldovai Gorge and found their brains were 45.1% larger than the mean cranial capacity for Australopithecus africanus (3.3–2.1ma) — an increase from around 450cc to 654cc (Figure 2) (Aiello & Wheeler 1995: 208) This is seemingly indicative of a relatively rapid rate of encephalisation in such an evolutionary short space of time (Tobius 1987: 741) (Smithsonian Institute 2015). This increase in cranial capacity could be the sudden ‘threshold’, or crossing of the cerebral rubicon at around 1.8ma that marks habilis out as having distinctly different behaviour and capabilities than its robust Australopithecine and primate ancestors. 
 We already know that the most rapid growth in brain size comes with the emergence of Homo erectus and the Acheulean assemblage of stone tools preceding the 1.8 ma threshold — which is coupled with the first signs of meat polish indications on the tools (Keeley & Toth 1981:264). However, if we accept that habilis is indeed part of the genus Homo, then perhaps they can be viewed as the first tentative beginnings of truly ‘human’ cognitions and behaviour.

Conclusion

There is therefore strong evidence that the early hominins 1.8ma, particularly Homo habilis and rudolfensis, were exhibiting signs of complexity in behaviour and cognition beyond that of apes. There are of course some ‘human’ characteristics which cannot be seen this early in the archaeological record, such as the development of language or the possibilities of symbolism reflected in hominin behaviour. However, the increase in cranial capacity would certainly have given these species superior cognition, in comparison not only with Australopithecines but also with modern chimpanzees. Although basic in technique when compared to the complexity of the following Acheulean industry, the Oldowan’s chaîne opératoire of intentional procurement of appropriate material, planned construction and multiple use shows forethought. Given all the reservations and difficulties with the nature and availability of the evidence, it is hard to deny that the signs that characterise humanity are already beginning to appear in our early hominin ancestors.