6.1. Brain Size, Language, Stone-Tools, and Fire
The predominantly directional increase in brain size in the lineages leading to H
over more than two million years, during most of the Pleistocene (~2.6 Mya to ~0.3 Mya) and across several human species [56
], is puzzling from an evolutionary theory point of view. A reversal of the growth trend at the end of the Pleistocene [57
] also requires explanations. In present-day humans, larger cortical size is robustly associated with higher IQ [59
]; a large brain relative to body mass has been shown to predict problem-solving ability in mammalian carnivores [60
Increased social complexity was hypothesized to be the cognitive challenge that drove brain size growth [61
]. Recently, however, ecological challenges, and in particular those related to foraging, have been proposed to better explain the need for brain expansion among primates [62
]. A reduction in gut size, muscle mass, or redirection of energy from locomotion, growth, and reproduction may compensate for the increased energetic cost of a larger brain [65
]. However, these compensations do not explain why a larger brain provided better fitness in the first place. Stanford and Bunn [70
] proposed that the initial increase in the Homo
brain size was driven by the need to develop hunting skills. Brain [71
] attributed the brain size increase to the need to avoid predation; however, the question remains what drove the further ~50% increase in brain size from H. erectus
to H. sapiens
. Establishing the energetic pressure that the decline in prey size inflicted on humans, we propose that the expansion of various cognitive abilities met the ecological challenge of obtaining calories and fat from smaller prey at acceptable energetic costs. Brain expansion allowed humans to partly or wholly mitigate the potential additional energetic expenses on locomotion by tracking and linguistic communication of prey location, and facilitating economic smaller prey acquisition and exploitation by accumulating and transferring knowledge, and maintaining fire, and producing shaped and complex tools.
As indicated in several HG studies, movement on the landscape represents the largest discrete energetic expenditure of HG groups [72
]. Therefore, tracking prey instead of relying on random encounters is a standard energy-saving behavior that could only have come about with an increase in cognitive skills, or the ability to deal with new information [74
]. Blurton Jones and Konner [76
] claimed that tracking is a cognitive process that mimics the scientific process, and used ethnographic research to argue that while tracking, hypotheses are formed and revised based on spoors’ information.
] describes two methods of tracking—systematic and speculative. Systematic trackers track successive spoors, a conceivably more efficient strategy for tracking larger animals because they naturally leave more conspicuous signs of their passage and do not flee (Figure 3
). On the other hand, speculative trackers skip some potential spoors and proceed to where they speculate that the animal has headed, such as a water hole, an area of shade, or a food patch. Speculative tracking is more suitable for hunting smaller animals, which leave less conspicuous signs of their passage. Speculative tracking advances the hunter more rapidly on a shorter route and improves the tracking process’s energetic efficiency. Liebenberg [78
] states that “Speculative tracking requires much experience. So, most trackers start as systematic trackers and only become speculative trackers once they have mastered the basic skills”. Additionally, the ability to identify fat-bearing animals, a critical ability when hunting smaller animals, also requires considerable experience and cognitive capacity [79
] (pp. 42, 43).
Language is a large consumer of cognitive resources [80
] and, hence, energy. We suggest that language increased fitness by facilitating energy savings in the face of prey size decline. Corballis [81
] argues that language evolved in humans to communicate events “displaced in space and time from the present”. A significant amount of energy can be saved by the quick and accurate exchange of information by group members about prey’s recent sightings; information that could not be communicated appropriately without language.
Interestingly, bees use “dance language” to point to a food source that is not evenly distributed and displaced in space from where they are at the time [82
]. In humans, the ability to also describe sighting time is essential as prey is more dynamic in the landscape than flower nectar. Additionally, language could help in the long-term retention and transfer of critical information concerning prey animals’ behavior and countless details regarding the nature of the world in which hunters operate, all of which help save energy during tracking and hunting. Much of the fireside conversation of hunters’ centers around natural phenomena and specific hunting experiences [76
]. In summary, we propose that the evolution of a larger, energetically costly brain was driven to a significant extent by selection for energetic savings capabilities that secured smaller animals’ acquisition at acceptable energetic costs.
Several researchers have claimed that increased mental capabilities facilitated technological innovations, such as the Lower Paleolithic cleavers or later multi-component projectile tools during the Pleistocene (e.g., [83
]), and were most probably oriented toward the acquisition and processing of large game. The bow and arrow, atlatl, and fluted points [86
] may represent inventions that were already improved by the initial expansion in H. sapiens
brain size. These hunting technologies were mostly employed to target relatively small animals [26
]. Transformations in stone-tool technologies could also be related to cognitive developments triggered by the need to acquire smaller and smaller prey.
The control of fire has been hypothesized as the reason for brain expansion in H. erectus
]; however, evidence for fire’s habitual use is much more common post-400 Kya (e.g., [90
]. A central hearth that was continuously and intensively used is a prominent feature in the late Lower Paleolithic site of Qesem Cave, Israel (dated 420–200 Kya), where dental remains of post-H. erectus
human lineage were discovered. Qesem Cave’s faunal assemblage is dominated by the ~100 kg fallow deer (Dama
) and is devoid of elephants, common in earlier Lower Paleolithic sites [93
]. It was argued that fire for roasting and cooking was intended to utilize the smaller animals more efficiently and was critical to the inhabitants’ adaptation. The control of fire is considered part of a suite of innovative behaviors at Qesem Cave that demonstrate a new level of cognitive complexity, triggered by the disappearance of megaherbivores. One of these behaviors, the production of tiny sharp flint items utilizing lithic recycling to execute high-precision cutting tasks, was recently also associated with a new strategy for processing small game [94
]. Moreover, the use of fire for roasting meat and extracting as many calories as possible from every food item continued progressively throughout the Paleolithic [95
], correlating with the decline in prey size. Finally, sharing of smaller animals might have required a higher level of inhibitory control, another improved capability of a larger brain [67
Neandertals also had a large brain, although they hunted large game alongside smaller animals. The comparison of cognitive abilities between Neandertals and H. sapiens
is a subject of continuous research. There is little argument that Neandertals’ brain structure was different, to some extent, from H. sapiens
], suggesting different functionality, which is the expected result of evolution under different ecological conditions.
Our mechanistic explanation for the correlation between the pace of brain growth and a decline in prey size during the Pleistocene can benefit from further testing. Initial indications of such a correlation can be found in East Africa where “brain expansion, independent of body size, appears to be most strongly expressed later, between 800 and 200 thousand years ago” [99
] (p. 10), roughly correlating with a decline in prey size during the East African Middle Pleistocene [6
Associating brain size increase with the mitigation of extra energetic costs that come with the need to hunt smaller prey can also explain the decline in brain size at the end of the Paleolithic period and beyond [57
]. In that period, plant consumption increased [2
], culminating in the domestication of plants and animals. Plants and domesticated animals do not escape so their acquisition does not require the same degree of knowledge and decision making under time pressure as hunting small prey does hence the lower cognitive requirements.
6.3. The Evolution of H. sapiens
The emergence of H. sapiens
in Africa around 300 Kya [108
] are contemporaneous with the onset of the Middle Stone Age mode of adaptation and, in East Africa, with the extinction of large-bodied grazing lineages and their replacement with related taxa of smaller body size [9
]. Potts et al. [9
] focused on the wet-dry climate variability, and the consequent need to cope with fluctuating resources as the drivers of changes at the onset of the MSA. However, we see carnivory specialization as a plausible solution to this and previous events of severe climate fluctuation [109
]. Environmental variation can initiate specialization rather than flexibility in animals’ behavior to reduce the experienced variation [110
], as a predator’s food sources are available in dry and wet conditions. Thus, the evolution of cognitive and cultural means of specializing in prey acquisition may be a viable and less costly solution to environmental variability than flexibility, which also has its costs [111
]. In support, reviewing 1087 extant taxa from 28 phyla, Román-Palacios, Scholl, and Wiens [112
] found that 63% are carnivores, and only 3% are omnivores. They state that their results “suggest that animals often specialize for carnivorous or herbivorous diet rather than being omnivores”.
Regarding mammals, analysis of a large (N = 139) dataset of mammals’ trophic levels [23
] shows that 80% of the mammals in the dataset are omnivores, but most of the omnivores (75%) consume more than 70% of their food from either plants or animals, leaving only 20% of the mammals in the dataset to be omnivore-generalists. Interestingly, while all of the 16 primates in the dataset were omnivores, 15 of the 16 were specialists. According to this somewhat counterintuitive point of view, the decline in prey size identified by Potts et al. [9
] might be the most significant phenomenon in the transition to the MSA. As mentioned, an identical phenomenon, the appearance of a new human lineage and a new cultural complex temporally coupled with the disappearance of the largest herbivore (the straight-tusked elephant), is evident in the Levant 400 Kya [10
]. The emergence of H. Sapiens
in Africa, a new, post-H. erectus
lineage in the Levant, and the concomitant new cultures in both places may represent adaptations to the acquisition and processing of smaller animals. Many physiological and behavioral characteristics of H. Sapiens
may also have been directed toward saving energy when hunting prey.
The increase in brain size as an adaptation towards efficient tracking and hunting of smaller game has already been discussed. Increased locomotive energetic efficiency may have been achieved by the lighter, agile body, which produced long lower limbs relative to bodyweight [113
]. Increased mobility can be a response to environmental variability as purportedly experienced at the onset of the MSA. For many animals, increased mobility “can functionally decrease environmental variation, especially if movement is coupled with choice behavior” [110
] (p. 149). Thus, in humans, increased locomotive efficiency may have partly mitigated the additional energetic expenditures associated with hunting a greater number of smaller prey animals. Better locomotive efficiency also leads to an improved endurance running capability when hunting smaller, fleeing animals. However, it is possible that despite the more efficient locomotion, H. sapiens
still had to adapt to higher metabolic expenses when prey size declined. The substantially greater basal metabolic rate and total energetic expenditures of H. sapiens
] may be, in part, an adaptation to the additional energetic expenses that were imposed on H. sapiens
by the need to obtain energy and fat from smaller prey.
Some of the face gracilization features in H. sapiens
] may have also been enabled by the decline in prey size. Neandertals’ robust frame has been attributed to the need to hunt large animals in close encounters [117
], and it can be argued that a robust brow ridge is a part of this robusticity suite in pre-sapiens
. Indeed, the reduced size of brow ridges in the Homo
genus over time [116
] could have been enabled by the decreased need to take down large animals at close encounters [26
The habitual control of fire was discussed in the post-H. erectus
context and applies to H. sapiens
as well. It was also mentioned that the development of projectile technology by H. sapiens
might have been intended for more energy-efficient hunting of smaller animals [26
6.4. The Extinction of the Neandertal
Until recently, many researchers agreed that in Europe, the Neandertal diet had a narrow breadth and focused on larger prey [117
]. A higher dietary plant content was postulated in more southern regions of the Neandertal’s presence, such as the Levant [127
]. Further, MIS 3 (~59–24 Kya) was a cold period leading to the Glacial Maximum, and cold regions such as tundra and taiga experience long periods of minimal vegetation, so it is reasonable to assume that Neandertals were also exposed to long periods of minimal vegetation in MIS 3 Europe.
Several researchers published a reconstruction of the Neandertal diet [121
]. Large and medium-sized herbivores dominate the Neandertal archaeological faunal record in Europe, including proboscideans and rhinoceroses [51
Stable isotope research (e.g., [118
] unilaterally supports a carnivorous profile for the Neandertal diet in western Europe (but see discussion and some reservations in [129
]). However, small animals and birds were also consumed by Neandertals (e.g., [136
In recent years, evidence for consumption of plants and cooking has emerged, based on plant residues in Neandertal dental plaque taken from fossils in Europe and Asia [124
]. A single study of five sediment samples of Neandertal coprolites from El Salt (Spain), around 50 Kya, found that Neandertals predominantly consumed meat but also had a significant plant intake [140
The Neandertal became extinct during Marine Isotope Stage 3 [141
] in parallel with Europe’s LQE [12
]. Because of the Neandertal’s heavier bodyweight and the cold weather, Neandertal total energetic expenditure (TEE) was estimated to be significantly higher than that of H. sapiens
]). In our model, higher TEE leads to higher obligatory fat consumption, especially in very cold, snow-covered conditions, when the availability of plant food is limited. For this reason, the Neandertal was more dependent on large animals with a high-fat level [143
] that lose less fat compared to smaller animals during periods of low primary production [144
]. Thus, in agreement with Geist [145
] and Stewart [146
], we hypothesize that the decline in prey size in Europe during the LQE was a significant driver of Neandertal extinction. It should be noted that there are many other hypotheses that attempt to explain the Neandertals’ extinction. They cover cultural and other aspects of their complex way of living and some of them remain plausible and can co-exist side by side with ours.