The prehistory of compassion

Beyond the biological traits that differentiate ancient species of hominins from extant humans, differences in social organization remain an important yet difficult issue to assess. Psychological features and interindividual relations, in particular, are among the least accessible aspects of past behaviors. Although the fossil and archaeological records do not easily allow us to tackle these kinds of issues, contrasting views on the cognitive capabilities and behavioral sophistication of fossil human species have often been expressed. In this issue of PNAS, Gracia et al. (1) provide new evidence on the survival of an abnormal individual with possible cognitive deficits from a group of pre-Neandertal Pleistocene hunter-gatherers, currently assigned to a geological age of >500 ka. The cranium SH14 from the Sima de los Huesos (Sierra de Atapuerca, Spain) is the earliest documented case of human neurocranial and brain deformity in the fossil record to date. Despite her/his pathological condition, this individual was not rejected at birth and survived until at least 5 years of age, apparently receiving the same attention as other children from the group.

Particularly extreme pathologies, allegedly necessitating some support by conspecifics to allow the survival of their bearer, have provided the ground for debates on the level of altruism and compassion reached by ancient hominins. Often underlying these debates is the notion that, in this respect, their behavior was similar to our own and different from that of apes. Among the Pleistocene hominins, attention has been focused on the Neandertals in particular. This group has provided an abundance of paleontological material mostly dating between ca. 200 ka and 30 ka, including nearly complete skeletons. Each of these rather complete skeletons displays one or more detectable traumatisms on the bones, which, in a few cases, resulted in significant impairments. One of the best known examples is that of a male individual from the site of Shanidar (Iraq) who survived an unrepaired fracture of the right arm above the elbow (2). Subsequently, his upper arm became atrophied and nonfunctional, and he may have lost his right hand and forearm entirely. In addition, this individual was likely partially blind and deaf, and had difficulties with locomotion. As the Shanidar 1 man apparently survived until an advanced age for a Neandertal (ca. 40 years), it has been argued that his survival was possible only because he received support from other adults in the group.

Similar claims for the social support of impaired individuals have been made, in particular, about edentulous adults who might have needed help with feeding from their companions. Significant antemortem tooth loss and alveolar bone loss are well-documented in Neandertals but less common and more limited in older periods of the Middle (0.78–0.13 my ago) and Lower (1.7–0.78 my ago) Pleistocene. In the Late Middle Pleistocene, all of the teeth from a portion of human mandible from Bau de l'Aubesier (France) were lost ante mortem or mechanically unstable at the time of death, with extensive bone loss and the development of abscesses along the dental arcade (3). Although this individual was still able to chew, his ability to masticate tough or hard food items would have been difficult and painful. A much older and even more complete edentulous specimen was yielded by the site of Dmanisi (Georgia), where the skull (D3444) and associated mandible (D3900) of an early form of Homo erectus, dated ca. 1.77 my ago, represent the earliest and best-preserved case of severe masticatory impairment in the hominin fossil record (4). Over its lifetime, this individual had lost all of its teeth except one, and thus, must have survived for a significant period by consuming only soft plant and animal foods that were easy to chew. The question is whether some external help was absolutely required for such an individual to have access to this peculiar diet (5).

As underlined by DeGusta (5), this kind of assessment can only be conducted in a comparative way. In fact, tooth loss surpassing that of Bau de l'Aubesier 11 and even approaching that of D3444/D3900 has been documented in apes (5, 6). Primatologists studying chimpanzees in the wild have also reported several cases of serious impairments. Individuals with snare injuries resulting in severe upper limb wounds or even complete amputations of one hand have been observed (7), demonstrating that these disabilities can be overcome by young or adult apes without much conspecific care. However, these severe pathologies reported in non-human primates result primarily from wounds inflicted over a lifespan or from the senescence of adults already well-integrated into their groups. What seems to be lacking in the ape repertoire is the survival of individuals with serious congenital abnormalities. In this respect, SH14 is quite interesting, even if she/he did not survive into adulthood. The impact of this individual's cranial malformation on her/his capabilities is difficult to assess, because, today, cognitive impairment occurs relatively infrequently in cases of craniosynostosis with only one affected cranial suture (8, 9). However, she/he certainly displayed abnormal anatomical features already visible in her/his first year of life. A similar case is represented by the Middle Pleistocene hominin from Salé (Morocco) (10). The Salé individual was likely a female who reached adulthood even though she suffered cranial distortion and muscular trauma related to a congenital torticollis. This deformation likely resulted from shortage of amniotic fluid and confinement of the fetus during gestation. Congenital torticollis causes a variable degree of asymmetry of the skull and face with reduced mobility of the neck and is often associated with other deformities (11). Like the craniosynostosis described in SH14, such postural deformities have a rather low incidence rate (2% in extant populations) (11) and one could speculate on the presence of these rare anomalies among the handful of fairly complete skulls known in the African and European Middle Pleistocene fossil record. In any case, the Salé and SH14 specimens demonstrate that individuals with congenital abnormalities or illnesses could survive for many years.

From an evolutionary perspective, the forms of altruism observed in animals in general and in non-human primates, in particular, have been primarily interpreted as either support to kin (helping those who carry the same genes) or support to those able to reciprocate the favor (helping oneself indirectly). This is in contrast to the trivial observation of humans helping others, even when the helper receives no immediate benefit and the person being helped is a stranger. However, claims have been made that the level of altruism displayed by chimpanzees could be much higher than what was once thought (12). For example, there have been reported cases of captive chimpanzees rescuing companions from drowning (13). Boesch and Boesch-Achermann (14) have also described a case of a wild adult male chimpanzee adopting an unrelated orphan. Recent experimental data confirm that, in some settings, young chimpanzees demonstrate an understanding of others' goals and an altruistic motivation help, regardless of whether this yields a reward or not (15). However, this incipient altruism seen in chimpanzees seems to disintegrate in competitive situations or when food sharing is involved. Interestingly, it has been observed that the food most often shared by wild chimpanzees is meat. Cooperative hunting (Fig. 1), as described in the Taï forest, for example, can result in meat sharing between hunt participants and nonparticipants (14). Because the increase in meat consumption is considered to be a major evolutionary change in early Homo, these hominins had to strengthen a behavior likely preexisting. Another adaptive reason for why humans had to amplify the incipient forms of altruism observed in apes—in particular, regarding immature individuals—is related to our peculiar life history. Among primates, humans are characterized by an extended growth period that allows us to build and maturate a big and complex brain. Although we start to reproduce much later than apes, we wean our children earlier and have shorter birth intervals. The result of this pattern is that after being weaned, children remain dependent for a longer time on mothers who can have other offspring. In the course of our evolution, this was made possible only by having the support of group members other than the mother (16). From this point of view, humans can be defined as “cooperative breeders.” When did the modern human life history pattern appear in the course of evolution? Although life history models have suggested that this could be as old as the rise in meat consumption, the paleontological evidence tells a different story. Early Homo and Lower Pleistocene H. erectus display a more rapid development than extant humans (17–19). Perhaps the large-brained species of Middle Pleistocene hominins, which the Salé and SH14 specimens belong to, had already acquired most of the human pattern. However, evidence has shown that later Neandertals still had more rapid dental development than extant Homo sapiens (20).

Fig. 1.

Chimpanzees sharing meat after a cooperative hunt in the Taï forest (courtesy of Christophe Boesch).

The level of altruism displayed by chimpanzees could be much higher than what was once thought.

Finally, the divide between apes and early humans might not be as large as one tends to think. Rather than considering ancient human altruism as proof of the moral values of our predecessors, one should instead see it as merely part of the spectrum of adaptations that have made humans such a prolific and successful species.