Dean Falk

Since Raymond Dart's first attempt to identify the lunate sulcus (“Affenspalte,” simian sulcus) in a fossil hominin endocast—that of the Taung child ( Australopithecus africanus )—paleoneurologists have debated this structure, which in the brains of monkeys and apes roughly coincides with the rostral boundary of the primary visual cortex. The classic view has been that the evolutionary expansion of the parietooccipital cortex “pushed” the lunate sulcus toward the back of the brain. However, there has been little consensus about how and when this might have happened during hominin evolution, as it has proven difficult to establish phylogenetic homology of potential lunate sulci in living humans with the lunate sulcus of great apes. Here we review the comparative neuroanatomical evidence and propose the hypothesis that the lunate sulcus underwent de‐opercularization, that is, the structures buried within the sulcus expanded and became part of the external cortical surface. During this process, the lunate sulcus became shallow, fragmented, and eventually obliterated. Specifically, rather than migrating toward the occipital pole during brain evolution, the lunate sulcus was a hotspot for the evolutionary expansion of annectant gyri and their eventual emergence on the parietooccipital cortical surface. We test the de‐opercularization hypothesis with an analysis of the parietooccipital endocranial region of early Homo from Dmanisi, Georgia, and conclude that in these fossils the lunate sulcus may have been in the evolutionary process of fragmentation as their brains became larger and their occipital lobes more caudally projected compared to earlier hominins.

The target article rightly questions whether the archaeological record is useful for identifying sea changes in hominin cognitive abilities. This commentary suggests an alternative approach of synthesizing findings from primatology, evolutionary developmental biology, and paleoanthropology to formulate hypotheses about cognitive evolution in hominins that lived during the three million years that preceded the record of material culture (the Botanic Age).

Human brains are three times larger, are organized differently, and mature for a longer period of time than those of our closest living relatives, the chimpanzees. Together, these characteristics are important for human cognition and social behavior, but their evolutionary origins remain unclear. To study brain growth and organization in the hominin species Australopithecus afarensis more than 3 million years ago, we scanned eight fossil crania using conventional and synchrotron computed tomography. We inferred key features of brain organization from endocranial imprints and explored the pattern of brain growth by combining new endocranial volume estimates with narrow age at death estimates for two infants. Contrary to previous claims, sulcal imprints reveal an ape-like brain organization and no features derived toward humans. A comparison of infant to adult endocranial volumes indicates protracted brain growth in A. afarensis, likely critical for the evolution of a long period of childhood learning in hominins.

Czech’s claims that my paper abounds with mistranslations, misrepresentations, and factual errors are refuted point-by-point, as is his declaration that the paper contains no relevant or new evidence. Asperger’s statements that Franz Hamburger saved him from the Gestapo are reaffirmed and supported with a personal communication from Asperger’s daughter, Dr. Maria Asperger Felder. Czech’s criticism of anonymous peer reviewers and his call for retraction of my paper are, at best, unconstructive. In light of the current resurgence of authoritarian governments that promote xenophobic and racist ideology in the United States, Europe, and elsewhere, it is essential that details about the Nazi euthanasia program continue to be recalled and deliberated, as they are in this exchange. I stand by my paper.

Recent allegations that pediatrician Hans Asperger legitimized Nazi policies, including forced sterilization and child euthanasia, are refuted with newly translated and chronologically-ordered information that takes into account Hitler’s deceptive ‘halt’ to the T4 euthanasia program in 1941. It is highly unlikely that Asperger was aware of the T4 program when he referred Herta Schreiber to Am Spiegelgrund or when he mentioned that institution 4 months later on the medical chart of another (unrelated) girl, Elisabeth Schreiber. Asperger campaigned vigorously from 1938 to 1943 to have his specialization, Curative Education, take priority in the diagnosis and treatment of disabled children over other fields that promoted Nazi racial hygiene policies. He neither disparaged his patients nor was he sexist. By 1938, he had identified the essentials of Asperger syndrome and described an unnamed boy whom he later profiled (as Ernst K.) in 1944. Rather than doing ‘thin’ research, Asperger made discoveries that were prescient, and some of his activities conformed to definitions of “individual resistance.”

The only direct source of information about hominin brain evolution comes from the fossil record of endocranial casts (endocasts) that reproduce details of the external morphology of the brain imprinted on the walls of the braincase during life. Surface traces of sulci that separate the brain’s convolutions (gyri) are reproduced sporadically on early hominin endocasts. Paleoneurologists rely heavily on published descriptions of sulci on brains of great apes, especially chimpanzees (humans’ phylogenetically closest living relatives), to guide their identifications of sulci on ape-sized hominin endocasts. However, the few comprehensive descriptions of cortical sulci published for chimpanzees usually relied on post mortem brains, (now) antiquated terminology for some sulci, and photographs or line drawings from limited perspectives (typically right or left lateral views). The shortage of adequate descriptions of chimpanzee sulcal patterns partly explains why the identities of certain sulci on australopithecine endocasts (e.g., the inferior frontal and middle frontal sulci) have been controversial. Here, we provide images of lateral and dorsal surfaces of 16 hemispheres from 4 male and 4 female adult chimpanzee brains that were obtained using in vivo magnetic resonance imaging. Sulci on the exposed surfaces of the frontal, parietal, temporal, and occipital lobes are identified on the images based on their locations, positions relative to each other, and homologies known from comparative studies of cytoarchitecture in primates. These images and sulcal identifications exceed the quantity and quality of previously published illustrations of chimpanzee brains with comprehensively labeled sulci and, thus, provide a larger number of examples for identifying sulci on hominin endocasts than hitherto available. Our findings, even in a small sample like the present one, overturn published claims that australopithecine endocasts reproduce derived configurations of certain sulci in their frontal lobes that never appear on chimpanzee brains. The sulcal patterns in these new images also suggest that changes in two gyri that bridge between the parietal and occipital lobes may have contributed to cortical reorganization in early hominins. It is our hope that these labeled in vivo chimpanzee brains will assist future researchers in identifying sulci on hominin endocasts, which is a necessary first step in the quest to learn how and when the external morphology of the human cerebral cortex evolved from apelike precursors.

In The Better Angels of Our Nature: Why Violence Has Declined, psychologist Steven Pinker cites mean ratios of war (battle) deaths suffered annually per 100,000 individuals as evidence for concluding that people who live in states are less violent than those who live or lived in “hunting, gathering, and horticultural societies in which our species spent most of its evolutionary history.” Because such ratios are blind to actual population sizes, it remains to be seen whether the apparent decrease in contemporary violence is an artifact of scaling factors. Here scaling of war deaths is quantified relative to actual population sizes for 11 chimpanzee communities, 24 human nonstates, and 19 and 22 countries that fought in World War I and World War II, respectively. Mean annual battle deaths expressed as percentages of population sizes scale inversely with population sizes in chimpanzees and humans, indicating increased vulnerability rather than increased violence in smaller populations. However, the absolute number of mean annual war deaths increases exponentially (superlinearly) and nearly identically with population sizes across human groups but not chimpanzees. These findings suggest that people evolved to be more violent than chimpanzees and that humans from nonstates are neither more nor less violent than those from states.