Autism and Human Atavism

One example lies with the partial transformation of the second pharyngeal arch into a copy of the first in mouse embryos (see Chapter 16), and another is seen in the partially atavistic recovery of abdominal limbs in the fleshy “prolegs” of lepidopterous larvae (see Chapter 21). It has also been shown that the larval stage of an interspecific hybrid between two sea urchin species with widely different metamorphosis patterns resembles a starfish juvenile more than it does either parent species (Raff et at., 1999).

A classifier was recently reported to predict with 70% accuracy if an individual has an autism spectrum disorder using 237 single-nucleotide polymorphisms (SNPs). Biomarkers, genetic or otherwise, that would facilitate earlier autism spectrum disorder diagnosis are crucial; therefore, these results warrant careful scrutiny. One potential confounder of such genetic studies is bias when cases and controls have different ancestral origins. Here, we show that the largest components of this classifier’s autism risk score distinguish populations, but do not separate cases from controls. In short, the frequencies of reported risk and protective alleles do not differ between related individuals with or without autism in independent data sets; instead, they reflect ancestral origin. Specifically, cases have more diverse ancestral origins within Europe than controls. The putative risk alleles are more common in the Northeastern than in the Northwestern Europe, whereas the putative protective alleles reflect the opposite trend. Likewise, we find that the autism risk scores based on the strongest SNPs do not differ between people with and without autism in an independent data set, but that they do differ between European populations.

We show that Neanderthals had significantly larger visual systems than contemporary AMHs (indexed by orbital volume) and that when this, along with their greater body mass, is taken into account, Neanderthals have significantly smaller adjusted endocranial capacities than contemporary AMHs. We discuss possible implications of differing brain organization in terms of social cognition, and consider these in the context of differing abilities to cope with fluctuating resources and cultural maintenance.

What evolutionary events led to the emergence of human cognition? Although the genetic differences separating modern humans from both non-human primates (for example, chimpanzees) and archaic hominins (Neanderthals and Denisovans) are known, linking human-specific mutations to the cognitive phenotype remains a challenge. One strategy is to focus on human-specific changes at the level of intermediate phenotypes, such as gene expression and metabolism, in conjunction with evolutionary changes in gene regulation involving transcription factors, microRNA and proximal regulatory elements. In this Review we show how this strategy has yielded some of the first hints about the mechanisms of human cognition.

Transitions from solitary to facultative, facultative to primitive, and primitive to advanced eusociality occur via exaptation, phenotypic accommodation and genetic assimilation. Multilevel selection characterizes the solitary to highly eusocial transition, but components of multilevel selection vary across levels of eusociality. Roles of behavioural flexibility and developmental plasticity in the evolutionary process equal or exceed those of genotype.

The eusocial naked mole-rat, whose face is dominated by prominent incisors, uses facial aggression to enforce reproductive suppression. In burrow-living mammals like the naked mole-rat in particular, and in rodents in general, somatosensory face representations in cortex are enlarged. Diversity of sensory domains mediating facial communication might belie underlying common mechanisms. As a case in point, neurogenetics has revealed strongly heritable traits in face processing and identified gene defects that disrupt facial interactions both in humans and rodents.

It has been proposed that worker behavior evolved from maternal care behavior. We explored this idea by studying gene expression in the primitively eusocial wasp Polistes metricus. Because little genomic information existed for this species, we used 454 sequencing to generate 391,157 brain complementary DNA reads, resulting in robust hits to 3017 genes from the honey bee genome, from which we identified and assayed orthologs of 32 honey bee behaviorally related genes. Wasp brain gene expression in workers was more similar to that in foundresses, which show maternal care, than to that in queens and gynes, which do not. Insulin-related genes were among the differentially regulated genes, suggesting that the evolution of eusociality involved major nutritional and reproductive pathways.

If the advantage to cooperate is large, such as in species where group selection may operate, eusocial behavior can emerge: the highly organized division of labor and reproduction seen in species such as ants, termites, and bees (Figure 1). Only two species of mole rats, a fossorial rodent, have evolved eusocial behavior in mammals, possibly due to the very specialized adaptive challenges in securing a stable food supply in those species. Other mammalian species have instead evolved more flexible and less stable forms of collective behavior–in primates, ones that involve deception and strategic behavior. It is an interesting question whether human evolution would eventually result either in extinction of our species or in a species with stably cooperative social behaviors reminiscent of eusociality.