Adam Preston wrote:Of course species are organisms which cannot interbreed successfully, yet didn't humans and Neanderthals do that?
Speciation is not defined by an instantaneous transition away from reproductive viability. There are actually stable "hybrid zones" between different species that allow advantageous mutations to spread further than the immediate clade of the original carrier. Humans and neanderthals were distinct enough to have very, very different brain organization:Evolution of the base of the brain in highly encephalized human species
(Dec 13, 2011)
• Two genetically different evolutionary lineages, Neanderthals and modern humans, have produced similarly large-brained human species. […] Three-dimensional geometric morphometric analyses of endobasicranial shape reveal previously undocumented details of evolutionary changes in Homo sapiens. Larger olfactory bulbs, relatively wider orbitofrontal cortex, relatively increased and forward projecting temporal lobe poles appear unique to modern humans. Such brain reorganization, beside physical consequences for overall skull shape, might have contributed to the evolution of H. sapiens' learning and social capacities, in which higher olfactory functions and its cognitive, neurological behavioral implications could have been hitherto underestimated factors.
• The significantly different evolutionary patterns in the modern human and Neanderthal lineages are shown in Figure 5. In H. sapiens cribriform expansion has occurred posteriorly. […] Cribriform plate increase is well observed comparing mean shapes of modern humans with its putative ancestors (both early Homo in Figure 4, and Mid-Pleistocene humans in Supplementary Fig. S2) but also with Neanderthals. The size of the cribriform plate is driven by the size of the olfactory bulbs due to coordinated embryological development. Adult morphology of the cribriform plate is achieved early in ontogeny (4 years in humans and probably even earlier in Neanderthals due to faster maturation rates). However, due to this very early maturation ontogenetic changes of adjacent and surrounding facial structures, growing much longer than the cribriform plate, are very unlikely to influence cribriform morphology by craniofacial integration. Moreover, the fact that large-faced Neanderthals showed smaller cribriform plates supports an interpretation in terms of neurological factors rather than by craniofacial integration. Furthermore, its specific increase in H. sapiens implies a unique evolutionary condition of a large cribriform plate atop a nasal cavity within an extremely reduced face. After all, nasal cavity and facial sizes are more related to respiration and mastication than to olfaction.
• The coincident evolutionary changes of structures comprising olfactory neuro-circuitry could be a novel feature in the evolution of H. sapiens, and, if confirmed, may have influenced some features of human behaviour. The olfactory neurological circuitry is highly integrated in cerebral, behavioural and immunological functions. Following the initial sensory process, axons from thousands of cells expressing odour receptors in the mucosa of the nasal cavity converge via the cribriform plate in the olfactory bulb. From there, olfactory signals are transmitted to the olfactory cortex (rhinal, pyriform cortex, medioventral to temporal lobe poles) and become relayed, on the one hand to higher cortical regions, where conscious thought processes are handled, and on the other hand to the limbic system, where emotional context is generated. Olfactory information thus projects to regions critical for mating, emotions, and fear (amygdala) as well as for motivation, high-level cognitive and emotional processes (orbital prefrontal cortex). It, thus, serves a role in central nervous system function above and beyond smell. In that respect, olfaction differs from other sensory modalities. Odour immediately triggers strong emotional evocations and provokes higher memory retention ('Marcel Proust Phenomenon') due to anatomical overlap of structures involved in memory process and olfaction pathways. Such associations with cognitive processes have been termed by Savic 'higher olfactory functions'. Smell, and linked higher olfactory functions, can thus be involved in modulating many different aspects of human behavior. It has been reported that people who are congenitally deaf or blind have intact reproductive–social capacities, whereas individuals with congenital anosmia usually do not.
• Moreover, olfaction has been linked to the immunological system. It is speculated that odour might be an important factor of attractiveness, that is, a mate selection criterion in human females, possibly selected for improving immunological fitness of the offspring, for example, in the case of the major histocompatibility complex [MHC]. Other recent research suggests that humans are also able to detect the 'scent of fear', potentially important in human social interaction.
• Our findings support previous hypotheses that modern humans show a different evolutionary trajectory because of the remaining significant shape differences between Neanderthals and H. sapiens after allometric size adjustment.
• Different evolutionary patterns likewise emerge from comparative genetic analyses, which—among other aspects—have shown evidence for positive selection of genes related to cognitive development, that occurred after the split of H. sapiens and Neanderthals. The same applies to roughly 4% of the 78 amino-acid configurations, which—ancestral in Neanderthals—are directly related to the olfactory system. Differences in the configuration of the olfactory sensory apparatus, and its previously discussed involvement into higher olfactory functions in social, and cognitive (memory) aspects could be part of this evolutionary process.
• Although different regions of the prefrontal cortex (frontal lobes) have been associated with higher integrative and social functions, (for example, decision making), regions of the temporal lobes are traditionally related to visual memory, language and to theory of mind. All of them are compatible with higher olfactory functions.
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Also relevant:Orbital prefrontal cortex volume predicts social network size: an imaging study of individual differences in humans
(Feb 1, 2012)
Path analysis in a sample of 40 healthy adult humans revealed a significant linear relationship between orbital (but not dorsal) [prefrontal cortex] volume and the size of subjects' social networks that was mediated by individual intentionality (mentalizing) competences. The results support the social brain hypothesis by indicating a relationship between PFC volume and social network size that applies within species, and, more importantly, indicates that the relationship is mediated by social cognitive skills.
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