A review of the current literature gives the distinct impression of a chimpanzee- centric view of evolution, with the last common ancestor similar to an extant chimpanzee. However, a significant point of departure is the perhaps more enduring aquatic phase of our evolution.
Current- day bonobo chimpanzees still practice some degree of wading and collection of aquatic foods. Pinnipeds are semi- aquatic animals, numbering about 30 species including gray and harbor seals (Phocidae), walruses (Odobenidae), and Otariidae (Cape fur seals, California sea lions) that have several brain capabilities that far exceed those of chimpan- zees. These include much more vocal flexibility, rhythmic ability, and vocal production learning. Harbor seals have been recorded imitating both human words as well as phrases [83]. Their vocal production learning capabilities stem from their relatively unique habitat and social organization, rendering them candidates for musicality and elementary speech production. The core brain processes that allow such abilities include a number of anatomical and brain circuitry features. These include, for example, imitation behav- ior, the pinniped vocal anatomy in terms of larynx and upper vocal tract air flow being similar to that in humans, and their ability to process both speech and music, the latter entrained in subcortical structures that are involved in rhythm perception and production.
Perhaps most importantly, pinniped working memory, covering both visual and auditory working memory, is particularly advanced. Auditory working memory is critical for speech and language, and pinniped auditory working memory exceeds that of non- human primates [84]. These findings imply that the pinniped brain would be a more appropriate comparative, extant model to study some of the unique human features such as speech, vocal learning, and rhythm production (allowing flexibility in vocalization), than primates [85].
The fundamentals of a brain and body bauplan that our primate lineage endowed us with include the following:
Brain anatomy
• multicomponent granular prefrontal cortex (Figure 2.7);
• expanded visual cortex and trichromatic vision.
Brain circuitry
• frontoparietal, further integration of sensorimotor systems;
• mirror neuron system.
Skeletal
• cranial size increase to ~450 cc;
• bipedalism.
Diet and metabolism
• frugivorous diet;
• fall-back foods – tubers;
• marine, riparian, lacustrine, riverine aquatic diet.
References 47
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51 Chapter
Even more profound than the brain size and gray matter increase is the remarkable surge in white matter fiber tract proliferation in the human lineage. During this phase, the major fiber tracts we measure and monitor today were formed. Specific examples include the extensive frontopontocerebellar (bipedality), frontoparietal (working memory) circuitry among mammals and Miocene apes, the mirror neuron tracts (copying actions, imitations, praxis), and the uncinate fasciculus (sociality, empathy). In addition, circuit modulators, the ascending neurotransmitter circuits, became repurposed. Eventually some of these circuits became exapted further into the so- called acquired cultural circuits.
The burgeoning brain size increases during human evolution included not only regional brain size increases, but also an even more impressive evolution of the cortical networks.
In particular, the prefrontal cortex connectivity with respect to other brain regions may be viewed as a major driver of the human mind and cognition [1]. The hallmark of the human brain and frontal function and dysfunction is the human connectome. The aver- age human brain contains ~2 × 1011 neurons, ~1 × 1012 glial cells, and 400 miles (700 km) of blood vessels. Perhaps the most striking vital statistic is the ~100 000 miles (160 000 km) of axons (nerve fibers) comprising the neuropil. A most remarkable and defining feature of the human brain in comparison to the primate brain is the dramatic escalation in the neuropil (interwoven dendrites, axons, glial cells) and consequently connectivity in and at a microscopic level in association with granular cortical areas [2,3].