fig1

Figure 1. Evolution and development of humans. (A) Evolutional changes in posture and brain in mammals, such as cats, monkeys, and humans. Through vertebrate evolution, the body axis of animals such as cats (red), monkeys (green), and humans (blue) changed from horizontal to vertical, and the frontal lobe area in each animal's cerebral cortex expanded considerably; (B) Changes in posture and gait with postnatal development and aging of humans. The antigravity function, indicated by the orange color, is facilitated by postnatal development, but declines due to aging. While humans exhibit a flexion posture immediately after birth, their antigravity ability develops over time. Specifically, at 1-2 months, the baby extends its neck and chest, and at around ten months, it begins to crawl. Then, at approximately 15 months, the baby starts to walk upright on two legs. Human antigravity capacity reaches its peak during adulthood. However, antigravity capacity declines with age, manifesting as a decreased walking speed and a flexed posture in old adults; (C) An open arrow indicates that postnatal brain maturation progresses from the lower to the upper brain areas. Specifically, spinal reflex neural circuits, including the CPGs, can be functional at birth. Then, the neural networks between the brainstem and spinal cord, including the PMRF and the RST, are maturated. The upper brainstem includes neural structures involved in innate motor functions such as swallowing, vocalization, and excretion, in addition to those contributing to the automatic posture-gait control, including the MLR and SLR. Maturation further progresses to involve the subcortical structures (the basal ganglia, limbic system, and thalamus), cerebellum, and cerebral cortical structures, operating higher brain functions. CPGs: Central pattern generators; PMRF: pontomedullary reticular formation; RST: reticulospinal tract; MLR: midbrain (mesencephalic) locomotor region; SLR: subthalamic (diencephalic) locomotor region; MNs: motoneurons.