The brain-to-body mass ratio of early mammals was about the same as that of today's opossum or hedgehog, about 3 times that of the average brain-to-body mass ratio of reptiles. After the extinction of the dinosaurs (65 million years ago), the brain volume of most mammals was maintained at this level. About 50 million years ago, the ancestors of horses, Archaeopteryx, which were about the same size as modern dogs, evolved a brain size comparable to that of modern possums. Subsequently, the brain of Archaeopteryx continued to grow until 20 million years ago, finally reaching the average level of modern mammals. Fortunately, though, there is one class of mammals that has not developed according to this law: we.
Primates seem to have larger brains than other animals as soon as they appear. Most mammals typically spend 5% of their metabolism on maintaining brain function, up from at least 9% in primates. Why can monkeys be so clever? The most primitive reason may have something to do with the fact that they have changed their nocturnal habits. They used to be active on the ground at night, and later learned to consume nutrient-rich foods in the canopy during the day. With the expansion and development of flowering plants, trees rich in flowers and fruits have become an important living resource for a variety of animals. Early primates needed clearer three-dimensional vision and greater physical flexibility in order to better feed on fruits and insects in the canopy. After millions of years of evolution, primates shortened their snouts and shortened the distance between their eyes, which allowed their binocular vision to overlap. As the brain enlarges, primates are better able to process the different images collected by their eyes to more accurately determine the depth of the environment. This three-dimensional visual perception is crucial for them to jump between branches. In addition, the stereoscopic vision produced by the eyes has many benefits, such as easier to spot hidden delicious insects, or to avoid predators in the dangerous dark night.
Primates' primary habitat is the top of the tree, where flowers and fruits grow, so they have evolved flexible arms and fingers that are easier to prey on insects and pick fruits. Slender hind limbs and grippable toes also help them grip the branches. Separate fingers, powerful thumbs and big toes are also important for gripping. Reptiles' claws evolved into fingernails, rough skin appeared on the surfaces of the hands and feet used for gripping to help them grip smooth objects, and we had fingerprints. At the same time, primates have far more flexibility than other animals in terms of forelimbs and shoulders. Upon closer inspection, scientists have found that the way food is fed in the mouth with its forelimbs is characteristic of all primates. However, without the development of flowering plants, primates could not have completed the evolution of this series of characteristics. Ferns, cycads and conifers were unable to provide so many honey-rich flowers, colorful fruits and nutrient-rich seeds that attracted insects and gave birth to the earliest insectivores.
After visual evolution, primates lost most of their backward vision, and their field of vision was confined to the front. So they came up with a key response – group travel. By grouping, individuals can take care of each other, observe in all directions, and beware of predators such as eagles, snakes, and arboreal cats. This social interaction, in turn, increases brain capacity. The more you understand what your peers mean, the more you can react appropriately and make your group work together more effectively. Overall, the more females in a group can interact effectively, the more advanced the neocortex development of the brain in individuals of this species.
At all stages of pregnancy, primate fetuses are almost twice the brain capacity of similar sizes in other mammals. In other words, brain development is a privilege held by primates.
—Robert Martin
As primates have evolved, there have been two more events in history that have proven that their brains have always grown. The first event was the discovery of fossils of the original Consur ancient ape about 20 million years ago. As one of the earliest apes, this forest ape not only had a larger brain than other apes of the same period, but also had no tail like other living apes (including gibbons, orangutans, gorillas, chimpanzees). The proto-Consur ancient ape walked upright for longer, so the lower limbs and back were also stronger. Another event was even more striking — apes gradually became the only mammals that could spin their forelimbs like a windmill, allowing them to hover and wander in the canopy. Of course, this is also due to the flourishing of flowering plants. Few other plants can flourish so much in the crown at the top. The flowering plants not only form a tropical forest with a dense canopy. In these forests, higher animals climb branches and hang back and forth between trees. In the process, our ancestors grew broader shoulders, slender, large-range arms, flexible elbow joints, and strong hands. Flowering plants are not only the cradles of primate birth, but have also been silently accompanying us to evolve highly flexible limbs.
Source: Environmental Science 365
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