The Handy Psychology Answer Book. Lisa J. Cohen

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      Comparisons of human, elephant, gorilla, macaque (a kind of monkey), dolphin, dog, cat, and mouse. Frontal lobes are proportionately larger and more convoluted (wrinkled) in species with greater intelligence.

      How has the olfactory bulb changed across human evolution?

      One of the most striking differences between the brains of humans and other mammals involves the size of the olfactory bulb, which is the part of the brain involved with smell. In many mammals, the olfactory bulb is a major portion of the entire brain. In fact it is present in even the most primitive vertebrates, such as fish. In humans it is a tiny little orb sandwiched between our limbic system and the bottom of our frontal lobe. This contrast illustrates our reduced reliance on the sense of smell in favor of vision, hearing, and analytic thinking, all functions supported by the cortex.

      THE BRAIN SINCE EARLY HOMINIDS

      When did human beings evolve from early hominids?

      About six million years ago, our ancestors and chimpanzees diverged from a common ancestor. The Australopithecus genus was one of the earliest forms of hominids. The Homo genus followed, with several species, such as Homo habilis, Homo erectus, and Homo neanderthalensis (Neanderthals) preceding or even overlapping with modern humans. Modern humans (Homo sapiens) evolved about 200,000 years ago.

      How do we compare our brains with those of extinct species?

      Because soft tissue decomposes quickly we cannot expect hominid brains to survive over the hundreds of thousands and even millions of years of evolution. Therefore, paleontologists must work with skeletal remains, using skulls and other bone fragments to draw inferences about the biology and behavior of early hominids. However, tools, animal bones, fossilized seeds, and even cave paintings (in the case of early modern humans) have been found alongside hominid skeletal remains, providing intriguing clues about the mental capacity of our predecessors.

      How has brain size changed from early hominids?

      Hominid skulls show a steady increase in cubic centimeters across evolution. Estimating from skull size and shape, Homo habilis had a brain size of 600 to 700 cubic centimeters and Homo erectus about 900 to 1,000 cubic centimeters. Homo sapiens (modern humans) have a brain size of about 1,400 cubic centimeters. Concurrent with the increase in brain size, paleontologists find an increase in the complexity of tools found with hominid remains. Larger brains apparently translated into more sophisticated tool use. Additionally, larger-brained hominids were adapted to more varied and/or harsher climates.

      Is there evidence of language in hominid skulls?

      Indentations on the inside of Homo habilis skulls (Homo habilis lived about two million years ago) suggest an enlarged area around the location of Broca’s area, a central region for speech production in modern humans. While we cannot know if this area was connected to speech in Homo habilis brains, we can suggest that at least a precursor to modern language regions of the brain was present at a very early point in hominid evolution.

      Has the frontal lobe increased?

      Along with the increase in skull size, the shape of the skull also suggests enlargement of the brain and of the frontal lobe in particular. Australopithecine skulls do not look much different from ape skulls. There is a prominent jaw, a small sloping forehead and a relatively small brain casing. Modern humans, in contrast, have flatter faces, very steep foreheads, and small jaws. Our foreheads, which lie just in front of the frontal lobe, cover about 50 percent of our faces. Likewise, our brain casing is greatly enlarged relative to the rest of our skull.

      What do we know about Neanderthals?

      The Neanderthals (Homo neanderthalensis) were a species of the Homo genus that lived from about 200,000-300,000 years ago until they became extinct around 30,000 years ago. Neanderthals were not the ancestors of modern humans but rather shared a common ancestor with our species. In that way they were more like our cousins. The ancestors of Neanderthals and modern humans broke off from a common ancestor about 400,000 years ago. As modern humans (Homo sapiens) date back to about 200,000 years ago, Neanderthals overlapped with modern humans for about 170,000 years, although for most of that time (about 140,000 years), the two species lived on different continents: Neanderthals in Europe and Asia and modern humans in Africa. In fact, modern genetic studies tell us that our two species interbred to some extent and that many modern humans carry a small amount of Neanderthal genes. Evidence suggests the interbreeding took place from 60,000 to 30,000 years ago across Europe and Asia.

      Neanderthal skeletons were first recognized as belonging to an earlier species in the nineteenth century. These skeletons had shorter, stockier bodies than those of modern humans, with heavier brows and larger jaws. Their brain casings were actually larger than ours. Originally it was thought that Neanderthals were much more primitive than modern humans—not much different from apes. However, advances in the science of paleontology as well as the new ability to decode the Neanderthal genome tells us much more about these fascinating relatives of ours. They ranged across Europe and Asia and ate a wide variety of foods, suggesting they could flexibly adapt to very different environments. They used sophisticated stone tools, hunted with wooden spears, and even buried their dead. While we have not yet established whether they used language or were capable of symbolic thought, the complexity of their tools suggests they had some form of complex communication. Scientists are not clear why Neanderthals became extinct or to what extent modern humans were a contributing factor to their disappearance.

      What does pedomorphy mean?

      Pedomorphy refers to an evolutionary process in which adult animals maintain the traits of juveniles. One fairly easy way for genetic mutations to produce physical changes in the animal is to adjust the timing of maturation. No new physical structures or behaviors need to be introduced; the animal simply maintains its youthful traits instead of shedding them when it reaches maturity. There is evidence that many advances in human evolution involve pedomorphy. For example, we are one of the few mammals that retain a high level of playfulness throughout adulthood. Secondly, the shape of our skull mirrors that of juvenile apes. The skulls of juvenile chimpanzees look more like the skulls of adult humans than the skulls of adult chimpanzees. Adult chimpanzees have small sloping foreheads, prominent jaws, and more horizontally aligned faces. Adult humans, on the other hand, have high foreheads, small jaws, and flat, vertically aligned faces, similar to juveniles of both species. Of note, juvenile chimpanzees have a larger brain to skull ratio than do adult chimpanzees. In humans, this favorable ratio is retained into adulthood.

      What does “phylogeny recapitulates ontogeny” mean?

      This is a nineteenth century idea put forward by a German zoologist named Ernst Haeckel (1834–1919) that development in childhood exactly parallels evolution. Phylogeny refers to evolution and ontogeny to development across the life span. Haeckel believed that every stage in the development of a human embryo exactly parallels the stages of human evolution. Besides having a faulty view of human evolution (e.g., there was no cow stage in human evolution), this theory oversimplified the processes of embryonic development. While Haeckel’s specific theory has been discredited, he was correct in pointing out important parallels between evolution and maturation. Careful understanding of our development across the life span does offer some clues as to our evolutionary history.

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