Human evolution… Homo floresiensis…Evolution of genus Homo..Anatomical changes….Encephalization

The reason for this encephalization is difficult to discern, as the major changes from Homo erectus to Homo heidelbergensis were not associated with major changes in technology. It has been suggested that the changes have been associated with social changes, increased empathic abilities and increases in size of social groupings.Human evolution is the evolutionary process that led to the emergence of anatomically modern humans, beginning with the evolutionary history of primates.The increase in volume over time has affected areas within the brain unequally—the temporal lobes, which contain centers for language processing, have increased disproportionately, and seems to favor a belief that there was evolution after leaving Africa, as has the prefrontal cortex which has been related to complex decision-making and moderating social behavior.Encephalization has been tied to an increasing emphasis on meat in the diet, or with the development of cooking, and it has been proposed that intelligence increased as a response to an increased necessity for solving social problems as human society became more complex. The human brain was able to expand because of the changes in the morphology of smaller mandibles and mandible muscle attachments to the skull into allowing more room for the brain to grow – in particular genus Homo – and leading to the emergence of Homo sapiens as a distinct species of the hominid family, the great apes.The human species eventually developed a much larger brain than that of other primates—typically 1,330 cm3 in modern humans, nearly three times the size of that of a chimpanzee or gorilla. The pattern of encephalization started with Homo habilis, after a hiatus with Anamensis and Ardipithecus species which had smaller brains as a result of their bipedal locomotion which at approximately 600 cm3 Homo habilus had a brain slightly larger than that of chimpanzees, and this evolution continued with Homo erectus (800–1,100 cm3), reaching a maximum in Neanderthals with an average size of (1,200–1,900 cm3), larger even than modern Homo sapiens. This pattern of brain increase happened through the pattern of human postnatal brain growth which differs from that of other apes (heterochrony). It also allows for extended periods of social learning and language acquisition in juvenile humans which may have begun 2 million years ago. However, the differences between the structure of human brains and those of other apes may be even more significant than differences in size. The study of human evolution involves many scientific disciplines, including physical anthropology, primatology, archaeology, paleontology, neurobiology, ethology, linguistics, evolutionary psychology, embryology and genetics.[1] Genetic studies show that primates diverged from other mammals about 85 million years ago, in the Late Cretaceous period, and the earliest fossils appear in the Paleocene, around 55 million years ago.Within the Hominoidea (apes) superfamily, the Hominidae family diverged from the Hylobatidae (gibbon) family some 15–20 million years ago; African great apes (subfamily Homininae) diverged from orangutans (Ponginae) about 14 million years ago; the Hominini tribe (humans, Australopithecines and other extinct biped genera, and chimpanzees) parted from the Gorillini tribe (gorillas) between 9 million years ago and 8 million years ago; and, in turn, the subtribes Hominina (humans and biped ancestors) and Panina (chimps) separated about 7.5 million years ago to 5.6 million years ago.Human evolution from its first separation from the last common ancestor of humans and chimpanzees is characterized by a number of morphological, developmental, physiological, and behavioral changes. The most significant of these adaptations are bipedalism, increased brain size, lengthened ontogeny (gestation and infancy), and decreased sexual dimorphism. The relationship between these changes is the subject of ongoing debate. Other significant morphological changes included the evolution of a power and precision grip, a change first occurring in H. erectus.Bipedalism is the basic adaptation of the hominin and is considered the main cause behind a suite of skeletal changes shared by all bipedal hominins. The earliest hominin, of presumably primitive bipedalism, is considered to be either Sahelanthropus[6] or Orrorin, both of which arose some 6 to 7 million years ago. The non-bipedal knuckle-walkers, the gorilla and chimpanzee, diverged from the hominin line over a period covering the same time, so either of Sahelanthropus or Orrorin may be our last shared ancestor. Ardipithecus, a full biped, arose somewhat later.The early bipeds eventually evolved into the australopithecines and still later into the genus Homo. There are several theories of the adaptation value of bipedalism. It is possible that bipedalism was favored because it freed the hands for reaching and carrying food, saved energy during locomotion,enabled long distance running and hunting, provided an enhanced field of vision, and helped avoid hyperthermia by reducing the surface area exposed to direct sun; features all advantageous for thriving in the new savanna and woodland environment created as a result of the East African Rift Valley uplift versus the previous closed forest habitat. A new study provides support for the hypothesis that walking on two legs, or bipedalism, evolved because it used less energy than quadrupedal knuckle-walking.However, recent studies suggest that bipedality without the ability to use fire would not have allowed global dispersal.This change in gait saw a lengthening of the legs proportionately when compared to the length of the arms, which were shortened through the removal of the need for brachiation. Another change is the shape of the big toe. Recent studies suggest that Australopithecines still lived part of the time in trees as a result of maintaining a grasping big toe. This was progressively lost in Habilines.Anatomically, the evolution of bipedalism has been accompanied by a large number of skeletal changes, not just to the legs and pelvis, but also to the vertebral column, feet and ankles, and skull.The femur evolved into a slightly more angular position to move the center of gravity toward the geometric center of the body. The knee and ankle joints became increasingly robust to better support increased weight. To support the increased weight on each vertebra in the upright position, the human vertebral column became S-shaped and the lumbar vertebrae became shorter and wider. In the feet the big toe moved into alignment with the other toes to help in forward locomotion. The arms and forearms shortened relative to the legs making it easier to run. The foramen magnum migrated under the skull and more anterior.The most significant changes occurred in the pelvic region, where the long downward facing iliac blade was shortened and widened as a requirement for keeping the center of gravity stable while walking;[15] bipedal hominids have a shorter but broader, bowl-like pelvis due to this. A drawback is that the birth canal of bipedal apes is smaller than in knuckle-walking apes, though there has been a widening of it in comparison to that of australopithecine and modern humans, permitting the passage of newborns due to the increase in cranial size but this is limited to the upper portion, since further increase can hinder normal bipedal movement.The shortening of the pelvis and smaller birth canal evolved as a requirement for bipedalism and had significant effects on the process of human birth which is much more difficult in modern humans than in other primates. During human birth, because of the variation in size of the pelvic region, the fetal head must be in a transverse position (compared to the mother) during entry into the birth canal and rotate about 90 degrees upon exit.The smaller birth canal became a limiting factor to brain size increases in early humans and prompted a shorter gestation period leading to the relative immaturity of human offspring, who are unable to walk much before 12 months and have greater neoteny, compared to other primates, who are mobile at a much earlier age.The increased brain growth after birth and the increased dependency of children on mothers had a big effect upon the female reproductive cycle,[and the more frequent appearance of alloparenting in humans when compared with other hominids.Delayed human sexual maturity also led to the evolution of menopause with one explanation providing that elderly women could better pass on their genes by taking care of their daughter’s offspring, as compared to having more children of their own.The genetic revolution in studies of human evolution started when Vincent Sarich and Allan Wilson measured the strength of immunological cross-reactions of blood serum albumin between pairs of creatures, including humans and African apes (chimpanzees and gorillas).The strength of the reaction could be expressed numerically as an immunological distance, which was in turn proportional to the number of amino acid differences between homologous proteins in different species. By constructing a calibration curve of the ID of species’ pairs with known divergence times in the fossil record, the data could be used as a molecular clock to estimate the times of divergence of pairs with poorer or unknown fossil records.In their seminal 1967 paper in Science, Sarich and Wilson estimated the divergence time of humans and apes as four to five million years ago,[54] at a time when standard interpretations of the fossil record gave this divergence as at least 10 to as much as 30 million years. Subsequent fossil discoveries, notably "Lucy", and reinterpretation of older fossil materials, notably Ramapithecus, showed the younger estimates to be correct and validated the albumin method.Progress in DNA sequencing, specifically mitochondrial DNA (mtDNA) and then Y-chromosome DNA (Y-DNA) advanced the understanding of human origins.[55][8][56] Application of the molecular clock principle revolutionized the study of molecular evolution.On the basis of a separation from the orangutan between 10 and 20 million years ago, earlier studies of the molecular clock suggested that there were about 76 mutations per generation that were not inherited by human children from their parents; this evidence supported the divergence time between hominins and chimps noted above. However, a 2012 study in Iceland of 78 children and their parents suggests a mutation rate of only 36 mutations per generation; this datum extends the separation between humans and chimps to an earlier period greater than 7 million years ago (Ma). Additional research with 226 offspring of wild chimp populations in 8 locations suggests that chimps reproduce at age 26.5 years, on average; which suggests the human divergence from chimps occurred between 7 and 13 million years ago. And these data suggest that Ardipithecus (4.5 Ma), Orrorin (6 Ma) and Sahelanthropus (7 Ma) all may be on the hominid lineage, and even that the separation may have occurred outside the East African Rift region.Furthermore, analysis of the two species’ genes in 2006 provides evidence that after human ancestors had started to diverge from chimpanzees, interspecies mating between "proto-human" and "proto-chimps" nonetheless occurred regularly enough to change certain genes in the new gene pool:A new comparison of the human and chimp genomes suggests that after the two lineages separated, they may have begun interbreeding… A principal finding is that the X chromosomes of humans and chimps appear to have diverged about 1.2 million years more recently than the other chromosomes.The research suggests:There were in fact two splits between the human and chimp lineages, with the first being followed by interbreeding between the two populations and then a second split. The suggestion of a hybridization has startled paleoanthropologists, who nonetheless are treating the new genetic data seriously.
H. floresiensis, which lived from approximately 190,000 to 50,000 years before present, has been nicknamed hobbit for its small size, possibly a result of insular dwarfism.[160] H. floresiensis is intriguing both for its size and its age, being an example of a recent species of the genus Homo that exhibits derived traits not shared with modern humans. In other words, H. floresiensis shares a common ancestor with modern humans, but split from the modern human lineage and followed a distinct evolutionary path. The main find was a skeleton believed to be a woman of about 30 years of age. Found in 2003, it has been dated to approximately 18,000 years old. The living woman was estimated to be one meter in height, with a brain volume of just 380 cm3 (considered small for a chimpanzee and less than a third of the H. sapiens average of 1400 cm3).[citation needed]

However, there is an ongoing debate over whether H. floresiensis is indeed a separate species.[161] Some scientists hold that H. floresiensis was a modern H. sapiens with pathological dwarfism.[162] This hypothesis is supported in part, because some modern humans who live on Flores, the Indonesian island where the skeleton was found, are pygmies. This, coupled with pathological dwarfism, could have resulted in a significantly diminutive human. The other major attack on H. floresiensis as a separate species is that it was found with tools only associated with H. sapiens.

Homo floresiensis
The hypothesis of pathological dwarfism, however, fails to explain additional anatomical features that are unlike those of modern humans (diseased or not) but much like those of ancient members of our genus. Aside from cranial features, these features include the form of bones in the wrist, forearm, shoulder, knees, and feet. Additionally, this hypothesis fails to explain the find of multiple examples of individuals with these same characteristics, indicating they were common to a large population, and not limited to one individual

en.wikipedia.org/wiki/Human_evolution

Posted by bernawy hugues kossi huo on 2017-05-14 15:34:06

Tagged: , floresiensis , H. , sapiens , Neanderthal , rhodesiensis , heidelbergensis , cepranensis , erectus , rudolfensis , georgicus , habilis , gautengensis , Evolution , genus , Homo , Anatomical , changes , GARDEN

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