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Humans

 

Humans (known taxonomically as Homo sapiens, Latin for "wise man" or "knowing man") are the only living species in the Homo genus of bipedal primates in Hominidae, the great ape family. Anatomically modern humans originated in Africa about 200,000 years ago, reaching full behavioral modernity around 50,000 years ago.

Human evolution is characterized by a number of important changes — morphological, developmental, physiological, and behavioral — which have taken place since the split between the last common ancestor of humans and chimpanzees. The first major morphological change was the evolution of a bipedal locomotor adaptation from an arboreal or semi-arboreal one, with all its attendant adaptations.

Humans have a highly developed brain, capable of abstract reasoning, language, introspection, and problem solving. The human brain is typically 1,400 cm³ in modern humans, which is over twice the size of that of a chimpanzee or gorilla. This mental capability, combined with an erect body carriage that frees the hands for manipulating objects, has allowed humans to make far greater use of tools than any other living species on earth. Other higher-level thought processes of humans, such as self-awareness, and rationality, are considered to be defining features of what constitutes a "person." The pattern of human postnatal brain growth differs from that of other apes (heterochrony), and allows for extended periods of social learning and language acquisition in juvenile humans.

Other significant morphological changes included the evolution of a power and precision grip, a reduced masticatory system, a reduction of the canine tooth, a shorter digestive tract, and the descent of the larynx and hyoid bone, making speech possible. An important physiological change in humans was the evolution of hidden oestrus, or concealed ovulation, which may have coincided with the evolution of important behavioral changes, such as pair bonding. Another significant behavioral change was the development of material culture, with human-made objects becoming increasingly common and diversified over time.

Humans and Chimps

Humans and chimpanzees diverged approximately six million years ago, making the chimpanzee the closest extant relative to modern humans, having DNA sequences about 98% identical to each other. Through comparison of the two genomes, the genetic differences that have accumulated since the human and chimpanzee diverged from a common ancestor have been cataloged. These changes constitute approximately 35 million single-nucleotide changes, five million ion insertion/deletion events, and various chromosomal rearrangements. Recent molecular studies have shown that there are differences in the expression of 19 regulatory genes between humans and chimpanzees, which many be largely responsible for the differences between the species. Due to the similar nature of the DNA, chimpanzees have been investigated with regard to medical application for humans. Humans and chimpanzees are similar with regard to many aspects of physiology and disease. There are several definite and likely differences, however, such as susceptibility to Alzheimer disease, epithelial cancers, and the progression of HIV to AIDS. Knowledge of chimpanzee biology remains incomplete, and will continue to provide useful information about the human body.

 

The great-apes

Figure 20. Image showing the relationship between the great apes.
"The Great Apes" by Merrilydancingape, Wikimedia Commons is licensed under CC BY-SA 3.0

 

Australopithecus

Australopithecus is a genus of hominids that is now extinct. Paleontological and archaeological evidence suggests that the Australopithecus genus evolved in eastern Africa around 4 million years ago, before spreading throughout the continent and eventually becoming extinct 2 million years ago. During this period various forms of australopiths existed, including: Australopithecus anamensis, A. afarensis, A. sediba, and A. africanus. Some debate remains among academics whether certain African hominid species of this time, such as A. robustus and A. boisei, constitute members of the same genus; if so, they would be considered robust australopiths, while others would be considered gracile australopiths. If these species do indeed constitute their own genus, then they may be given their own name, the Paranthropus.

It is widely held that the australopiths played a significant role in human evolution. Eventually, one species evolved into the Homo genus (Africa, circa 2 million years ago)—the same genus contained within species like Homo habilis, H. ergaster, and eventually the modern human species, H. sapiens sapiens. Gracile australopiths, widespread throughout Eastern and Northern Africa around 3.5 million years ago, share several traits with modern apes and humans. The earliest evidence of fundamentally bipedal hominids exists at the site of Laetoli in Tanzania. This site contains hominid footprints remarkably similar to those of modern humans, and have been dated to 3.6 million years. Until recently, the footprints were generally classified as australopith since that was the only form of pre-human documented in that region at that time. However, some scholars have considered reassigning the footprints to a yet unidentified very early species of the genus Homo.

The brains of most species of Australopithecus were roughly a third smaller in size than that of a modern human. Also, most species of Australopithecus were diminutive and gracile, standing between 1.2 to 1.4 m tall (approx. 4 to 4.5 feet). Several variations of Australopithecus experience a considerable degree of sexual dimorphism[IC81], in this case males being larger than females. Modern hominids do not display sexual dimorphism to the same degree—in particular, modern humans display a low degree of sexual dimorphism, with males being only 15% larger than females, on average. An Australopithecus male, on the other hand, might possibly have been up to 50% larger than his female counterpart. Despite continuing debate on the subject, new research suggests that sexual dimorphism may not have been this pronounced.

 

Australopiteco

Figure 21. Skull of Australopithecus, with circle representing the size of the brain.
"Skull of Australopithecus africanus (Mrs Ples)" by User:Editor at Large, Wikimedia Commons is in the Public Domain 

 

The fossil record also suggests that Australopithecus is the common ancestor of the distinct group of hominids now called Paranthropus (the "robust australopiths"), and most likely of the genus Homo (that of modern humans). Although the intelligence of these early hominids was likely as sophisticated as modern apes, the bipedal stature[IC82] is the key evidence distinguishing this group from previous quadruped primates. The morphology of Australopithecus contradicts previous theory among scientists that large brains preceded bipedalism. If A. afarensis is definitively the hominid responsible for the footprints at Laetoli, it strengthens the notion that A. afarensis was small-brained but biped. Fossil evidence such as this has made it clear that bipedalism far predated large brains

 


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