Scientific evidence shows that the physical and behavioral traits shared by all people originated from apelike ancestors and evolved over a period of approximately six million years. One of the earliest defining human traits, bipedalism -- the ability to walk on two legs -- evolved over 4 million years ago.
Evolution Before about it was widely thought that distinctively hominin fossils could be identified from 14 to 12 million years ago mya. However, during the s geneticists introduced the use of molecular clocks to calculate how long species had been separated from a common ancestor.
The molecular clock concept is based on an assumed regularity in the accumulation of tiny changes in the genetic codes of humans and other organisms. Use of this concept, together with a reanalysis of the fossil record, moved the estimated time of the evolutionary split between apes and human ancestors forward to as recently as about 5 mya.
Since then the molecular data and a steady trickle of new hominin fossil finds have pushed the earliest putative hominin ancestry back in time somewhat, to perhaps 8—6 mya.
Possible pathways in the evolution of the human lineage. Announced inthis specimen is dated to the period between 7 and 6 mya.
The distinctive mark of Hominini is generally taken to be upright land locomotion on two legs terrestrial bipedalism. The skull of S. The most remarkable aspect of this skull is the broadness and flatness of its face—something previously associated with much more recent hominins—in conjunction with a smaller, ape-sized braincase.
This specimen also has small canine teeth compared with those of apes, thus aligning it with the hominins in an important functional regard. Sahelanthropus, then, emphasizes an evolutionary pattern that seems to have been a characteristic of the tribe Hominini from the very start—a pattern that aligns it with what is observed in most other evolutionarily successful groups of mammals.
Human evolution, it appears, has consistently been a process of trial and error. As flattering to the modern human ego as this picture may be, it is evidently quite wrong. Instead, human evolution has been throughout its long history a matter of experimentation, with new species being constantly spawned and thrown into the ecological arena to compete and, more often than not, become extinct.
Viewed this way, H. Fossils found since the early s have begun to hint at just how complex the hominin bush was in the three million years or so following the time of Sahelanthropus. Three other new genera of early hominins ArdipithecusOrrorin, and Kenyanthropus dating from 6 to 3 mya have been recovered from Kenya and Ethiopia.
Furthermore, during the latter half of the 20th century, new species were added to the long-established genera Australopithecus and Paranthropus, both known from South and East African sites. This early radiation diversification of hominins, of which the latest survivors lived as recently as about 1.
In general, they were relatively small-bodied, with large chewing teeth but reduced sometimes highly reduced canine teeth. They had small ape-sized braincases and rather protruding faces.
Replica of a 3. The design of her pelvis and feet are suggestive of bipedal locomotion. However, other skeletal elements indicate that she spent much of her time clambering through the branches of trees.
These fossils, along with the slightly older trails of footprints found at LaetoliTanzania, prove that early hominins were upright bipeds when on the ground.
However, they also retained many reminders of their tree-dwelling ancestry, especially their rather long arms, short legs, narrow shoulders, and long grasping extremities. All these features would have made them agile upright foragers among tree branches, where they presumably sought food by day and sheltered at night even though they moved on two legs while on the ground.
The environments in which these early hominins lived suggest that 1 they were still comfortable in the forest and 2 they were largely active at the forest edges and in the woodlands where the forest graded into more open savanna —a type of habitat that was expanding in their African homeland after about 7 mya as climates became drier and more seasonal.
A trail of footprints probably left by Australopithecus afarensis individuals some 3. Moreover, although these ancient forms were clearly members of the same larger group, discerning exactly how any of them may have been connected to later species is problematic because of incomplete fossil evidence or different interpretations of the same evidence.
Homo may have originated as early as about 2. A variety of incomplete or broken fossils from the period between about 2. Taken together, this hominin assemblage makes a rather odd assortment that is based more than anything else on a modest increase in the size of the brain compared with that of Australopithecus and its relatives.
Even more important in the assignment of these fossils to Homo may be the occurrence in the same geologic deposits of very primitive stone tools. Decades later, the species responsible for producing the first stone tools remains unknown, but it likely was relatively small-brained, with a body proportioned quite differently from that of H.
Paleontologists speculate that, barring the finding of an as-yet-undiscovered species of Homo, the tools were likely constructed by members of Australopithecus or Kenyanthropus. Cranial remains dating to slightly less than 2 mya have been discovered at Koobi ForaKenya.
These are thought to belong to the same species as the remarkably complete 1. The nature of the association between the two finds is not yet completely evident, as even partial hominin skeletons are almost vanishingly rare as researchers delve deeper into the past to a time before the introduction of burial practices.In this system, modern humans are classified as Homo sapiens.
Evolution occurs when there is change in the genetic material -- the chemical molecule, DNA -- which is inherited from the parents, and especially in the proportions of different genes in a population.
Homo sapiens neanderthalensis (also Homo neanderthalensis) Neandertal (or Neanderthal) man existed between , and 30, years ago. The average brain size is slightly larger than that of modern humans, about cc, but this is probably correlated with their greater bulk.
Homo sapiens is one of several species grouped into the genus Homo, but it is the only one that is not extinct. See also human evolution. The name Homo sapiens was applied in by the father of modern biological classification (see taxonomy), Carolus Linnaeus.
The timeline of human evolution outlines the major events in the development of the human species, Homo sapiens, and the evolution of the human's ancestors.
It includes brief explanations of some of the species, genera, and the higher ranks of taxa that are seen today as possible ancestors of modern humans. Human evolution is the evolutionary process that led to the emergence of anatomically modern humans, beginning with the evolutionary history of primates – in particular genus Homo – and leading to the emergence of Homo sapiens as a distinct species of the hominid family, the great apes.
Although human evolution can be said to involve all those species more closely related to H.
sapiens than to the apes, the adjective human is usually applied only to H. sapiens and other members of the genus Homo (e.g., H. erectus, H. habilis).