The Origin and Evolution of the Genus Homo Origin of the modern human genus, Homo

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The Origin and Evolution of the Genus Homo
Origin of the modern human genus, Homo, is one of the most intriguing and controversial questions in paleoanthropology. The oldest fossils of our genus are at least 2.3 to 2.5 million years old. The evolution of the modern human genus can be divided roughly into three periods: early, middle, and late. Species of early Homo resembled the early australopiths in many ways. Some early Homo species lived until possibly 1.6 million years ago. The period of middle Homo began perhaps between 1.8 million and 2.0 million years ago, overlapping with the end of early Homo. Species of middle Homo evolved an anatomy much more similar to that of modern humans but had comparatively small brains. The transition from middle to late Homo evolved large and complex brains and eventually language. Culture also became an increasingly important part of human life during the most recent period of evolution.
The key change usually considered to signal the origin of Homo is an increase in brain size, measured by the volume of the inside of the brain case (cranial capacity). The average cranial capacity of modern humans (Homo sapiens) is 1350 milliliters (about 5 cups), although the range of variation is large, around 1000 to 2000 cc. In the possible ancestors of Homo (Australopithecus afarensis and A. africanus) brain size was about 350 to 500 ml. What size, it may be asked, defines the difference between the brains of Homo and Australopithecus?
The paleoanthropologist Louis Leakey originally argued that the origin of Homo related directly to the development of toolmaking--specifically, the making of stone tools. This once popular idea of "man the toolmaker" considered toolmaking to require certain mental skills and fine hand manipulation that may exist only in members of our own genus. Indeed, the species name Homo habilis (meaning "handy man") refers directly to the making and use of tools.
However, several species of australopiths lived at the same time as early Homo, making it unclear which species produced the earliest stone tools. Recent studies of australopith hand bones have suggested that at least one of the robust species, Australopithecus (Paranthropus) robustus, could have made tools. In addition, during the 1960s and 1970s researchers first observed that some nonhuman primates, such as chimpanzees, make and use tools, suggesting that australopiths and apes that preceded them probably also made some kinds of tools. Furthermore, several early human lineages (including early and later australopiths and possibly Homo) lived at the time of the oldest known stone tools, around 2.5 million years ago. So, scientists are not sure which early humans were responsible for the gradual proliferation of stone tools starting around that time.
Still, according to some scientists, early Homo was probably the toolmaker since handheld tools for cutting and pounding were most useful to these smaller-toothed humans, whereas intensive chewing of food inside the mouth was the hallmark of the robust australopiths. Furthermore, stone tools like the oldest known ones continued well after the early australopiths died out.
Some scientists think that a period of environmental cooling and drying in Africa set the stage for the evolution of Homo. According to this idea, many types of animals suited to the challenges of a drier environment originated between about 2.8 million and 2.4 million years ago, and these included the first species of Homo. A toolmaking human might have had an advantage in obtaining alternative food sources as vegetation became sparse. The new foods might have included underground tubers and roots and meat obtained through scavenging or hunting. However, the period in question consisted of several fluctuations between dry and wet environments, not just a change to dry. Thus brain enlargement, early stone tool use, and expansion of diet all may have been ways of adapting to unpredictable and fluctuating settings rather than just dry, cool ones. Also, the supposed pulse of species originations and extinctions is not well documented. In short, the exact causes of the origin of Homo are poorly known; future fossil discoveries in this key time period should help in understanding the earliest origin of our genus.

Early Homo

Paleoanthropologists generally recognize two species of early Homo, Homo habilis and Homo rudolfensis, who overlapped in time and appear to have co-existed in the same region with other early human species. The new species of smaller-toothed humans had evolved as early as 2.5 million years ago. Scientists cannot always tell whether these fossils belong to late-surviving gracile australopiths or early representatives of Homo. The two groups resemble each other because Homo likely descended directly from an early species of australopith (perhaps A. afarensis or A. africanus)

Homo habilis

In the early 1960s, at Olduvai Gorge, Tanzania, paleoanthropologists described a newly discovered group of early human fossils that showed a cranial capacity of 590 to 690 cc. Based on this brain size, which was above the range of that known in australopiths, the scientists argued that a new species, Homo habilis, should be recognized.
H. habilis lived in eastern and possibly southern Africa between about 1.9 million and 1.6 million years ago, and maybe as early as 2.4 million years ago. Although the fossils of this species somewhat resemble those of australopiths, H. habilis had smaller and narrower molar teeth, premolar teeth, and jaws than did its predecessors and contemporary robust species.
H. habilis also had more modern-looking feet and hands capable of producing tools. Many of the earliest stone tools at Olduvai have been found with H. habilis fossils, suggesting that this species made them.
Scientists have noticed a high degree of variability in body size as more fossils of early Homo were discovered. This could mean that H. habilis had a large amount of sexual dimorphism. However, the differences in size actually exceeded those expected between males and females of the same species, and this finding has helped convince many researchers that another species of early Homo had lived in eastern Africa at around the same time.

Homo rudolfensis

This second species of early Homo was given the name Homo rudolfensis, after Lake Rudolf (now Lake Turkana), northern Kenya. The best-known fossils date from about 1.9 million years ago. Paleoanthropologists have not yet determined the entire time range during which H. rudolfensis lived.
This species had a larger face and overall skull than did H. habilis. The cranial capacity of H. rudolfensis averaged about 750 ml. H. rudolfensis also had fairly large teeth, approaching the size of those in robust australopiths. Scientists have found several modern-looking thighbones that date from between 1.8 million and 2 million years ago and may belong to H. rudolfensis. These bones suggest a body size of 1.5 m (5 ft) and 52 kg (114 lb.).

Middle Homo

By about 1.9 million years ago, the period of middle Homo had begun in Africa. Until recently, paleoanthropologists recognized one species in this period, Homo erectus. Many now recognize three species of middle Homo: H. ergaster, H. erectus, and H. heidelbergensis. However, some still think H. ergaster is an early African form of H. erectus, or that H. heidelbergensis is a late form of H. erectus.
Quite recently a fourth middle Homo species has been described, Homo georgicus. Discovered in Georgia (not the state but the country north of Turkey), the skull and limb bones have been dated to about 1.8 mya. At around 600 ml (very close to H. habilis), this is the smallest and most primitive hominid skull ever discovered outside of Africa. It has always been thought that the first hominid to leave Africa was Homo erectus (or ergaster), and that this had happened after erectus had attained the modern body shape and full adaptation to bipedality shown in the erectus and ergaster fossils. The discovery raises the possibility that the Georgian hominids might have evolved from habilis-like ancestors that had already left Africa. That in turn would cause re-evaluation of theories about why hominids first left Africa.
The skulls and teeth of early African Homo ergaster populations differed subtly from those of later H. erectus populations from China and the island of Java in Indonesia. These subtle differences seem to parallel the differences that occurred between later humans, including our own species, and H. erectus. Since this appears to be the case, the early African species may be more closely related to modern humans. Homo heidelbergensis has similarities to both H. erectus and the later species H. neanderthalensis, and many paleoanthropologists refer to it as a transitional species between middle Homo and the line to which modern humans belong.

Homo ergaster

The oldest known appearance of Homo ergaster is in Africa around 1.9 million years ago. This species had a rounded cranium, prominent brow ridge (bony, protruding ridge across the brow above the eyes), small teeth, and other features that it shared with the later H. erectus. Many paleoanthropologists consider H. ergaster a good candidate for an ancestor of modern humans because it also had certain modern skull features, including relatively thin cranial bones. Specimens of H. ergaster are especially well known in the time range 1.6 to 1.7 million years ago.
The most important fossil find of this species is a nearly complete skeleton of a young male, dated 1.6 million years old, from West Turkana, Kenya. The sex of the skeleton is determined from the shape of the pelvis and by its brow ridges, and an age of 9 to 12 years at death is known by the pattern of tooth eruption and bone growth. It is not known how the boy died. The "Turkana boy" had long leg bones adapted for long distance walking. The length of his arms, legs, and trunk were proportioned as in modern humans, in contrast with the apelike short legs (and long arms) of H. habilis and A. afarensis. This skeleton is remarkable for the evidence it offers of an early human fully committed to bipedality, with no signs of significant tree climbing. H. ergaster had an elongated body, indicating that it was adapted to hot, tropical climates, just as modern humans from the tropics also tend to have long, slender bodies. An adult height of about 6-ft and a body weight of 150 lbs. is estimated from the Turkana skeleton, assuming that the body underwent an adolescent growth spurt as modern human teenagers usually do.
Most paleoanthropologists used to believe that human evolution consisted of a single line that evolved progressively over time, an australopith species followed by Homo erectus, then Neanderthals, and finally modern Homo sapiens. But now it is thought that as many as five different species of early human, including robust australopiths, inhabited Africa about 1.9 million years ago. Since hybridization rarely succeeds between species with significant skeletal differences, only one of these species could have been the ancestor of modern humans. H. ergaster is widely accepted as an ancestor, although it arose from earlier populations of Homo, possibly H. habilis or H. rudolfensis. It appears that periods of species diversity and extinction have been common during human evolution, a similarity to the evolutionary histories of other organisms. Modern H. sapiens has the distinction of being the only living human species today.

Homo erectus

Paleoanthropologists now know that humans first evolved in Africa and lived only on that continent for at least the first two million years of our evolutionary history. But this finding was not clear to scientists until quite recently. In fact, the first discoveries of early human fossils in the 1800s were in Europe. Later discoveries came from Asia and included fossils from the Indonesian island of Java. The first finds from Java were in 1891 by Dutch physician Eugene Dubois. Dubois named this early human Pithecanthropus erectus, or "erect ape-man". Today paleoanthropologists refer to this species as Homo erectus. H. erectus was the first human species known to have spread in large numbers beyond the African continent.
H. erectus appears to have evolved in Africa from earlier populations of Homo ergaster, and then spread to Asia between 1.8 million and 1.5 million years ago. The youngest known fossils of this species, from the Solo River in Java, have been dated to about 50,000 years old. So this species was very successful, both widespread (Africa and Asia) and long-lived, having survived for more than 1.5 million years.
H. erectus had a low and rounded braincase that was elongated from front to back, a prominent brow ridge, and an adult cranial capacity of 800 to 1,250 cc, an average twice that of the australopiths. Its bones, including the cranium, were thicker than those of earlier species. Prominent muscle markings and thick, reinforced areas on the bones of H. erectus indicate that its body could withstand powerful movements and stresses. Its body was well adapted for bipedal walking. Although its teeth were much reduced in size from Australopithecus, its lower jaw was still quite thick and rugged looking.

Homo heidelbergensis

Many paleoanthropologists believe that early humans migrated into Europe by 800,000 years ago, and that these populations were not Homo erectus. A growing number of scientists refer to these early migrants to Europe -- who predated both Neanderthals and H. sapiens in the region -- as H. antecessor and H. heidelbergensis.
Scientists have found few human fossils in Africa for the period between 1.2 million and 600,000 years ago, during which H. heidelbergensis or their ancestors first migrated into Europe. Populations of Homo ergaster (or possibly H. erectus) appear to have lived until at least 800,000 years ago in Africa, and possibly until 500,000 years ago in northern Africa. When these populations disappeared, other massive-boned and larger-brained humans -- possibly H. heidelbergensis -- appear to have replaced them. Scientists have found fossils of these stockier humans at sites in Ethiopia, South Africa, Tanzania; and Zimbabwe.
There are at least three different ideas about these fossils. Some scientists place the African fossils in the species H. heidelbergensis, and think that this species gave rise to both Neanderthals (in Europe) and H. sapiens (in Africa). Others think that the European and African fossils are distinct, and that the African fossils belong in their own species (not H. heidelbergensis), which gave rise to H. sapiens. Still others prefer the long-held view that H. erectus and H. sapiens form a single evolving lineage, and that the African fossils should be placed in the category of archaic H. sapiens. According to this last view, H. erectus was the direct ancestor of modern humans, but the first two views give that role either to H. heidelbergensis, saying that the species spread through Europe and Africa, or to a separate African species. The main point is this: There is a growing number of fossils from Asia, Africa, and Europe that are intermediate between early H. ergaster and H. sapiens, and this makes it hard to decide how to divide up the variation in the bones and to determine which group of fossils represents the most likely ancestor of later humans.

Why Did Humans Spread Out of Africa?

Humans evolved in Africa and lived only there for as long as 2, or possibly 3, million years. So scientists wonder what finally triggered the first human migration out of Africa (a movement that coincided with the spread of early human populations throughout the African continent). The answer to this question depends, in part, on knowing exactly when that first migration occurred.
Some studies claim that sites in Asia and Europe contain crude stone tools and fossilized fragments of humanlike teeth that date from more than1.8 million years ago. Although these claims remain unconfirmed, small populations of humans may have entered Asia prior to 1.7 million years ago, followed by a more substantial spread between 1.7 million and 1 million years ago. The first major habitation of central and western Europe, on the other hand, does not appear to have occurred until between 1 million and 500,000 years ago.
By the time of the earliest humans, the world’s continents were in essentially the same positions they now occupy, so continental drift had no impact at all on human dispersal or the origin of races. Migrations were the result of several factors. First, the fall and subsequent rise in sea level occurred repeatedly, especially over the past 2.8 million years, coinciding with the expansion and melting of glaciers. When sea level fell, coastal land area expanded, which included the development of land bridges between continents and islands. Land expansion allowed new areas to be colonized. Second, climate change led to the movement, expansion, and contraction of habitats that were favorable to early humans and other organisms. Migration from one region to another may have simply occurred as early humans tracked climate conditions or habitats to which they were already adapted. Finally, the origin of new adaptive behaviors, such as the ability to control fire or improvement in language communication, may have also resulted in the ability of populations to expand into new types of habitat.
Scientists once thought that advances in stone technology could be correlated with the earliest human spread beyond Africa. However, these advances do not seem to be related. By 1.6 million years ago early humans began to make new kinds of tools commonly known as handaxes and cleavers. But this new technology (called Acheulean) was apparently not responsible for the spread, as the earliest human presence in Asia is older than the first handaxes. Also, most of the tool kits from East Asian sites more than 200,000 years old are made up of simply shaped cores and flakes rather than symmetrical handaxes.
It's been suggested that the early Pleistocene spread of humans was part of a wider colonization of new regions by meat-eating animals, like lions and hyenas. The dispersal of these African carnivores to Eurasia also occurred during the early Pleistocene, between 1.6 million and 780,000 years ago. Meat-eating may have allowed H. erectus to move through many different environments without having to learn the diverse poisonous plants in different regions. The long dispersal to eastern Asia, however, may have been gradual and occurred through the lower latitudes and environments similar to Africa's. Even a very minor expansion of populations each generation (such as 1 mile every 20 years) would have allowed East African H. erectus to reach Southeast Asia in only 150,000 years. Careful comparison of fossil animals, stone tools, and early human fossils unearthed from African, Asian, and European sites will help to test these ideas.

Late Homo

The origin of our own species, Homo sapiens, is one of the most hotly debated topics in paleoanthropology. One distinctive group of fossil humans, the Neanderthals, and their relationship to modern humans has been at the center of the debate. Traditionally, paleoanthropologists have classified as Homo sapiens any fossil human younger than 500,000 years old with a braincase larger than that of H. erectus. Many scientists who believe that modern humans descend from a single line dating back to H. erectus use the term "archaic Homo sapiens" to cover a wide variety of fossil humans that predate anatomically modern H. sapiens. Therefore, Neanderthals are sometimes classified as a subspecies of archaic H. sapiens -- H. sapiens neanderthalensis. Other scientists think that the variation in archaic H. sapiens actually falls into clearly identifiable sets of traits, and that any type of human fossil exhibiting a unique set of traits should have a new species name. According to this view, the Neanderthals belong to their own species, H. neanderthalensis.

Neanderthals and Other Archaic Humans

The Neanderthals lived in areas ranging from western Europe through central Asia between about 200,000 and 36,000 years ago, although recently discovered fossil and stone-tool evidence suggests that Neanderthals may have persisted until 28-24,000 years ago.
The distinction between Neanderthals and modern humans was supported early on by a faulty reconstruction showing bent knees and a slouching gait. This reconstruction was responsible for the standard picture of the Neanderthals' supposedly crude caveman lifestyle. This image turned out to be mistaken. The Neanderthals walked fully upright without a slouch or bent knees. Their cranial capacity was large, around 1500 ml (slightly larger on average than the brains of modern populations, a difference probably related to their large bodies and lean muscle mass). They were also culturally sophisticated compared with earlier humans. They made finer tools and were the first humans known to bury their dead and to have symbolic ritual. The practice of intentional burial is one reason why Neanderthal fossils, including a number of skeletons, are quite common compared to earlier forms of Homo.
Nevertheless, Neanderthals differed from modern populations in certain ways. Their skulls showed a low forehead, large nasal area, projecting cheek region, double-arched brow ridge, weak chin, and an obvious space behind the third molar (in front of the upward turn of the mandible, or lower jaw). Their bodies were distinguished by these traits: heavily-built bones, occasional bowing of the limb bones, broad scapula (shoulder blade), hip joint rotated outward, long and thin pubic bone, short lower leg and arm bones relative to the uppers, and large joint surfaces of the toes and long bones. Together, these traits made a powerful, compact body of short stature -- males averaged 1.7 m (5ft 5 in) tall and 84 kg (185 lb.), and females averaged 1.5 m (5 ft) tall and 80 kg (176lb).
The short, stocky build of Neanderthals conserved heat and helped them withstand cold conditions that prevailed in temperate regions beginning about 70,000 years ago. The last known Neanderthal fossils in western Europe are approximately 36,000 years old, and recent dates of Neanderthals from central Europe are 28,000 to 29,000 years old. Neanderthals produced sophisticated types of stone tools known as Mousterian, which involved creating blanks (rough forms) from which several types of tools could be made.
At the same time as Neanderthal populations grew in number in Europe and parts of Asia, other populations of nearly modern humans arose in Africa and Asia. These fossils, considered to be from archaic humans, are distinct from but similar to those of Neanderthals. Fossils from Chinese sites display the long, low cranium and large face typical of archaic humans, yet they also have features similar to those of modern people in the region. And at a cave site in Morocco, scientists have found fossils with the long skull typical of archaic humans but also the modern traits of a somewhat higher forehead and flatter midface. Fossils of humans from East African sites older than 100,000 years -- such as in Tanzania and Kenya -- also seem to show a mixture of archaic and modern traits.

Anatomically Modern Homo sapiens

The oldest known fossils that possess skeletal features typical of modern humans date from between 130,000 and 90,000 years ago. Several key features distinguish the skulls of modern humans from those of archaic species. These features include a much smaller brow ridge, if any; a globe-shaped braincase; and a flat or only slightly projecting face of reduced size. Among all mammals, only humans have a face positioned directly beneath the frontal lobe (forward-most area) of the brain. As a result, modern humans tend to have a higher forehead than did Neanderthals and other archaic humans. The cranial capacity of modern humans ranges from about 1,000 to 2,000 ml, with the average being about 1,350 ml.
Scientists have found both fragmentary and nearly complete cranial fossils of early anatomically modern Homo sapiens from the sites in Sudan, Ethiopia, South Africa, and Israel. Based on these fossils, many scientist conclude that modern H. sapiens had evolved in Africa by 130,000 years ago and started spreading to diverse parts of the world beginning on a route through the Near East sometime before 90,000 years ago.

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