What are the stages of hominids??

Answer 1

Your book has the answer

Answer 2

Thats a loaded question... hominids encompass all human ancestors that were not purely ape. This includes anything in the genus homo, the austrolopithecines, neanderthals, etc.

Answer 3

There are two basic directions in the evolution of hominid dentition. The first is for larger back teeth and is associated with A. robustus and A. boisei who specialized their diet towards vegetation. Coupled with the large molars, massive muscular structures to aid chewing developed in these early hominids. These hominids shared the ability for bipedal walking. The reasons for the development of bipedalism in these early hominids would have been the same as for later hominids. They shared the freedom of the hand from locomotion, as well as the ability of the trunk to be controlled on the hindlimbs during bipedal postures. Therefore, their hands could be used in many other activities besides walking.

The other direction in the evolution of the dentition of hominids was for smaller teeth and a parabolic shape to the palate. The trend for smaller teeth was one which began with early Homo and continued through to modern Homo sapiens. Despite a reduction in the size of dentition for H. erectus, their teeth were still larger than ours. The decrease in the size of the teeth is thought to be related of a diet that included a wide variety of foods. There was less emphasis on plant foods that required heavy chewing.




Every animal has its niche, a space that it fills and a role it plays in an ecosystem. It is likely that early Homo straddled the carnivore and herbivore niches. Niches can shift over time as various environmental factors change. Hominids may have been forced shift from time to time. Their diurnal, mobile, mixed ground-tree scavenging niche may have been well suited through flexibility. Dry season scavenging and wet season foraging may have created a highly flexible adaptive niche for early hominids. This probably provided early Homo with a distinct advantage as different types of hominids competed with each other for survival.





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Specialization
A. boisei represents a specialized direction for hominids. The massive face, large premolars and molars, and sagittal crest each mark this specialized form. This "human cuisinart" was a hominid for the same reasons that we are. Yet, the specialized adaptation for chewing may have led to extinction. The role of competition with H. erectus can be explored as a reason for the failure of this specialized hominid.

The east African and south African robust australopithecines probably shared similar habitats and behavior. During the period that the robusts existed, spanning from at least 2.5 m.y.a. to around 1.0 m. y. a., the climate changed regularly with fluctuating cold and warm spells on a global scale. After a million years ago, temperatures dropped steadily. This sharp cooling trend may have impact the robusts' ability to survive in several ways. First, its likely they did not possess the fire-bearing capabilities of homo erectus. Second, they were dependent on a highly specialized plant diet that may have undergone modification in a cooler climate; H. erectus enjoyed a much more varied and flexible diet. This may have been a key to survival.

Even without climate change, herbivores have an evolutionary disadvantage. They need to eat more to get the same amount of nutrition that carnivores or omnivores do from their richer diet. The robust australopithecines may have found their dry habitats offering less and less to eat over the millennia, especially if their hard, tough foods were of low quality.

A Contrast in Adaptations.....





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Increasing Brain Size
It is now clear that bipedality developed before larger brain capacity. It is equally clear that A. africanus is earlier than A. robustus in southern Africa. The face of A. africanus is not as massive as the robust forms, and there is no sagittal crest. In overall size, the back teeth of A. africanus are still larger than those of modern humans but not as large as those for A. robustus or A. boisei. The brain size is relatively small (440 cc.). This is only slightly larger than that for a modern chimpanzee ( 282 to 500, avg. 385 cc.). The average cranial capacity for H. habilis is 660 cc., which is 50 percent larger than that of A. africanus. The cranial capacity for H. habilis ranges from 509 to 810 cc. This overlaps with the range of the early H. erectus (804 to 900 cc.; average 851 cc.) . Later H. erectus were to have cranial capacities near 1200 cc. with still later archaic Homo sapiens having capacities over 1600 cc.



There are obvious advantages to increased brain capacity. As an evolutionary trend for one branch of the hominids, the increasing size of the brain meant greater behavioral flexibility to adapt to different environmental circumstances. These would include the ability to manufacture more complex tools, construct sturdier shelters, and use fire for warmth, protection and cooking. It also was easier to transmit innovative behaviors from one generation to the next.

Large brains also have costs. A longer gestation period and extension of the period of care for the infant following birth is required. Much of the development of the human brain occurs after the birth. There is a period of critical care for a human infant for a period of one to two years after birth. The growth of the brain requires a high level of energy intake for the mother not only during the longer gestation (pregnancy) but for a relatively extended period following. Those hominids with greater brain size and effective energy intake for infant development and care may have been better able to survive and reproduce. Therefore, the trend for increased brain capacity may have been coupled with a trend for increased nuitritional intake. (Read the following on "growing young".)

At the same time that we recognize that there was a trend for greater brain size in some hominids, there was another group of hominids that did not develop a larger brain. This is the A. robustus and A. boisei division. There is very little difference between even the gracile and robust forms of australopithecines. The average for the graciles (A. africanus) is about 442 cc. while the robust forms (A. robustus and A. boisei) have an average of 516 cc. Given the greater body size of the robust forms, this is an insignificant difference. If there is a continuum between forms such as represented by WT17000 and the later A. boisei, it is possible to look at very little change in brain capacity for one line of the hominids over nearly a two million year period.

The development of a "thinking machine" as Alan Walker has put it is a significant trend that followed the development of bipedality. The success of a generalized hominid in contrast to a more specialized one should be considered as you evaluate trends in hominid natural history.

Increasing brain size can be correlated with complex development and an increased relience on tools. With increased ability to adapt by means of manipulating the environment with technology, H. erectus appears to be successful in living in a number of different environments. One must continue to focus on the adaptive processes underlieing hominid evolution. Homo clearly is part of an adaptive process.

Answer 4

The hominids
AFRICAN HOMO ERECTUS: HOMO ERGASTER

H. ergaster existed between 1.8 million and 1.3 million years ago.

Like H. habilis, the face shows:

protruding jaws with large molars;
no chin;
thick brow ridges;
long low skull, with a brain size varying between 750 and 1225 cc.
Early H. ergaster specimens average about 900 cc, while late ones have an average of about 1100 cc. The skeleton is more robust than those of modern humans, implying greater strength.

Body proportions vary:

Ex. Turkana Boy is tall and slender, like modern humans from the same area, while the few limb bones found of Peking Man indicate a shorter, sturdier build.
Study of the Turkana Boy skeleton indicates that H. ergaster may have been more efficient at walking than modern humans, whose skeletons have had to adapt to allow for the birth of larger-brained infants.

Homo habilis and all the australopithecines are found only in Africa, but H. erectus/ergaster was wide-ranging, and has been found in Africa, Asia, and Europe.


ASIAN HOMO ERECTUS

Specimens of H. erectus from Eastern Asia differ morphologically from African specimens:

features are more exaggerated;
skull is thicker, brow ridges are more pronounced, sides of skull slope more steeply, the sagittal crest is more exaggerated;
Asian forms do not show the increase in cranial capacity.
As a consequence of these features, they are less like humans than the African forms of H. erectus.

Paleoanthropologists who study extinct populations are forced to decide whether there was one species or two based on morphological traits alone. They must ask whether eastern and western forms are as different from each other as typical species.

If systematics finally agree that eastern and western populations of H. erectus are distinct species, then the eastern Asian form will keep the name H. erectus. The western forms have been given a new name: Homo ergaster (means "work man") and was first applied to a very old specimen from East Turkana in East Africa.

HOMO GEORGICUS

Specimens recovered recently exhibit characteristic H. erectus features: sagittal crest, marked constriction of the skull behind the eyes. But they are also extremely different in several ways, resembling H. habilis:

small brain size (600 cc);
prominent browridge;
projection of the face;
rounded contour of the rear of skull;
huge canine teeth.
Some researchers propose that these fossils might represent a new species of Homo: H. georgicus.

HOMO ANTECESSOR

Named in 1997 from fossils (juvenile specimen) found in Atapuerca (Spain). Dated to at least 780,000 years ago, it makes these fossils the oldest confirmed European hominids.

Mid-facial area of antecessor seems very modern, but other parts of skull (e.g., teeth, forehead and browridges) are much more primitive. Fossils assigned to new species on grounds that they exhibit unknown combination of traits: they are less derived in the Neanderthal direction than later mid-Quaternary European specimens assigned to Homo heidelbergensis.

HOMO HEIDELBERGENSIS

Archaic forms of Homo sapiens first appeared in Europe about 500,000 years ago (until about 200,000 years ago) and are called Homo heidelbergensis.

Found in various places in Europe, Africa and maybe Asia.

This species covers a diverse group of skulls which have features of both Homo erectus and modern humans.

Fossil features:

brain size is larger than erectus and smaller than most modern humans: averaging about 1200 cc;
skull is more rounded than in erectus;
still large brow ridges and receding foreheads;
skeleton and teeth are usually less robust than erectus, but more robust than modern humans;
mandible is human-like, but massive and chinless; shows expansion of molar cavities and very long cheek tooth row, which implies a long, forwardly projecting face.
Fossils could represent a population near the common ancestry of Neanderthals and modern humans.

Footprints of H. heidelbergensis (earliest human footprints) have been found in Italy in 2003.

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Phylogenic Relationships
For almost three decades, paleoanthropologists have often divided the genus Homo among three successive species:

Homo habilis, now dated between roughly 2.5 Myrs and 1.7 Myrs ago;
Homo erectus, now placed between roughly 1.7 Myrs and 500,000 years ago;
Homo sapiens, after 500,000 years ago.
In this view, each species was distinguished from its predecessor primarily by larger brain size and by details of cranio-facial morphology:

Ex. Change in braincase shape from more rounded in H. habilis to more angular in H. erectus to more rounded again in H. sapiens.
The accumulating evidence of fossils has increasingly undermined a scenario based on three successive species or evolutionary stages. It now strongly favors a scheme that more explicitly recognizes the importance of branching in the evolution of Homo.

This new scheme continues to accept H. habilis as the ancestor for all later Homo. Its descendants at 1.8-1.7 million mears ago may still be called H. erectus, but H. ergaster is now more widely accepted. By 600,000-500,000 years ago, H. ergaster had produced several lines leading to H. neanderthalensis in Europe and H. sapiens in Africa. About 600,000 years ago, both of these species shared a common ancestor to which the name H. heidelbergensis could be applied.

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"Out-of-Africa 1" model
Homo erectus in Asia would be as old as Homo ergaster in Africa. Do the new dates from Dmanisi and Java falsify the hypothesis of an African origin for Homo erectus? Not necessarily.

If the species evolved just slightly earlier than the oldest African fossils (2.0-1.9 million years ago) and then immediately began its geographic spread, it could have reached Europe and Asia fairly quickly.

But the "Out-of-Africa 1" migration is more complex. Conventional paleoanthropological wisdom holds that the first human to leave Africa were tall, large-brained hominids (Homo ergaster/erectus). New fossils discovered in Georgia (Dmanisi) are forcing scholars to rethink that scenario completely. These Georgian hominids are far smaller and more primitive in both anatomy and technology than expected, leaving experts wondering not only why early humans first ventured out of Africa, but also how.

[edit]
Summary
Homo ergaster was the first hominid species whose anatomy fully justify the label human:

Unlike australopithecines and Homo habilis, in which body form and proportions retained apelike features suggesting a continued reliance on trees for food or refuge, H. ergaster achieved essentially modern forms and proportions;
Members also differed from australopithecines and H. habilis in their increased, essentially modern stature and in their reduced degree of sexual dimorphism.
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