Hominids of the Acheulean Period

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.

Facial reconstruction of Turkana Boy

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

Replica skull of "Peking Man", an 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.

Facial reconstruction of Homo Heidelbergensis

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.

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.

A hypothesised family tree for Homo, proposed in 2012

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.

"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.

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.