Homo erectus

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September 29, 2010

Remains of Homo erectus are found throughout Africa and in western and eastern Asia (as far east as the island of Java in Indonesia).  Other fossils, assigned by some scholars to this species, have been found in Europe, as far north as England.  Homo erectus had a long tenure; the earliest Homo erectus fossils are dated to roughly 1.8 million years ago, while the earliest fossils assigned to this species date to roughly 300 thousand years ago (ka).  Homo erectus is important to the study of human evolution because it is the first species to be found outside of Africa and displayed many anatomical features (especially brain size and aspects of the postcranial skeleton—i.e., the parts of the skeleton below the head) that reflect evolution toward the pattern seen in Homo sapiens.

The taxonomy (the naming and assignment of species) of Homo erectus is controversial.  Some scholars maintain that important differences exist between the Asian and African representatives of this species.  In particular, these scientists contend that features in the cranium (skull minus lower jaw)—e.g., large teeth, sagittal keels (narrow areas of thickened bone extending from just behind the brows to the back of the skull) and massiveness of the neurocranium (the part of the skull that covers and protects the brain) and face—are found only in Asian H. erectus fossils.  To these paleoanthropologists, this evidence suggests that the Asian and African sample represent separate species; the name ‘Homo ergaster’ is given to the African fossils to formalize this species-level distinction.  However, as other scholars argue, many of these traits are also found in some H. erectus fossils in Africa, suggesting that the entire sample constitutes a single species, Homo erectus.  On this website (and for the remainder of this essay), the latter interpretation will be employed—i.e., ‘Homo erectus’ will be used to describe the entire sample (African and Asian).

Many of the features that distinguish Homo erectus from other hominin species—both earlier and later species—are seen in the skull.  The average brain size of Homo erectus is estimated to have been roughly 900 cubic centimeters (cc.), which is larger than Homo habilis, but smaller than that of Homo heidelbergensis and other later forms.  The size of the Homo erectus brain is negligibly larger than in Homo habilis when it is considered as in relation to body size—i.e., brain size increased substantially in Homo erectus, but, because body size also increased, the relative size of the Homo erectus brain is not considerably larger than that of Homo habilis.  The absolute in brain size, however, caused changes in the brain case; for instance, the braincase is higher than in Homo habilis, but lower than in later hominin species.  The Homo erectus braincase is also very long relative to its height, giving the skull a football-shape when viewed from the side.  The braincase and the face and jaws of Homo erectus were very heavily built, with thick bones and extreme thickenings along some of the skull sutures (where two skull bones connect).  For instance, the browridges were massively built and continuous across the face and large, bony prominences existed in the back of the skull (the occipital torus and angular torus).  Due to these prominences, the cranium of Homo erectus is pentagon-shaped when viewed from behind, with the widest area coinciding with the bottom of the cranium.

In addition to their use to define the species, cranial remains have also been used to study the way that Homo erectus grew and developed into adulthood.   Despite extensive study, scholars disagree about how the evidence bearing on this subject should be interpreted.  Evidence from the teeth of Homo erectus fossils suggests that Homo erectus had not yet evolved the unique growth schedule exhibited by Homo sapiens.  Using evidence from fossil crania, however, some scientists contend that H. erectus possessed otherwise uniquely human features, such as the juvenile growth spurt.  Thus, it is currently unclear if the growth pattern found in Homo sapiens evolved in Homo erectus or in a later species.

The postcranial remains of this taxon have traditionally been interpreted as being very similar to those of Homo sapiens.  The only notable difference is that the postcranial remains of Homo erectus are generally thicker and more massively-built than those of H. sapiens; all other features, though, point to striking similarities between the two species.  The stature of Homo erectus, for example, has been suggested to be very similar to living humans and the hindlimb (leg) of Homo erectus is much longer than in earlier forms for which good evidence exists.  Therefore, the proportion of the hind- to forelimb length (leg to arm) in Homo erectus is considered to be almost identical to that of Homo sapiens.  Because of these similarities, most scholars believe that Homo erectus was the first hominin species that was capable of a bipedal form of locomotion indistinguishable from that of Homo sapiens.  Recent research, however, suggests that key fossils assigned to Homo erectus were much shorter than previously thought and argues that modern human stature may not have been present in Homo erectus.  To wit, recent fossil finds in Kenya suggest that Homo erectus was quite variable in size and that sexual dimorphism (differences between males and females within a species) may have been greater than previously believed.

Whether it was due to its possession of human-like gait or not, Homo erectus is the first hominin species to have been found outside of Africa.  Many sites bearing Homo erectus fossils have been discovered in China and Indonesia.  In general, these sites are later than Homo erectus sites in Africa.  At a site called Dmanisi in the Republic of Georgia, however, fossil assigned to Homo erectus have been found in sediments dated to 1.7 mya.  The fossils at Dmanisi suggest that bands of Homo erectus began migrating out of Africa at a time very close its origin.  Interestingly, though, the postcranial remains from Dmanisi are more primitive (more similar to earlier species; e.g., the hindlimb was relatively longer) than those found at many other sites.

The ability of Homo erectus to colonize land outside of Africa was also largely dependent on the new technology this species invented to deal with its surroundings.  Homo erectus was the first species to actively control fire; the first good evidence for controlled fire is from a Homo erectus site in Israel and is dated to roughly 780 ka.  Homo erectus also invented new technology for making stone tools.  The types of tools used by this species are called the Acheulean industry.  The Acheulean industry consists largely of handaxes (large, teardrop-shaped artifacts) and cleavers (large artifacts with chips removed on either side of one end to create a sharp edge), but other tool types are also included.  Handaxes are particularly emblematic of the Acheulean industry.  These artifacts are bifacially worked (chips removed from both sides of the original rock), creating sharp edges on most surfaces.  Handaxes have been traditionally considered to be tools, but a growing minority of scientists now argues that it was the flakes of stones that were removed from the original rock that were the tools that were ultimately utilized for hunting, butchery, and other tasks.  Handaxes are found much less abundantly at sites in eastern Asia.  This fact may be due to underrepresentation of Homo erectus site in the region (due, in turn, to lack of scientific efforts in the region and/or a real dearth of sites) or to the fact that other materials (e.g., bamboo) that do not preserve in the archaeological record replaced handaxes.  It is also possible that Homo erectus lost the tradition of making handaxes as it migrated from Africa to eastern Asia.  No consensus has been reached on this subject.

The evolutionary relationships between Homo erectus and other hominin species are not fully resolved.  Many scholars believe that Homo erectus evolved from Homo habilis, probably in East Africa.  Recent finds in Kenya, however, have increased the temporal range of Homo habilis, suggesting that the two species overlapped greatly in time and causing some scientists to question the direct linkage between Homo habilis and Homo erectus.  It is generally agreed, however, that Homo heidelbergensis evolved from Homo erectus in Africa and spread across the Old World replacing later populations of Homo erectus, in much the same way that Homo sapiens replaced Homo heidelbergensis and Homo neanderthalensis three-quarters of a million years later (see essays on Homo sapiens and Homo neanderthalensis).