Hominid Species Time Line

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Homo erectus: developmental trends

The importance of this species (or group of apparently related species) cannot be over-emphasized, as their evolution shows the progressive increase in brain size that defines our genus.  (The generalizations below apply equally to H. antecessor and H. heidelbergensis.)

Over the long lifetime of this species, brain size increases from around 850 cc in early specimens to 1200 cc about half a million years ago.  The later is within the lower range of modern human brain sizes.  This growth in brain size means that the larger brained individuals of this species exhibit a fifty-percent increase in brain size over the Homo habilis/ergaster/rudolfensis.  [A recent re-dating of African finds shows that the earliest erectus/ergaster specimens have bigger brains than either H. habilis or H. rudolfensis.] 

Although the largest brain sizes of H. erectus fall within the range of modern humans, the H. erectus brain is configured somewhat differently than our own, particularly in the areas related to speech.

Individual specimens of Homo erectus vary widely but trends are visible over time. These artist's reconstructions of three faces from skulls show variations over a period of about a million years. The oldest, approximately 1.5 million years in age, is on the left, and the most recent, approximately .5 million years old, on the right. Seen together, these specimens show the developmental trends very clearly. Over time, the face evolved toward a flatter, more vertical plane with a larger and more rounded cranium as brain size increased. It is important to remember that reconstructions such as these are accurate only as to bones and the larger muscles that attached to them.  Features such as nose structure are only partly inferable from surviving evidence. Ears, hair, skin color and texture, etc. are conjectural.

Decrease in Sexual Dimorphism

Among early Homo erectus/ergaster specimens, males are much larger than females, as is typical of early hominids. After a million years ago, however, the size ratio of the sexes gradually changes and becomes roughly the same as in modern humans. Females at that point tended to be approximately 80% as large as males, the same as in modern populations. Scientists speculate that this size change in later H. erectus/antecessor/heidelbergensis signals important behavior changes in the species. The increase in the size of females may also mark a physical adaptation to the difficult task of giving birth to large-brained infants. 

At some point later in time, a mutation (that we modern humans have) slowed down the growth of the fetal brain so the head would not grow too large before the child was safely outside the womb.  This was a momentous, if currently un-datable, change in human life, because the mutation also slowed our whole maturation process. Modern humans have the longest juvenile stage of any life form; at some point our ancestors became slow developers too, but we don’t know precisely when.

This shift in the size ratio of the sexes and the slowing of our maturational process may be among the most important milestones in human development.