I believe that a profound, natural demarcation separates what we might call
the adaptive complex of true Homo (big-brained forms, starting with
Homo rudolfensis) from the australopithecine adaptive complex, including
not only Australopithecus and Paranthropus but also "Homo"
habilis, the species that may warrant placement in a new, as yet
unnamed, genus of australopithecines. In other words, the advent of true
Homo ushered in a new phase in human evolution. The salient traits
of the australopithecine adaptive complex were these:
· a relatively small brain
· an apelike schedule of postnatal maturation
· semiarboreal adaptations, with limited ability to run on the ground
· a large disparity in size between males and females
· promiscuous sexual behavior rather than pair-bonding
· the ability of species to coexist with other hominid species (violation of the single-species hypothesis)
· the failure to manufacture stone tools traits:
The contrasting adaptive complex of Homo includes these
· a very large brain with an expanded prefrontal cortex
· delayed postnatal development compared to that of an ape
· anatomical adaptation to an essentially terrestrial life (some humans still occasionally climb trees)
- a modest difference in size between the two genders (at least in advanced species)
Pair-bonding between the sexes (at least in advanced species) the inability
of the two species to coexist for very long (adherence to the single-species
hypothesis)
· the fabrication of stone tools or more advanced artifacts
The notion that the human family falls naturally into two quite different
adaptive complexes runs counter to the views of those anthropologists who
continue to insist that "Homo" habilis was a valid
member of our genus that included both the smallbrained fossil forms generally
agreed to represent this species and the big-brained individuals now recognized
as Homo rudolfensis. This lumping of quite different crania obviously blurs
the distinction between the two adaptive complexes that I have delineated.
The workers who still unite the quite different crania focus heavily on
the teeth of "Homo" habilis, which to a degree resemble
those of unquestioned Homo; these workers, in my view, pay too little
attention to the small brain of "Homo" habilis,
the apelike proportions of its limbs, the absence of advanced balancing
structures in its ears, and the tiny size of its females. All of these features
have profound implications.
Clearly, human evolution crossed a major threshold when Homo emerged about two and a half million years ago, even though "Homo" habilis and Paranthropus lived alongside the new genus for some time thereafter. The new adaptive complex was all of a piece. The delayed development and large brain were, of course, interrelated, and they were incompatible with everyday tree climbing. The advanced brain led not only to tool manufacture but also indirectly to pair-bonding and, hence, to a reduced difference in the average body size between males and females. Homo soon put its brain to good use, in part by constructing advanced social structures that could not function if males frequently contested with one another for females. Thus, sexual selection that had favored especially large males came to an end.
Evolution as a Compromise
While developing the theme that human evolution has not followed a simple, linear path from the earliest australopithecines to Homo sapiens, I have emphasized the compromises that evolution has negotiated along the way. Many of these are still with us, leaving us far less perfect animals than we might wish to be.
The adaptive compromises embodied in the skeleton of Australopithecus are the evolutionary results of living a double life. Australopithecus was both a reasonably good climber, so that it could have made good use of copses of trees as protective home bases, and also a reasonably good walker, so that it could have foraged effectively for food on the ground and migrated between home bases efficiently. But, as the product of evolutionary compromise, it could neither match the acrobatics of an ape in a tree nor run as we can on the ground.
For Homo I have discussed adaptive compromises of a different kind. They represented the inability of the evolutionary process to perfect even a creature that has committed itself totally to life on the ground. One of these compromises was the evolutionary descent of the voice box. While this change facilitated speech by providing space for movements of the tongue, it prevents us from breathing while we eat and allows food occasionally to block our windpipe. Another compromise was the expansion of the pelvis that permitted both Homo neanderthalensis and Homo sapiens to give birth to large babiesbut only at some loss of mechanical advantage for hip muscles during two-legged locomotion.
Retardation of our development after birth was the most profound trade- off of all, however. In the magnitude of both its positive and negative effects, it exceeded all other compromises in human evolution. By extending the high rate of brain growth that all primates experience before birth, it endowed infants of early Homo with a huge brain not long after they were born. (Recall that my calculations show even Homo rudolfensis experiencing delayed development much like ours.) One negative result of this giant positive step was energetic: the larger brain required many calories to sustain its activityliterally, food for thought. Much more problematical, however, was the period of time that Homo ended up spending in a state of infantile helplessness. (Recall that here, as in our degree of brain expansion immediately after birth, we modern humans rank number one among all living species of mammals.) Since its inception Homo has been forced to tend and transport and protect embryolike infants.
In descendants of Homo erectus, natural selection extended the interval of rapid brain growth further by the widening of the pelvis, so that the high rate of early brain growth could continue longer inside the womb. One way of viewing the overall extension of the fetal growth pattern is to think of modern humans as having a gestation period of twenty-one months, nine inside the womb plus twelve in the outside world during which the brain continues to grow at the high fetal rate in babies that remain highly immature.
My measurements suggest that Homo erectus expanded its brain at the rapid fetal rate by about the same percentage as we modern humans do after birth. Having entered the world with a smaller body and brain, however, it ended up with a smaller body and brain at the stage when the rapid rate of brain growth ceased a stage that modern human infants reach at an age of about one year. Homo erectus was probably pushing juvenilization to the limit, just as our species was likely doing before the advent of medical facilities that nurture premature babies. In fact, the birthing of immature babies must have created major ecological problems for early members of the human genus. Babies substantially more embryonic than ours at birth would likely be intolerable in nature because they would suffer very high rates of infant mortality and would impose an impossible burden of dependency on parents.
The origin of our large brain exemplifies evolutionary opportunism at its best. In effect, natural selection took advantage of a preexisting aspect of developmenta fetal pattern of growthand extended it into the postnatal interval. Presumably all that was required for evolution to start down the path it actually tookthe path of delayed developmentwas for one key mutationor, at most, a very fewto arise within a population of Australopithecus and then spread throughout it. The odds probably had been enormous against natural selection's enlarging the australopithecine brain dramatically by moving along any other path than this one. The path to the big brain that evolution followed was worth the ecological and energetic sacrifices it entailed.
Qualities of the New Brain
We must not overlook the fact that, when evolution created Homo, it did not simply enlarge the brain but also changed its structure. The brain grew much more than was required for managing the muscles of a slightly larger body, so that the excess brain tissue was available for expanded cognition. Sectors of the brain devoted to particular thought processes vied for unclaimed neurons by a remarkable sorting process. Early in life, neurons and connecting fibers proliferated rampantly throughout the brain but then withered away if they proved not to be well connected. Only those that were put to good use received sustenance. By this process the neural networks of Homo took form. The brain of early Homo undoubtedly differed in many ways from that of Australopithecus. Unfortunately, the interior surface of a skull records so little of the convoluted topography of the brain that fossils frustrate us with meager information about the details of the reorganization. What we can easily see is that in the evolution of Homo the prefrontal cortex of the brain expanded as the forehead became elevated to become a tall, nearly vertical facade for the front of the brain and the prefrontal cortex is where we think.
Lateralization the installation of new functions on only the left or right side of the brainalso came into play. When a new function arose on only one side of the brain, then the function that had previously been positioned there survived in the corresponding region on the other side. Expansion of the entire brain during this displacement process then provided more space for the original function on the side where it remained. The evolution of this formidable asymmetrical brain with its massive prefrontal cortex endowed Homo with a level of cunning, creativity, and socialization that the world had never seen before.
Would that we could visit with earliest
Homo to get inside its head. Sadly, we will never even know the extent
to which it could communicate with its comrades by sign language or verbal
utterances. Even for the more advanced Neanderthals, language is at issue.
Because the simple Neanderthal culture may not have required sophisticated
communication, Christopher Stringer and Clive Gamble, experts on these animals,
have doubted whether they employed advanced syntax. Perhaps language, as
we modern humans know it, is our unique preserve. Even so, early Homo
must have engaged its large brain for advanced communications and social
interactions far beyond anything that Australopithecus ever achieved.