(Taken from Steven Stanley's Children of the Ice Age, pages 172 - 179)
While it is possible to imagine many ways in which the large brain of Homo
would have been useful, its evolution also entailed a highly detrimental
side effect singularly immature infants. Parents now had to feed and protect
not only themselves but also their feeble, virtually immobile offspring.
Some mothers would have had more than one needy child in tow. These and
other special problems of child rearing must have beset members of the human
genus since its inception.
Conspicuous as the problems imposed by helpless infants may be, even for modern humans, some prominent scientists have downplayed them, assuming that because the retardation of early human development is so profound it must actually have redeeming value. These scientists have then been at pains to explain why helplessness and dependency early in life should be beneficial simply because it evolved. In a book entitled Growing Young, the anthropologist Ashley Montagu enumerated several ostensibly valuable attributes of delayed human maturation. Among these, he cited the provision of a long interval for learning from elders, for bonding to a family unit, and for remaining cooperative in a complex society.
The fact is that we learn very little during our first two years of life and cooperate largely through unwilled passivity. It certainly would benefit parents if our bodies could mature more quickly during this early interval. Furthermore, it is not necessary that we remain physically immature in order to remain attached to older people as dependent trainees. As college students prove, we remain more or less tractable and educable long after we reach physical maturity; in other words, we often choose to remain socially and, in effect, ecologically dependent long after we have the bodies of adults. In general, in the emotional and cultural aspects of our lives, we cannot be deriving a net benefit from retarded physical development.
The evolution of the large brain of Homo by means of a problematical slowing of development amounted to a profound tradeoff. It must stand as one of the most remarkable evolutionary compromises in the history of life. On the negative side were the physical and mental deficiencies of immature offspring that, from the earliest days of Homo, constituted a great ecological handicap for parents. From the beginning extended child rearing has robbed parents of time that they could otherwise have spent gathering food, making tools, or constructing shelters, and it has restricted mobility and complicated confrontations with enemies. On the positive side were the vast benefits of the new brain. In the game of natural selection, the positive value of the large brain clearly outweighed the negative side effects of infantile immaturity. Otherwise, quite simply, our brain would never have evolved.
For natural selection to create the large brain of Homo, the many benefits conferred by the incipient brain not only had to outweigh the problems imposed by helpless infants but also those imposed by the high rate of metabolism of the large brain itself. Recall that brain tissue requires an enormous supply of energy. Fatty meat and bone marrow are rich sources of energy, and, as Robert Martin of the Anthropological Institute of Zurich has observed, we can imagine that early Homo turned to them increasingly as its brain evolved toward larger size. The brain itself would have played an important role in the capture of animals that supplied the meat and marrow. In other words, the large brain of early Homo must have played an important role in stoking its own metabolic furnace.
One might wonder why evolution failed to follow some different path to the large brain that would have entailed no great ecological sacrifice. In fact, all such paths were probably highly improbable. A delay of the developmental process was probably the only mechanism by which evolution could have readily produced the marked encephalization. By employing this mechanism, natural selection made use of a pattern of growth the high rate of brain growth in utero that was already present in the ancestral animal. All that was required was a change in timing. This mode of evolution also had the advantage of producing a big brain early in life, so that advanced learning could begin during childhood.
Why Evolve an Enormous Brain?
How exactly might a very large brain have improved the chances of survival
for the incipient Homo if it required so many calories and burdened
its owner with highly immature infants? One way to begin to answer this
question is to ask another. What basic mental traits separate modem humans
from lower animals and what might those traits have accomplished for early
Homo? The traits that require large frontal lobes of the brain presumably
began to appear during the emergence of Homo. We have seen that generative,
or creative, ability is an especially salient trait of this kind. From it,
early on, should have come the potential to fabricate tools, including weapons
that could have warded off carnivores that were swifter, stronger, and innately
better armed than members of the human genus. In fact, I suggest that the
need for self-defense while living freely on the ground was the primary
driving force behind the natural selection that created the large brain
of Homo. This is not to say that the brain had little value in the quest
for food a quest that may have become more difficult once the transitional
population was living fully on the ground. Having access to fewer of the
resources that grew in trees, this group probably faced greater food shortages
than Australopithecus had normally encountered. As we can see from
the history of our own species, at some point tools facilitated the acquisition
and processing of both meat and edible plant materials on the ground.
From the new generativity would also have come the capacity to construct a language, perhaps largely through signing at first. Early Homo, using its newly expanded frontal lobes, was probably the first creature on earth that could generalize from experiences of its past in order to devise strategies for the future. As brainpower advanced, societies would have used the experiences of earlier generations to mold their strategies, because advanced communication gave rise to an oral tradition. By combining its ability to communicate with unprecedented creativity, early Homo presumably engaged in social cooperation that was not rigidly instinctive, like that of its four-legged enemies, but unconstrained and versatile.
Pair-bonding
When they engaged in hunting or
self-defense, only cooperation with one another would have enabled
the males in a troop of early Homo to substitute brains for the brawn they
lacked. This need for rapport among males probably accounts for the origin
of pair-bonding, which is the semipermanent attachment of one male to one
female. In species of primates that lack such bonding, males compete intensively
for females. Sometimes they actually fight, but the danger of injury to
both opponents has led evolution to produce substitute behavior, in which
potential combatants simply face off in aggressive postures, with one eventually
intimidating the other. The result is what Darwin labeled sexual selection
the process described earlier in which members of one gender that win contests
for mating rights produce a disproportionate number of offspring.
The "height-is-might" principle comes into play in sexual selection. More often than not, the larger of two contesting males is the more imposing and wins the right to mate with the one or more females who are the object of competition. As a result, in species where promiscuity rather than pair-bonding is the rule, males are usually much larger than females. As generation has followed generation in the evolution of such species, the genes of big males have accumulated preferentially. Living primates that mate promiscuously illustrate the results of such sexual selection for large males.
In most primate species that practice pair-bonding, males are closer to females in size. The current American divorce rate notwithstanding, pair- bonding is an innate pattern for H. sapiens, and on average modem men are only slightly more than 25 percent heavier than modem women. The fact that men are larger at all is probably related to their early role in hunting and, perhaps, warfare. On average women are probably just big enough to give birth to somewhat larger than average male babies.
According to the criterion of body size, Australopithecus afarensis the species to which Lucy belonged probably did not engage in pair-bonding. Lucy weighed only about 30 kilograms (66 pounds), but other skeletons assigned to her species represent animals that were nearly twice as heavy and are presumed to have been males. The disparity seems to have been smaller for Ausralopithecus africanus, the younger species that was probably ancestral to Homo. Even within this southern species, however, males may have been as much as 70 percent heavier than females. At the Australopithecus stage of our family history, from more than 4 to about 2.3 million years ago, promiscuous mating apparently was the norm.
Human ancestors may have had little reason to engage in pairbonding until members of a troop were cooperating in complex ways. Males could have worked more compatibly within hunting parties if, instead of vying with each other for females on the home front, each understood that a particular mate awaited him on return from a hunting expedition.
It is also easy to see how pair-bonding would have benefited Homo in rearing its physically immature offspring. Natural selection probably favored any male who became part of a nuclear family in order to help train his own offspring. These progeny were more likely to survive and reproduce, passing on their father's genes, than were offspring that a father left in the care of an unaccompanied mother during their lengthy childhood. Likewise, natural selection probably favored females who were inclined to enter common-law marriages, which favored dependent children with two devoted parents.
How Human was Homo rudolfensis ?
Oldowan tools and fractured bones associated with them point to a major
shift from the australopithecine pattern of subsistence at the time Homo
came into being. Cut marks on bones that have been found with manufactured
stone flakes reveal that the makers of the tools presumably populations
of H. rudolfensis butchered large animals, especially antelopes.
The toolmakers evidently put sharp flakes to use in skinning the animals
and in cutting through their tendons and muscles to separate flesh from
bone. Some of the bones also bear the unmistakable tooth marks of carnivores.
In some fashion human ancestors and four-legged meat eaters were sharing
carcasses, but it is not clear which group usually got there first. Which
ordinarily was the killer and which, the scavenger? Attempts to resolve
this question have stalled in the absence of definitive evidence. If the
troops of H. rudolfensis were the killers, they were big-game hunters.
If, instead, they were habitual scavengers, they must have had the wherewithal
to intimidate other large flesh eaters. In either case they were engaging
in effective aggressive activity that they could only have undertaken by
pooling both their prodigious mental resources and their limited physical
powers to best animals that were swifter and stronger than they. Their butcher
sites reveal that they were also collaborating in order to process meat.
The rendered meat presumably formed part of an omnivorous diet much like
that of hunting and gathering cultures of our own species.
Given the evidence of cooperative carnivorism for H. rudolfensis, we can imagine how pair-bonding would have benefited this species by reducing disruptive rivalry between males. We also have seen how pair- bonding would have aided the species in rearing its immature infants. It would be satisfying to find evidence for pair-bonding within H. rudolfensis in the relative body sizes of its two genders. Unfortunately, although we have some indications here, the data are thin. The mature male pelvis from which I estimated the size of a male of the species, both at birth and in adulthood, has a hip socket that would have housed a femoral head (the ball at the upper end of the thighbone) about 47 millimeters across; this happens to be only about 2 millimeters smaller than the average diameter for adult modern American males. The KNM-ER 1481a thighbone, which is of nearly the same vintage as the pelvis, has a well-preserved femoral head that is just a shade larger that the average for adult modern American females. It is tempting to conclude that the fossil pelvis belonged to a male and the fossil thighbone, to a female and then to assert that H. rudolfensis resembled us in the relative sizes of its genders. This, however, would be going too far. Within each gender of any species of the human family, there has been enough variation of thighbone proportions to allow a modest possibility that the fossil femur, like the pelvis, belonged to a male simply a much smaller one. Only future discoveries can reveal whether H. rudolfensis fits the typical body-size pattern for pair- bonding species. If I were to bet, it would be for the presence of pair- bonding in this advanced creature, a product of a giant evolutionary step toward the modem human animal.