On Tue, 3 Oct 1995 23:06:26 -0400 Glenn wrote:
>Stephen wrote:
SJ>.I fail to see how becoming "among the most helpless of any of the
>animals" conferred a selective advantage *before* the human brain had
>grown large enough to compensate by enabling higher intelligence.
>The same Blind Watchmaker, who is able to bring the eye to near
>perfection, must have let that one through? :-)
GM>There is data which would say that man did not become the most
>helpless of animals until after the brain had expanded. I think the
>following says that your assumption is wrong. All the fossil
>children may have grown up faster than ours. Our teeth leave growth
>marks which can be used to tell how old a child is.
It was Eiseley (an anthropologist) who claimed that being "among the
most helpless of any of the animals" was a precondition for the human
brain's expansion. Indeed, there would be no point to a human baby
becoming helpless after the human brain had expanded. The
helplessness was necessary to allow postnatal emryonic growth rates
(see Gould below)
GM>"The perikymata aging technique has already challenged the
>conventional view of growth in australopithecines...thus the first
>known australopithecine fossil, the Taung child, would have been
>closer to three years of age at death than to the six years commonly
>estimated." Chris Stringer and Clive Gamble, In Search of the
>Neanderthals, (New York: Thames and Hudson, 1993), p. 85
Thanks to Glenn for this information. However,two things: 1. the
"australopithecines" were at the very beginning of this brain
expansion, so arguments based on their alleged growth rates are of
limited value; and 2. Stringer and Gamble's quote above indicates
theirs is a minority view, ie. it is not what is "commonly
estimated"?
GM>"The young age of the Devil's Tower child makes its brain size --
>estimated at about 1400 ml, close to the modern adult average -- even
>more remarkable. The development of the child's molar teeth would
>also have been well-advanced for a three-year-old. This specimen
>thus suggests that Neanderthal children grew up quite fast." Chris
>Stringer and Clive Gamble, In Search of the Neanderthals, (New York:
>Thames and Hudson, 1993), p. 86
Another two things: 1. Perhaps this is an older child than a "three
year old" (see previous); and 2. The being "among the most helpless of
any of the animals" refers to the human child's *first* year of growth
(see Gould below) . There would be nothing in this that would rule out
rapid growth after the first year. Indeed it might be expected.
Gould shares Eiseley's "assumption" too, that human babies'
helplessness was necessary to allow post-natal brain expansion:
"Whatever the explanation, no one will deny that primates are the
archetypical precocial mammals. Relative to body sizes, brains are
biggest and gestation times and life-spans are longest among mammals.
Litter size, in most cases, has been reduced to the absolute minimum
of one. Babies are well developed and capable at birth. However...we
encounter one obviously glaring and embarrassing exception-namely us.
We share most of the precocial characters with our primate
cousins-long life, large brains, and small litters. But our babies
are as helpless and undeveloped at birth as those of most altricial
mammals...Why did this most precocial of all species in some traits
(notably the brain) evolve a baby far less developed and more helpless
than that of its primate ancestors? I will propose an answer to this
question that is bound to strike most readers as patently absurd:
Human babies are born as embryos, and embryos they remain for about
the first nine months of life. If women gave birth when they "should"
-after a gestation of about a year and a half-our babies would share
the standard precocial features of other primates." (Gould S.J.,
"Human Babies as Embryos", "Ever Since Darwin", Penguin: London,
1977, pp71-72)
"...In the previous essay, I argued that a (if not the) major feature
of human evolution has been the marked slowing up of our development.
Our brains grow more slowly and for a longer time than those of other
primates, our bones ossify much later, and the period of our childhood
is greatly extended. In fact, we never reach the levels of
development attained by most primates. Human adults retain, in
several important respects, the juvenile traits of ancestral primates-
an evolutionary phenomenon called neoteny. Compared with other
primates, we grow and develop at a snail's pace; yet our gestation
period is but a few days longer than that of gorillas and chimpanzees.
Relative to our own developmental rate, our gestation has been
markedly shortened. If length of gestation had slowed down as much as
the rest of our growth and development, human babies would be born
anywhere from seven to eight months (Passingham's estimate) to a year
(Portmann and Ashley Montagu's estimate) after the nine months
actually spent in utero." (Gould, pp72-73)
"...during their first year, human babies share the growth patterns of
primate and mammalian fetuses, not of other primate babies. (The
identification of certain growth patterns as either fetal or postnatal
is not arbitrary. Postnatal development is not a mere prolongation of
fetal tendencies; birth is a time of marked discontinuity in many
features.) Human neonates, for example, have not yet ossified the
ends of limb bones or fingers; ossification centers are usually
entirely absent in the finger bones of newborn humans. This level of
ossification corresponds to the eighteenth fetal week of macaque
monkeys When macaques are born at twenty-four weeks, their limb bones
are ossified to an extent not reached by humans until years after
birth. More crucially, our brains continue to grow at rapid, fetal
rates after birth. The brains of many mammals are essentially fully
formed at birth. Other primates extend brain development into early
postnatal growth. The brain of a human baby is only one-fourth its
final size at birth." (Gould, pp73-74)
"...The culprit in this tale is our most important evolutionary
specialization, our large brain. In most mammals, brain growth is
entirely a fetal phenomenon. But since the brain never gets very
large, this poses no problem for birth. In larger-brained monkeys,
growth is delayed somewhat to permit postnatal enlargement of the
brain, but relative times of gestation need not be altered. Human
brains, however, are so large that another strategy must be added for
successful birth-gestation must be shortened relative to general
development, and birth must occur when the brain is only one-fourth
its final size." (Gould, p75).
My question therefore remains, how would the Blind Watchmaker know to
let a human baby become "among the most helpless of any of the
animals" in order for it to later develop higher intelligence?
God bless.
Stephen
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