Crossing Centiles

Every organism exhibits a series of growth and development trajectories it inherits from it's ancestors.  In many cases, these characteristics are the products of natural selection.  Life history theory is an approach in evolutionary ecology and biology that examines the timing, energetics and trade-offs involved in the growth and development of organisms.  The analysis can be inter-specific--for instance, comparing humans with Chimps--or it can be intra-specific--analyzing why bean plants in the shade grow less than their sun-exposed counterparts.

In many ways humans are a prototypical primate: highly social, abstract thinking, opposable-thumb-gripping mammals.  In some ways, however, we are quite unique. Take childhood for instance.  We may assume that childhood is a universal stage through which every mammal passes, but this would be incorrect.  Growth and development expert Barry Bogin has described childhood as the human adaptation.  Every other mammal passes from the infancy straight into the juvenile period.  So what is childhood?

Childhood represents a stage of post-weaning feeding dependency.  That is to say, whereas most other mammals gain feeding independence upon being weaned from their mama's, humans are dependent on elders for a good 5-6 more years.  Notable exceptions can be seen in lions, hyenas and dogs, which still are dependent on their mothers for a while longer after they have been weaned (think about the skills and guts required to hunt prey).  But even these exceptions are more similar to the general mammal pattern than they are to the human period of childhood.

Various definitions abound, but the most holisitic one in my opinion defines childhood as:

1) The period from 3-7 years of age in which,
2) Growth in height levels off at about 5cm a year
3) Brain growth is finalized (at year 7)
4) Locomotor efficiency of adults is reached (at age 7 again.  Prior to this, children can be upto 30-40% less
    efficient in their gait)
5) The first permanent molar erupts, and
6) A series of cognitive developments take place, resulting in more sophisticated thinking and motor skills,  
    and as
7) A stage in which play is paramount.

This prolonged period of growth and development results in later first-births in our species, compared to Chimps and Gorillas (our closest relatives).  And yet, our average fertility is substantially higher than that of any other ape (and for those of you wondering "WTF?", in evolutionary terms fertility is essential for the transmission, selection, fixation, etc, of traits).  The take home message: Even though it takes us longer to become independent, able-bodied individuals, this prolonged learning period (aka "childhood") has conferred us with tremendous evolutionary advantages vis-a-vis our closely related, but now extint hominin relatives.

This introduction on childhood sets the stage for a brief (I promise) reflection and discussion of infancy as exhibited by one Santiago Jose Ruiz Brelsford.  Point "2" listed above highlights a pattern of childhood: in the absence of adverse environmental or social stimuli, human children grow at remarkably predictable rates and patterns.  As such, childhood growth and development data are often used as solid indicators of a society's well being.

Infancy is a rather different matter.  Remember when you visited your pediatrician and s/he would say to you,"yup, you will probably be X cms tall as an adult." I would think, "cheeky sumofabliff, my dad is way taller than that..." (a discussion on male obsession with size today is for another post).  Well this sumofantich was right, but he had to wait until I was a child to pass his forecast.  Infants go through a process of centile crossing before they become "canalized" into their centile for height once in childhood.

All theory and no data makes Darwin cry.

Take Santiago.  At birth-cute sonofmywife--his head circumference was at the 3rd percentile for newborns according to the World Health Organization reference points.  Today, some 62 days later (whose counting?) he's at the 97th percentile.  He's height-for-age changes have been less dramatic, going from the 50th to closer to the 60th percentile; his weight for length has increased similarly from 50th to 70th percentile.

At our last visit our doctor uttered music to our ears: "Healthy, breastfed baby...not much to worry about at all." After the first week I stopped feeling nervous at his every noise.  He's a tough son, a happy one.  He's big and cute and healthy, and he exemplifies all the Homo sapiens characteristics despite various Neanderthals in his blood line. Every family's got 'em.

Phylogenetic constraints and inertia gonna rock you to sleep.  What is noticed at times, and I seek to break from the urge to disclose it.  The "tyranny of the concept", wrote Adorno.  I like to think of it more like the tyranny of characterization.  But that's some other post for some other sumofasandwich.