Why do humans care for their disabled children?

I recently came across an article in Discover magazine that talks about the discovery a fossil a disabled child half million years old that was cared for and nurtured for 10 years.  You can find the article here:
The fossil is 530,000 years old and belongs to the species Homo heidelbergensis.  It is a 10 year old child with a birth defect that would have caused severe mental and physical handicaps.  The fact that the child lived for 10 years is strong evidence that we developed the will and capacity to care for severely disabled offspring long before we became fully human.  Such behavior of course is not evolutionarily adaptive.  In fact, the effort that must have been exerted to care for this child who would no doubt have no chance of producing offspring of his own, would have had to be much greater than the effort required for the care of a child without such impairments.  This effort must have extended to the tribe not just to the mother.
In evolutionary biology, a trait is adaptive if it is heritable and it bolsters reproductive success.  The fact that caring for a disabled child is not adaptive means that it is truly a selfless act. 
This article struck a personal note for me because I have a 9 year old little boy with severe mental and physical disabilities.  How is it that my love for my children is equal when from an evolutionary perspective, one might assume that I should invest all of my resources into the child with the potential of passing on my genes?  Am I just conforming to some cultural norm?  Boyd and Richerson (see my paper on cultural evolution in my culture evolves post below) talk about an arms race between cultural and biological evolution.  The capacity for culture on balance is adaptive and improves the individual's chances of survival and passing on their genes.  However, every adaptive trait has some kind of a compromise involved because evolution cannot produce perfect solutions to problems.  Culture is an extreme example of this.  Our capacity for culture, allows us to escape the tyrannical demands of our selfish biological interests.  Because of culture, we are not necessarily chained to the demands of reproduction and survival.
A cultural explanation may be part of what's going on but cannot be the whole answer.  Clearly, 530,000 years ago there were no organized religions guilting this tribe into caring for their disabled members.  In fact, the child was probably a burden to the rest of the tribe so it is doubtful that the parents would have been judged harshly for choosing to abandon the child. So, it seems to me that these parents did what they did purely out of love.
Even in  many modern cultures, having a disabled child may be looked down upon as a shameful thing.  This may compel some parents to abandon their child at an institution and keep the whole thing a secret.  Nevertheless, there are many examples of parents raising their disabled children even when they are looked down upon and shamed by their culture (I need to do some research to get some clear examples of this.)  So, cultural conformity is not the explanation.  Despite being maladaptive I believe there is a strong biological tendency for parents to care for their disabled children.  Parents that make these sacrifices do so out of love, and not necessarily because of cultural conformity.
If this is true, then how did such a tendency that transcends cultures escape the tyrannical hatchet of natural selection?  I believe that the answer may be connected to the uniquely human trait of caring for our elderly.  Such care is biologically adaptive.  Tribal elders were a vast resource for vital knowledge in ancient societies.  Tribes that cared for their elderly eventually out-competed those that didn't.  We learned to have compassion for and to take care of those who could not care for themselves.  Care for the elderly has clear biological advantages that have been selected for.  Is it possible that the same cultural mechanisms that evolved to care for our elderly are the same ones that compel us to love our children unconditionally and to experience love in its purest form?