LOS ANGELES, CA (Catholic Online) - Hawkes was keen to further her theory behind "grand-mothering."

A thumbnail sketch of Hawkes' and her colleagues study goes like this.

In ancient times, the common ancestral mothers of humans and great apes couldn't move on to have another child until their most youngest child was no longer breast-feeding. Hawkes notes that breast feeding acts as a contraceptive, in order to prevent a mother being overburdened with children she can't support.

This period could take a long time, longer than it now does. Infants need to be quite old before they can feed themselves. In primitive ape-man tribes, nobody else was going to get hold of suitable food and feed it to the youngest members of the tribe.

In ancient times, female apes would generally die once they were past childbearing age. Only a few genetically different individuals might live on for longer. According to Hawkes and her colleagues, these older ape ladies in many cases would get hold of suitable infant food and pitch in to help feed their descendants.

These primitive children would thus be weaned earlier, allowing their mothers to have another child sooner, so spreading the grandma's unusual long-life genes faster and conferring an evolutionary advantage.

This is why human females can live so long past their reproductive lifespan, because it helps to spread their genes, according to theory.

Various computer simulations have sought to confirm -- or disprove this idea. One recent study cast doubt on the idea that older mothering members of tribes extended their families' longevity. Hawkes and her colleagues were quick to counter this argument.

"The researchers were conservative, making the grandmother effect 'weak' by assuming that a woman couldn't be a grandmother until age 45 or after age 75, that she couldn't care for a child until age 2, and that she could care only for one child and that it could be any child, not just her daughter's child," came the reply.

"Based on earlier research, the simulation assumed that any newborn had a 5 percent chance of a gene mutation that could lead to either a shorter or a longer lifespan."

In addition, Hawkes argued that the simulation begins with only one percent of women living to grandmother age and able to care for grandchildren, but by the end of the 24,000 to 60,000 simulated years, the results are similar to those seen in human hunter-gatherer populations: about 43 percent of adult women are grandmothers.

How the male lifespan is affected by one's mother and mother-in-law being around longer is unclear. It doesn't seem to have been considered very important by the study authors, and to be fair may only have become an issue in modern times.

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