If you believe that humans were always 100% monogamous, even back in hunter-gatherer times, then I can see where you're coming from. If we use observations of other higher primates as a guide, the offspring are raised by closely related small groups, where males don't provide significant nurturance to individual young.
How did we go from the group model to the Ozzie and Harriet model? What was the defining event or set of circumstances that led to the concept of "my son"? Would those circumstances have necessitated enforced virginity until a marriage contract, together with female fidelity that we do not observe in other higher primates?
I did qualify that statement with the word, “notion.”
The higher primates are aware of the relationship of father to son.
http://icb.oxfordjournals.org/cgi/content/abstract/25/3/873
and
http://www.nature.com/nature/journal/v425/n6954/full/nature01866.html
(Behind a paywall, here’s the abstract)
Nature 425, 179-181 (11 September 2003) | doi:10.1038/nature01866; Received 15 April 2003; Accepted 30 June 2003
True paternal care in a multi-male primate society
Jason C. Buchan1, Susan C. Alberts1,2, Joan B. Silk3 and Jeanne Altmann2,4,5
1. Department of Biology, Duke University, Box 90338, Durham, North Carolina 27708, USA
2. Institute of Primate Research, National Museums of Kenya, Nairobi, Kenya
3. Department of Anthropology, UCLA, Los Angeles, California 90095, USA
4. Department of Ecology and Evolutionary Biology, Princeton University, Princeton, New Jersey 08544, USA
5. Department of Conservation Biology, Brookfield Zoo, Brookfield, Illinois 60513, USA
Correspondence to: Susan C. Alberts1,2 Email: alberts@duke.edu
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Abstract
Although male parental care is rare among mammals1, adult males of many cercopithecine primate species provide care for infants and juveniles. This care is often in the form of grooming, carrying, support in agonistic interactions, and protection against infanticide2, 3. For these behaviours to be interpreted as true parental care, males must selectively direct care towards their own offspring and this care must result in fitness benefits4. With the exception of males defending probable offspring from infanticide5, male primates living in multi-male, multi-female social groups have not been shown to selectively direct care towards their own offspring6, 7. We determined paternity for 75 juveniles in a population of wild savannah baboons (Papio cynocephalus) and collected data on interventions in agonistic disputes by adult males on behalf of juveniles as a form of male care. Here we show that adult males differentiate their offspring from unrelated juveniles and selectively support their offspring in agonistic disputes. As support in agonistic disputes is likely to contribute to rank acquisition and protect juveniles from injury and stress2, 3, 5, this can be considered true parental care.