Blaffer Hrdy estimates that cooperative upbringing began
at the beginning of the Pleistocene epoch (about 1.8 million years ago) when the African Homo erectus appeared. The author drew this conclusion from the relatively small difference in size between the sexes compared to the much older Australopithecines.
The male Homo erectus were
only 18 percent larger than the females. This degree of sexual dimorphism is only slightly more pronounced than in modern humans. (p. 384)
The plausible interpretation of Blaffer Hrdy would mean that the hominids were looking after their young together long before Homo sapiens. Therefore, the use of human communication by the helpers and thus the whole group of Homo sapiens is very plausible.
Carel van Schaik and Karin Isler also emphasise the importance of the helpers in caring for the young:
In most mammal species, the mother is on her own during the strenuous gestation and nursing periods, so any help will save the mother energy.
If not only the father but also other members of the group help by protecting, warming, carrying or feeding the young, the burden on the mother is considerably reduced.
In our own species, Homo sapiens, the help comes from fathers, other adult men and older siblings of young children, among others. However, grandmothers probably provide the most important assistance. The menopause has produced a whole class of sterile helpers in our population, which is almost unique in mammals. (pp. 164s)
In her work on grandmothers (the grandmother hypothesis), Kristen Hawkes highlighted the importance of post-menopausal women. All of these authors emphasise the contribution of all that have a relationship with the offspring i.e. the whole social group to the raising of the next generation.
I conclude that the whole group learned new intentional communication with the infant and then used it among themselves.