About thirty thousand years ago there was a huge increase in grandmothers. At the same time the human population experienced explosive growth.
Scientists studying hominid fossils found that in the Early Upper Paleolithic period, thirty thousand years ago, there were more people living into old age than ever before. They made this observation based on the fossils’ teeth. It’s possible to estimate how long they lived by the amount of wear on their teeth. Old age in the study is defined as twice the age of sexual maturity, or about thirty years. While we don’t see very many thirty year old grandparents these days, it was probably common then.
Throughout the first three million years of the study, from Australopithecines through Homo Erectus to early modern humans, only a small percentage of people lived long enough to help raise their grandchildren. Then there was a population explosion of grannies. And grandpas, too. Five times as many people were living beyond the age of thirty. At the same time there was a marked increase in the population of modern humans in general. And the archeological record shows that their social structures were becoming more complex as well.
This supports the Grandmother Hypothesis. Anthropologists believe that the older members of human societies allowed their cultures to develop. They increased the accumulation of useful knowledge and its transmission down the generations. Their continuing presence helped to build bridges between families and clans. And grandmothers’ experience improved the success of child bearing and child rearing.
When this is put together with continuing improvements in areas like tool-making, language and food preparation, it helps to explain why modern humans did so well populating the planet. Having all those older, wiser people to rely on gave us an advantage.
Something else that contributed to our success is that we are better at cooperating than are other primates. Scientists can infer that from fossil evidence from as long ago as 3-4 million years. The bones indicate a small sexual dimorphism, or size difference, between hominid males and females. In other primates like gorillas and baboons, which have fierce mating competition among males, the sexual dimorphism is much greater. Our ancient hominid ancestors differed in size between the sexes by less than 15% on average, the same as our modern ratio. The toning down of competition and an increase in group cooperation seem to have been successful traits for us hominids.
All we needed then was more grandmothers.