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Grandmothering and the Evolution of Homo Erectus

The article “Grandmothering and the evolution of Homo erectus”, O’Connell et al hypothesize that Homo erectus evolved because climate changes brought about changes in their diet from meat to tubers. This change was accompanied by the need for more females to be involved in foraging and food sharing practices in order to provide for the juvenile in the group, who were unable to feed themselves. When grandmothers helped their daughter’s children with meeting their nutritional requirements, it freed the mother for the next pregnancy, thus increasing her fertility. The longer the grandmother lived post-menopausal, the more children the daughter was able to conceive, thus leading to selection in favor of long life spans. The authors have based their hypothesis on the study of Hadza women’s foraging and food sharing habits.

The article “Grandmothers and the evolution of human longevity” furthers this hypothesis for modern humans. In this article, Hawkes uses comparison between humans and chimpanzees to justify the grandmother hypothesis and explain human fertility and longevity. According to Hawkes, very few chimpanzees reach the age of terminal fertility and chances of them living past their child bearing age is even smaller, with only about 5% living beyond the age of terminal fertility. In contrast, about one-third of adult human females live past their child bearing age, even in hunter-gatherer societies. Based on this, Hawkes proposes that the remarkable longevity of the humans is due to the grandmother foraging for herself as well as the juvenile children of her daughter.

Critique

The article “Grandmothering and the evolution of Homo erectus” challenges the conventional theory of the evolution of Homo erectus according to which Homo erectus evolved as a result of big game hunting and parental provisioning. Before proposing an alternate theory, the authors discuss in details the loopholes of the existing theory suggesting that hunting could not have possibly been behind the evolution H. erectus because similar behavior among chimpanzees has not led to paternal provisioning. Besides, the highly unpredictable nature of big game hunting would have made it impossible to feed an entire family on a regular basis. Having thus discredited the conventional theory the others go on to propose their own theory. The critique of the conventional theory as done by the authors is extremely comprehensive and explains the theory in details before discussing the loopholes. It sets the stage for the introduction of the “grandmother hypothesis.”

The “grandmother hypothesis” is based on the study of modern hunter-gatherer tribes. The theoretical framework as proposed by the authors is that when grandmothers help with the foraging, it allows the younger woman to wean their child early and have more children. Theoretically, this would favor selection of post-menopausal longevity which would have led to the evolution H. erectus.

The tribe studied by the authors in order to come up with the hypothesis is the Hadza tribe of northern Tanzania. Here the authors have assumed that foraging habits of the Hadza women would be similar to that of H. Erectus and would be a good guide in understanding their evolution. The methodology may not be perfect, but works for the purpose of the hypothesis. Study of the foraging habits of the Hadza woman shows that when food is abundantly available, they allow the children to provision for themselves. However, when “resources easily taken by children” by children are scarce, they have to provision for the offspring as well. This means that they should be able to forage enough to meet the nutritional requirements of themselves as well as one other person. This becomes difficult when the woman is pregnant or nursing. In such circumstances, the grandmother starts providing for the young weaned children. Based on this study, the authors have hypothesized that ancestral grandmothers would have to remain fit for a longer period in order to provide for the young children, which would have led to the selective evolution of long post-menopausal life over apes.

Having thus argued in the favor of the grandmother hypothesis for humans, the authors go on to apply it to the H. erectus. In order to find a fit between the hypothesis and the evolution of the H. erectus, the authors try to check if available fossil and archaeological records show a sudden change in climate coincident with the changes in the life histories of the hominids. The longevity of the H. erectus is estimated by studying their brain size, dental eruption schedule and adult body size and found to be longer for H. erectus compared to broader hominoid. This change in H. erectus life histories seems to correspond with a trend towards cooler climates. The authors also believe that this change in climate would have led to increased consumption of tubers and cross-check it with the archaeological and fossils evidence. Based on this, they hypothesize that since obtaining tubers is difficult for juvenile and needs the strength of adults, grandmothers would have helped in provisioning, leading to the evolution of H. erectus.

Here the authors first study the foraging habits of modern humans to come up with the grandmother hypothesis and then check the hypothesis against the available fossil and archaeological evidence to find a fit and conclude the it also led to the evolution of the H. erectus. Although several objections can be raised to the argument, it is detailed and comprehensive enough to be acceptable as a valid basis for the hypothesis. The authors also predict some of the objections which might be raised against the hypothesis and try to answer them.

The paper has put forward a new hypothesis for the evolution of humans and H. erectus. This is a radical departure from the classical hunting hypothesis and may be met with stiff opposition. The hypothesis needs to be further examined and some of the findings which form the basis of the hypothesis need to be tested before it can gain wider acceptability. However, the paper does lay open a line of thought which could be pursued by future scholars.

The second article, “Grandmothers and the evolution of human longevity” is written by Kristen Hawkes, one of the co-authors of the first article. The first article first proposed the grandmother hypothesis and then used it to explain the evolution of H. erectus. However, the grandmother hypothesis itself needs further examination before it becomes widely acceptable. The second article tries to further the grandmother hypothesis by comparing life histories of chimpanzees with modern humans.

The article tries to answer why humans tend to have long post-menopausal life histories, a fact unique to humans. On comparison with chimpanzees, we find that the decline in their fertility corresponds with that of humans, however, they do not live long after menopause, that is, if they do manage to reach menopause at all. Hawkes attributes this unique longevity of humans to “selection pressures” which favored longer life spans because when grandmothers took care of the juvenile, the mother was free to produce more offspring. Thus by linking human longevity to the grandmother hypothesis, Hawkes provides another basis for the hypothesis

Both the articles discuss in details a new hypothesis while pointing out the weaknesses of the traditional theory. In this regard, they can be considered extremely bold since widespread acceptance of the grandmother hypothesis is still a long way away. The authors seem to predict all the possible arguments against the hypothesis and have carried out intensive research in various fields to support their hypothesis. This results in well presented and well argued papers with few flaws. The theoretical framework of the hypothesis is extremely strong. Using the foraging habits modern day hunter-gatherer tribes and cross-referencing it with fossil and archeological evidence, helps makes their hypothesis stand on firm ground. Future studies need to be carried out to further validate the hypothesis and help it gain wider acceptance.