It seems that our diet as well as our large brain, long life span and high fertility are key elements that made humans an evolutionary success. In a new study published in the journal PLoS ONE, researchers have now shown that by becoming omnivores, our ancestors were able to give birth to a greater number of offspring.
The specific impact of carnivory on human evolution, life history and development remains controversial. Researchers say they have shown in quantitative terms that dietary profile is a key factor influencing time to weaning across a wide taxonomic range of mammals, including humans ::::
The research emphasizes the high degree of similarity of relative time scales in mammalian development and life history across 67 genera from 12 mammalian orders and shows that the impact of carnivory on time to weaning in humans is quantifiable, and critical.
Since early weaning yields shorter interbirth intervals and higher rates of reproduction, with profound effects on population dynamics, our findings highlight the emergence of carnivory as a process fundamentally determining human evolution.
Past research has tried to explain the relatively short breast-feeding period of humans based on social and behavioral theories of parenting and family size. For an average human baby, the duration of breast-feeding is two years and four months. This is not long in relation to the maximum lifespan of our species – around 120 years. It is even less if compared to our closest relatives – female chimpanzees suckle their young for four to five years, whereas the maximum lifespan for chimpanzees is only 60 years.
An early weaning time has implications not only for the development of offspring but also for the time that elapses between births and so for the reproductive rate of the female. This, in turn, can affect population dynamics and the fitness of the species – a more reproductively active female is more likely to pass on her genes to future generations. A longstanding hypothesis suggested that the reduced weaning time in humans was due to the adoption of a carnivorous diet by early hominids some 2.6 to two million years ago.
Although various hypotheses existed, until now they had yet to be quantitatively tested. In the current study, the researchers built a mathematical model to investigate the factors that are important in determining the time to weaning for a range of mammals. For this model they used data on nearly 70 different mammals, including information on diet and brain size. The animals were also categorised into herbivores, omnivores, and carnivores to analyse the effect of diet on timing of weaning – a species was considered carnivorous if at least 20% of its diet consists of meat.
“The model shows that the young of all species cease to suckle when their brains have reached a particular stage of development on the path from conception to full brain-size”, said Psouni. “Carnivores, due to their high quality diet, wean earlier than herbivores and omnivores. The model thus demonstrates that diet is an important factor in determining time to weaning.”
Although further research is required into the mechanism behind this interaction, Psouni suggested two possible explanations: It could be that “either the high quality diet of the female gives a higher quality milk or that the offspring itself starts eating meat relatively early during development,” she said. “Given that human milk in content resembles that of great apes, the second hypothesis is a bit more likely – earlier weaning affects fertility as lactation and suckling is known to inhibit ovulation. Thus the conclusion that early weaning allows for more births within a woman’s reproductive life.”
This result provides qualitative data to support a much-debated area of research. Craig Stanford, an anthropologist at the University of South Carolina, who was not involved in the study, praised the research but highlighted one concern:
“The only problem is the quantity of meat and its nutrients that early humans consumed is much in debate but almost certainly very small until the emergence of Homo sapiens”, he said. If this is correct, weaning time and interval between births would not have been significantly shortened between some of the early hominid species suggested by the researchers.
However, he added: “The research certainly does provide a hypothesis to explain why apes reproduce so slowly relative to humans. A female chimp gives birth at best every four years in the wild”