Michael Gurven is an anthropologist interested in demography1. His interest goes back to evolutionary times. He proposes that since homo-sapiens first emerged we have had a “natural lifespan” of 70 years. That is a life span when free of “extrinsic” factors such as war and the bubonic plague. He makes his case based on his understanding of the few remaining “hunter gather” tribes. He looks at their lifestyle. To this he adds historical facts and some archeology.

A Death Curve for a Hunter Gather Community?

How would you put together such a curve? Many tribes do not count beyond 10. They do not have modern societies’ fixation with birthdays. They don’t know when they were born and do not care. Don’t forget that the most famous “Happy Birthday” song was not written until the start of the twentieth century.

Anthropologists do have a way of creating such curves. They build a family tree by talking to everyone in the community. Children cannot be born before their parents. Women cannot bear children before puberty. If they are lucky, they can then “anchor” their chart by tying someone to a specific event. “X was born or died the year of the earthquake” or “the year the mission opened”. From this they can infer the ages at which people have died.

The curves have been created for many isolated communities. Some are pure hunter- gathers, some also engage in slash and burn horticulture. The pattern is the same. Child mortality is extremely high. Sometimes up to 30% or even 50%. However, if you can survive to 15 the curves show that the most likely age at which you will die is around 70. Deaths before the peak are much higher. The environments in which these tribes live are hostile. There is a drop in the curve after the age of 15 but it does not drop far.

Historical Records Support the Biblical Lifespan

We can use what written records there are to model lifespan. I can trace my family of farmers and farm labourers back to Jerimiah in about 1625. All that through parish records kept by the churches. There are no records for “common people” earlier than that.(see last weeks Newsletter)

Beyond that the data only exists for specific groups. Kings and Nobles trace their ancestry back centuries. Between 1350 and 1425 their lifespans were low. They were more like the curves found for the worst modern day subsistence tribes. If you made it to 15 British nobles could expect to make it to 48. However, if you made it to 60 you could expect another 9 to 14 years. The monasteries provided another source of written records of ages and deaths. “Three score years and ten” was the most likely age to naturally die. If you lived in Ancient Rome and made it to sixty, you could expect to live another 13 years.

The Evolutionary Drive

In other Newsletters I have discussed the unique feature of humankind. Most female primates die very few years after menopause. Humans do not, we live for many years after we are no longer reproductive. We are no longer of value to evolution so why did it let us evolve to live on? Uniquely we live in multi-generational communities. Grandmothers and grandfathers are valuable to the community. They support the human ability to have more children and out compete other primates. The community can support more children than an individual female or couple. (Newsletter #202 The Grandmother Hypothesis)

Gurven points out that implicit in this social model are assumptions of a longer life. A life longer than we might expect. These communities have discovered that survival means sharing. If you have food, you share it because someday you may need your neighbours to share it with you. These are not just family groups.

Anthropologists have studied individual consumption and production of food. How it varies over different ages. In tribe after primitive tribe, they have found that for both men and women it is not until the age of 20 that they are self-sufficient. This is the age at which an individual can provide as many calories to the community as they consume. It takes a long time, for example, to become an effective hunter in the jungles of the Amazon. The anthropologists held archery “Olympics”. These showed that it took until the age of 40 for hunters to become really proficient. After a peak at the age of 40 the effectiveness of food provision declines with age. For hunter gathers around 65 it goes negative again. For those with some farming it stays positive.

Humans prospered by discovering that they should live in a community. This made them more competitive in evolutionary terms. They could have more children, faster if everyone helped to provide food. Evolution is not stupid. Such social models can only survive if they are fair. On balance individuals have to get as much out of the system as they put in. If they only cover their own needs at twenty. If peak production is when you are forty, how long do people have to live? The answer is much closer to “three score years and ten” than we might imagine.

1. “Seven Decades, How we Evolved to Live Longer”. Michael D Gurven. Princeton Press