A new explanation for the MP/UP transition: a chicken-and-egg story?

[Jacques Cinq-Mars]

All,

Take it for what it’s worth, but I like to think (I hope) that Caspari’s last sentence is not to be taken at (scientific) face value and I am certainly looking forward to get confirmation of this by reading the actual article.

Jacques Cinq-Mars

EurekAlert! – 5 July, 2004

Contact: Diane Swanbrow
swanbrow@umich.edu
734-647-9069
University of Michigan

Old is young, study finds
Longevity evolved late for humans.

ANN ARBOR, Mich.---Researchers at the University of Michigan and the University of California at Riverside have discovered a dramatic increase in human longevity that took place during the early Upper Paleolithic Period, around 30,000 B.C.

In their study of more than 750 fossils to be published July 5 in the Proceedings of the National Academy of Sciences, anthropologists Rachel Caspari and Sang-Hee Lee found a dramatic increase in longevity among modern humans during that time: the number of people surviving to an older age more than quadrupled.

Caspari, an assistant research scientist at the U-M Anthropology Museum, said this increase in the number of relatively old people likely had a major impact, giving modern humans a competitive edge that ensured their evolutionary success.

For the study, the researchers analyzed the ratio of older to younger adults in hominid dental samples from successive time periods: later australopithecines, Early and Middle Pleistocene Homo, Neandertals from Europe and Western Asia and post-Neandertal Early Upper Paleolithic Europeans. They used a new analytical resampling technique allowing them to assess the significance of differences in rates of molar wear.

In the study, they defined "old" to be at least double the age of reproductive maturation, which is also the time when the third molars typically erupt. "While the age of reproductive maturation may have varied in early human groups, if it were 15, then age 30 would be the age at which one could theoretically first become a grandmother," Caspari noted.

Other scientists have argued that the presence of grandmothers confers an important evolutionary advantage since they heavily invest their knowledge and other resources in their reproductive-age daughters and their daughters' offspring.

By calculating the ratio of old-to-young individuals in the samples from each time period, the researchers found a trend of increased survivorship of older adults throughout human evolution. It's not just how long people live that's important for evolution, but the number of people who live to be old, Caspari and Lee pointed out.

The increase in longevity that occurred during the Upper Paleolithic period among modern humans was dramatically larger than the increase identified during earlier periods, they found. "We believe this trend contributed importantly to population expansions and cultural innovations that are associated with modernity," they wrote.

A large number of older people allowed early modern humans to accumulate more information and to transmit specialized knowledge from one generation to another, they speculated. Increased adult survivorship also strengthened social relationships and kinship bonds, as grandparents survived to educate and contribute to extended families and others. Increased survivorship also promoted population growth, the authors explain, since people living longer are likely to have more children themselves, and since they also make major contributions to the reproductive success of their offspring.

"Significant longevity came late in human evolution and its advantages must have compensated somehow for the disabilities and diseases of older age, when gene expressions uncommon in younger adults become more frequent," the authors noted.

"There has been a lot of speculation about what gave modern humans their evolutionary advantage," Caspari said. "This research provides a simple explanation for which there is now concrete evidence: modern humans were older and wiser."

###

For more information on the U-M Anthropology Museum, visit: http://www.umma.lsa.umich.edu/
For the Proceedings of the National Academy of Sciences, visit: www.pnas.org

[Dale Hoogeveen]

Hi Jacques,

I suppose it wouldn't take much additional longevity to expand reproductive periods which could easily expand populations enabling more individual cultural storage capacity and perhaps a greater breadth of individually stored experience to enrich total culture.  But that is a lot of if's between longevity and "wiser".

I also have a reservation about direct mother/daughter cultural reinforcement , considering that humans still show a lot of patrilocal dispersal.  Daughters often reassociate with mothers in many but not even all modern cultural settings for that kind of support to be sure, but that process has to be questioned as assumptive in more ancient cultures.  Actually there is still more universal daughter/"mother-in-law" interaction, especially when the younger woman is begining to reproduce, I think.   There is no proof that paternity was formalized into husband/father roles at that point either.  We do not know at what point in human history that  formalized identification of fatherhood/formalized male sexual access rights, etc.  became important cultural issues.  So I do not think we can legitimately add husband/father to the mix quite at any point even in UP culture.  "Mother-in-law" at that would have been more of sponsor/mentor'/foster mother, I would think, "mother-in-law" in actuality only because the older woman's son(s) would likely be amoung the local reproducing males.

It also seems to me that 30 is a bit old for the eruption of third molars.   Hopefully that has not prejudiced the assignment of greater age than is warranted.

What extended age does allow is for more succeeding generations of women to reproduce at the same time as their parent's generation of women does which provides for greater likelihood of childhood peer groups with parentage of dramatically mixed age status and therefore likely mixed social standing, perhaps enabling even the children of the youngest mothers to participate in the more experienced, higher status nuture provided to some of their peers.  That should improve reproductive success rates.

UP population densities were still so extremely low that local groups had to have been very small.  That throws all the children of all the local mothers into quite compact local age group structures out of which those peer groups would form.

Childhood cultural assimilation is likely to be the application point in all of this that is most important.  

One of the things about expanding populations that is almost never mentioned that I can see, is that there is as much as anything an expansion of excess pubescent males, many of whom are likely excluded from as much reproductive participation as they are inclined to desire, which could be either by lack of experience or by lack of status - likely both.  At some point population densities expand enough so that peer groups of such young men take on an identity of their own.  In modern cultures we have male teen gangs, most of which are not even actually criminal, but nearly all are trouble prone.  At some point increasing population densities produce enough of these easily frustrated young men that their volatile tempers could no longer be managed or placated individually as would be much more possible in the lowest density settings.  

Dale

All,

Take it for what it’s worth, but I like to think (I hope) that Caspari’s last sentence is not to be taken at (scientific) face value and I am certainly looking forward to get confirmation of this by reading the actual article.

Jacques Cinq-Mars

[colin]

750 fossils in this study may sound like quite a lot - but they are apparently spread across time from australipithicus onwards. While it may be the case that longevity  increased over time, there are surely simply  not enough fossils out there to pinpoint increased longevity to around 30,000 BC. It seems likely that this is more prompted by the "old story" that something significant  (magic gene anyone?) happened around the MP/UP  transition (in europe at least)  and then looking for an explanation of what it might be.
Cheers
Colin

[Dale Hoogeveen]

All in all agreed that it is a very small sample over such a span of time.  

The source of the paper is very interesting, however.   I would not normally expect Caspari to be advancing a magic gene theory, if that is what this is.  I rather doubt that it is.  There is probably more to this that is waiting to come out.

In modern contexts aging population ratios normally result from decreased reproduction, not improved reproductive success.  Fewer children naturally increases average age.  The density numbers would have been very different at 30,000 ya however, very much nearer the point that decline in reproductive success would quickly mean local extinction.

What's next:  to put this in a context of restabilizing gene pools that followed the reproductive upset of a genetic mixing?    

Dale
 

750 fossils in this study may sound like quite a lot - but they are apparently spread across time from australipithicus onwards. While it may be the case that longevity  increased over time, there are surely simply  not enough fossils out there to pinpoint increased longevity to around 30,000 BC. It seems likely that this is more prompted by the "old story" that something significant  (magic gene anyone?) happened around the MP/UP  transition (in europe at least)  and then looking for an explanation of what it might be.
Cheers
Colin

[trehinp]

The source of the paper is very interesting, however.   I would not normally expect Caspari to be advancing a magic gene theory, if that is what this is.  I rather doubt that it is.  There is probably more to this that is waiting to come out.

In modern contexts aging population ratios normally result from decreased reproduction, not improved reproductive success.  Fewer children naturally increases average age.  The density numbers would have been very different at 30,000 ya however, very much nearer the point that decline in reproductive success would quickly mean local extinction.

Dale
 

You're right Dale, I also doubt that it would be a genetic change.

In addition to decreased reproduction ratios in modern days populations, better nutrition and medical care has augmented significantly the life expectancy, at least in the developped world populations.

One must also take into account the fact that even is our ancestors had a higher reproduction ratio, the infant mortality must have been quite high too... This is also linked to the longer life expectancy nowadays where infant mortality has been reduced drastically in the past half century...

So it could also have been the case that some feeding habits imrovement, replicated in the population by imitation, may have produced a similar effect in the palaeolithic.

Perhaps a link with the researches on palaeolithic feeding habits (at least what we can guess about them) may be established...

Paul Trehin

[Dale Hoogeveen]

Hi Paul,

I think we have to be very careful about diet.  Pre agricultural hunter gatherer diets were often higher qualitiy than what was produced by the farming cultures that replaced them.  That is signaled in a lot of areas by increasing numbers of malnutrition markers on bones and teeth following a transition to agriculture, although agricultural diet is obviously capable of carrying heavier population densities.  

Farming produces more food but starchy grains as staples are simply not as complete in protein, complex fats or vitamins and trace minerals.  Maize (corn) is one of the most obvious examples of starch becoming a main dietary component in grain based diets resulting  in exclusion of broader based nutritional factors.  Rice and wheat produce very similar types of declines in dietary quality in return for greater abundance of available food.

Dale