Modern Human Bone Density Drop Tied to Lifestyle Shift

Title: Modern Human Bone Density Drop Tied to Lifestyle Shift
Website: Discovery News
Author: Charles Choi
Date: December 29, 2014
http://news.discovery.com/human/evolution/modern-human-bone-density-drop-tied-to-lifestyle-shift-1412291.htm

Summary:
Scientist have observed that modern day humans' bones are much less dense than our ancestors. Originally, they figured that modern-day humans' bones would just be less dense than those of Neanderthals and our primate ancestors, but upon more in-depth research, they have found that our bone density also differs with the modern humans from before the current Holocene Epoch, which started around 12,000 years ago; they also had denser bones than us. The lesser dense bones in recent modern humans explain the rise of some bone conditions such as osteoporosis.

As for an explanation as to why this shift occurred, there are two main hypotheses. One is simply that the modern humans have fallen into bad dietary habits and have been consuming less calcium than our ancestors. The other hypothesis is that physical activity has changed. To support this, a group of paleontologists compared the hip bones in groups of old humans- groups of agriculturalists and groups of mobile-foragers. They found that these mobile-forager groups, who would be more physically active, had the stronger hip bones, suggesting that physical activity was a factor in the bone density of humans.

Relevance:
This relates to ideas of evolution and natural selection that we talked about in class. Stronger bones in the mobile-forager groups is an adaptation that would be genetically advantageous for these humans, given that the groups would have to move a lot, so the denser, stronger hip bones would be of advantage to them. These humans may have been more likely to survive to reproductive age and support the population growth, as the weaker-boned mobile-foragers would not be as fit to survive the lifestyle. Thus, natural selection would occur and the population would eventually be made up of denser-boned mobile-foragers.

As for the explanation of the differences in bone density in humans today, this evolution occurred partially because of genetic drift, and partially because of natural selection. As humans transitioned from hunter-gatherer nomads to more permanent living arrangements, there may no longer be a benefit for the denser bones, which relates to the idea of adaptation. As we talked about, Darwin's findings contradicted the once-believed idea that species remained unchanged. But Darwin said that species could adapt to their environments, and also that these adaptations could evolve as well as they became more or less useful to the species. This relates to the bone density research because of the change in bone density after the adaptation became less useful. Genetic drift comes into play here, because thousands of years ago, perhaps it was by chance whether the thinner or denser bone genes were inherited from the gene pool . But it could also be do to natural selection, because the denser bones may actually have been slightly detrimental. When they no longer needed the bones to walk long distances, it is possible that the denser, heavier bones actually decreased mobility, causing natural selection to play out, with evolution favoring the humans with lesser dense bones. Either way, it is clear that some evolutionary change has caused this shift in human bone density over time, similar to ideas we have discussed in class about Darwin's finches and other evolutionary phenomena.






Hannah Gearan