Did rapid brain evolution make humans susceptible to Alzheimers?

March 29, 2010 By Alvin Powell, Harvard University
The puzzling question, Prof. Bruce Yankner said, is why humans develop the severe disabilities of Alzheimer’s disease.

(PhysOrg.com) -- Of the millions of animals on Earth, including the relative handful that are considered the most intelligent -- including apes, whales, crows, and owls -- only humans experience the severe age-related decline in mental abilities marked by Alzheimer's disease.

To Bruce Yankner, professor of pathology and neurology at Harvard Medical School (HMS), it’s pretty clear that evolution is to blame.

“Something has occurred in evolution that makes our susceptible to age-related change,” Yankner said in a talk last night sponsored by the Harvard Museum of Natural History as part of its “Evolution Matters” lecture series.

Yankner, whose HMS lab studies brain aging and how getting old gives rise to the pathology of Alzheimer’s and Parkinson’s diseases, said Alzheimer’s is one of the most rapidly emerging diseases of this century. As medical science lengthens human lifespan, the proportion of the population that is elderly is growing. Considering that as many as half of those over age 85 develop Alzheimer’s, there is a growing urgency to understand the disease more fully and to develop more effective interventions.

“It is clear that and decline is one of the emerging health threats of the 21st century,” Yankner said.

Yankner said that scientific evidence shows that some — beginning in middle age and accelerating after age 70 — is normal as we grow older. This decline is also seen in other animals, including mice and monkeys. It is marked by wide variation among individuals, with some individuals maintaining similar to those much younger.

The puzzling question, Yankner said, is why humans develop the severe disabilities of Alzheimer’s disease. Studies of other creatures show no sign of similar conditions even in our closest animal relatives. That means susceptibility to Alzheimer’s evolved recently, likely during a period marked by a rapid increase in our brain size. Size alone probably isn’t the determining factor, though, Yankner said, since other animals are known to have even larger brains, including whales, elephants, and even our extinct relative the Neanderthal.

Instead, he said, it is likely that brain complexity and the new large number of cells in the human brain have something to do with it.

Recent research, in Yankner’s lab and elsewhere, has used genetic tools to probe the differences between young and old brains in humans, monkeys, and mice. The work shows that gene function in the aging brain slows — dramatically in ones with Alzheimer’s — and that the genes that shut off the most are those that protect the brain against genetic damage from environmental and other factors.

Yankner said he believes that cognitive decline is due to a slow accumulation of genetic damage in the aging brain, with Alzheimer’s showing the most severe form of this damage, called double strand breaks. Though the source of the damage is not yet clear, one culprit, he said, may be the accumulation of metals in the brain over time, particularly iron.

Neurons use more energy than most other cells, Yankner said. With the brain’s increase in complexity over time, its energy demands also rose. Iron plays a key role in a cell’s energy-producing mitochondria, and so iron accumulation leading to genetic damage could be a byproduct of our neuron-rich, energy-gobbling brains.

“Aging is a balance between wear and tear and repair. Where you wind up in that balance determines how you do,” Yankner said.

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5 / 5 (2) Mar 29, 2010
Or, it could be that Alzheimer's is due to a genetic defect which spreads through the population because it does not cause disability until after the reproductive years.

Or, because this seems to be a recent problem, perhaps it is due to environmental stress (various chemicals, etc. in use today).

In fact, until we find the cause, speculation of all types is just speculation.
not rated yet Mar 29, 2010
There is also the question of how many people diagnosed with Alzheimer's may actually have mad cow disease. The symptoms are very much the same, and the testing procedures, at least in the US are laughable.

Looking at it from this stand point, as beef consumption rises while testing standards decline... It doesn't take a genious to see it may be a factor in the over all spike in Alzheimer's cases.
not rated yet Mar 29, 2010
There are two causes: genetics and/or environment.

We could blame Alzheimer's on genetics, but how many human-specific diseases have, in the past, been caused by environment?

A step from here would be to eliminate common factors in environment between humans and the aforementioned intelligent animals. Anything that's left over is a potential cause of human-specific ailments.
not rated yet Mar 30, 2010
Increase in brain size, language and nature's demand to share information have contributed to a number of mental illnesses. You can include anxiety, stress, schizophrenia, depression, dissociative identity disorder and some forms of epilepsy.

D J Wray
Packaged Evolution: The Intelligent Universe
not rated yet Mar 30, 2010
Perhaps the susceptibility was purposely built in, along with a lower lifespan, by someone or something that decided to create a shorter life span human model.

According to known theory and known information, that explanation fits the 'facts' or data as well as anything else, no matter how psychologically upsetting the idea may be.

After all, we can, ourselves, at the highest levels of genetic modification and manipulation, in laboratories in the black ops levels and beyond, ..we can accomplish far more and as easily as breathing. For what is in the mainstream and known is always far from being the cutting edge.

It has always been that way. The best we can do is nowhere near the mainstream or nowhere near being in the world of public knowledge...and those who work in the 'top' of such areas are deeply and fully aware of such things.

And this path of thought leaves us with such potential questions and potential answers. No matter how unsavory - the potential is real.

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