Friday, 12 February 2016

Celebrating International Darwin Day: What can evolution teach us about neurodegenerative diseases?

Charles Darwin

On this day (12th February) in the year 1809, the naturalist Charles Darwin was born. Two hundred and seven years later, we are still celebrating him and his contribution to science - the process of evolution by natural selection. Darwin was the first to theorise that all forms of life had common ancestry and that species had diverged due to nature driving adaption to various environmental pressures, such as temperature, habitation, competition, food supply and predators. 

An excellent example of this can be seen in a comparison of the polar bear to its closest relative – the brown bear. Between 350,000 and 6 million years ago, these bears were just one species which then became separated into different geographical regions. These ancestors when first arriving in snowy conditions were various shades of brown, but those with lighter coloured and thicker fur will have had a better chance of successfully hunting in the snow, surviving the cold and therefore producing offspring, the critical factor of natural selection.

Brown bear and polar bear originate from the same species

The next generation of bears also had had a range of fur colour and thickness -but overall a greater proportion is more likely to have lighter coloured, thicker fur. In this way, traits more likely to improve reproductive success are enriched in populations, and those that are detrimental are not passed onto the next generation. Over thousands to millions of years, the accumulation of these small changes between generations can produce drastic differences when comparing modern species in the present day. We now know these changes in physical characteristics are due to alterations in DNA, which encodes the genetic information to produce all forms of life.   

This includes humans too; our closest relative is the chimpanzee! We share 99% of our DNA with chimps, and it is estimated our species diverged approximately 5-8 million years ago. That 1% makes a big difference though, and one of the greatest differences has been the rapid development of our brains. Not only are our brains larger, but we have developed particular regions of the brain and processes that greatly improve our cognitive function, fine control of our hands, memory, and ability to socialise and communicate.

Showing the primate lineage and estimated time since sharing a common ancestor (millions of years ago; mya) and estimated brain size comparison among primates. The human brain is, by far, the largest primate brain. The blue region indicates the prefrontal cortex, the region of the brain responsible for personality traits and other complex cognitive behaviour.

In 1989 a scientist named Rapoport first suggested that problems in the brain leading to Alzheimer’s Disease were occurring in the brain regions and processes developed recently in the human lineage. He also suggested that perhaps these new regions were more susceptible to errors as they had had less time to “perfect” than earlier established regions of the brain, common to a broad range of the animal kingdom. More recently, a group in California found that Frontotemporal Dementia (FTD), a disease which affects human aspects of social behaviour and emotion, was caused by a loss of spindle neurons; a type of cell evolved only in apes and humans and much more abundant in human brains. 

Whilst evolution has shaped the development of our higher brain function, which has proved greatly advantageous to our species, the associated neurodegeneration has only been recently exposed through improvements in lifespan allowing an aged population never previously seen in chimpanzees or early populations of humans. Evolution through natural selection has been seemingly unable to affect the frequency of neurodegenerative diseases, as the late-age onset means it has usually no bearing on whether or not someone has children and passes on their DNA to the next generation. 

There is a fine balance between the gain of higher capacity for dexterity, gait, memory, language skills etc. that distinguishes us from our ape cousins, and a vulnerability to neurodegenerative diseases as we age. An evolutionary perspective on neurodegenerative diseases might help to further understand underlying problems and improve our model systems, but clearly highlights the importance of taking the matter of combating neurodegenerative diseases into our own hands!

By Phillippa Carling

Phillippa is a postdoctoral researcher funded by Parkinson’s UK in Professor Oliver Bandmann’s group, working on mitochondrial dysfunction in idiopathic Parkinson’s disease. You can find out more about her work on ResearchGate and LinkedIN.

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