Tuesday, March 26, 2013

Evolution in a terrarium?

Our understanding of ecology and evolution has historically been highly stratified; with conceptual divisions and blinders between humans and the environment, between disciplines, and between substantive foci.

As a highly anthropocentric species, for a long period of time people thought that the rules of nature somehow didn’t apply to humans.  Arguably, even today, many who study human evolution do so in the absence of thinking about how evolution occurs in all of the other organisms on our planet.  To be fair, it is difficult to really delve into a subject without focusing on it – but I also think that a narrow approach can be dangerous too.  Furthermore, it’s probably easier to see evolution occurring in your favorite organism if that organism doesn’t have generation times that are as long as yours…

And this “us versus the rest of the world” type of thinking has extended both ways.  Not only do we evolve, but we are constantly modifying our environment.  We’ve probably been doing that for a very long time.  Given that most other organisms modify their environments – beavers build dams, ruminants alter landscapes by selectively grazing in certain areas – I would say that it’s silly to think that we haven’t been altering our landscapes for a long period of evolutionary time.

For a period of time in U.S. history, when manifest destiny and western expansion were the battle cry of the day, it wasn’t uncommon for people to arrive in environments, new to them, and to think of those places as pristine, untouched landscapes.  This is what much of our modern environmental movement grew out of.  We thought of these Western landscapes as things that hadn’t yet been marred by human hands, and we needed to keep them pristine for the next generation.  This was and is a flawed view of the relationship between humans and environment.

Long before Euro-Americans showed up in the American West there were native peoples living in these wide open spaces.  There were just a lot less of them when the new Americans showed up because new pathogens had already moved across the landscape, at a speed that would have been impossible for the new Americans to match, and completely decimated much of the population.  These beautifully manicured landscapes weren’t untouched.  They had been manipulated, lived in, and exploited for thousands of years prior to European arrival here.

The arrow goes the other way too.  We are modified by our environment.  Returning to my favorite subject (pathogens), the story of the sickle cell trait and malaria is a textbook example of how the environment, this time in the form of parasites spread through mosquitoes, has actually left an imprint on the human genome [1].  And considering that much of the evidence about the evolutionary history of falciparum malaria suggests a leap into humans around 10k years ago, humans may have started the ball rolling per se with the onset of agriculture.  The real story, however, is probably much more complex than the one(s) that anthropologists usually tell.

Humans began changing their environment to such an extent that it could have led to a population increase in a specific type (or types) of mosquito.  These mosquitoes were already in the environment, and would have already been capable of spreading a parasite that had also already been in the environment.  That parasite has recently been shown to occur in high prevalences in wild gorillas, meaning that agriculture wasn’t necessary for it to exist in human populations, but several lines of evidence suggest that agriculture could have been sufficient for it to be a major factor in human populations [2].  Furthermore, malaria parasites have coevolved with their hosts, with a multitude of adaptations that allow them to survive our (and other organism’s) immune responses [3]

Humans made a change to their landscape: agriculture.  That change may have reduced the risks associated with starvation, but also may have increased the population sizes of some pest species: mosquitoes.  And it wasn’t just any type of mosquito that would have been influenced, but a type or types which are capable of effectively spreading a parasite: falciparum malaria.  And that parasite has developed the ability to sexually reproduce in certain types of mosquitoes and to maintain population sizes within human hosts that don’t immediately kill everyone who is infected, therefore allowing the life cycle to continue. 

These coevolutionary relationships aren’t even close to being limited to the malaria story.  For example, a non-trivial portion of the human genome is made up of endogenous RNA viruses.  We’ve been shaping our environment, and it has been shaping us, for a very long time. 

This story isn’t just about pathogens though, because through domestication humans have transformed plants and animals too.  The list of such “domesticated” organisms is, like the list of pathogens with coevolutionary relationships with humans, too long to give proper space to here.  But think about the special relationships between humans, cats and dogs (they’ve arguably domesticated us).  Or between humans and yeast (see a nice blog post here), or with maize or potatoes, or cows and sheep.  The not so funny thing is, that while we know about all of these modifications that humans have made to the environment, and that the environment has made to humans, they almost always get studied in a highly stratified way.  As if each of these components occur by themselves, in a terrarium, not interacting with everything else. 

Plants get studied in botany departments or in schools of agriculture.  Insects are either studied by agricultural scientists (worried about what they’ll do to plants) or biomedical scientists (worried about what they’ll do to people).  Biomedical scientists who focus on the actual pathogens, in my view, tend to ignore the human component.  (Of course, a lot of interventions are based on killing something in the epidemiological cycle, and most people don’t want to take out the human host for the sake of the other components of the epidemiological cycle).  And finally, people who study human evolution are just as bad (or worse) than everyone else on this list; mostly ignoring the major evolutionary pressures (disease and nutrition) that have undoubtedly shaped who we are so that we can instead focus on topics that are sexy (to humans, that is).  Even though we know that humans do not live in terrariums.  Or most of us anyway.    

1. Kwiatkowski DP: How malaria has affected the human genome and what human genetics can teach us about malaria. American Journal of Human Genetics2 2005, 77:171–190.

2. Liu W, Li Y, Learn GH, Rudicell RS, Robertson JD, Keele BF, Ndjango JBN, Sanz CM, Morgan DB, Locatelli S: Origin of the human malaria parasite Plasmodium falciparum in gorillas. Nature 2010, 467:420–425.

3. Evans AG, Wellems TE: Coevolutionary genetics of Plasmodium malaria parasites and their human hosts. Integrative and comparative biology 2002, 42:401–7. 


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