Barnacles? What about, say, Primates?

On Nov 8, here, I wrote about Darwin's work on the forms and evolution of barnacles.  I asserted that he had a deterministic view of the process of evolution, that I think reflected the Newtonian legacy of science at the time.  Darwin described the mode of sexual reproduction in different species of barnacles, as observed today, as if they were at various stages along the way to a most 'mature' kind of final stage of fully separate adult males and females.   

Darwin wrote several books on barnacles, with typical detail and care, and I cited a fine book by Rebecca Stott (Darwin and the Barnacle) that summarized the story very well and which I highly recommend.  The idea, basically, was that various stages of barnacle reproduction in the past had led to fully separate sexual reproduction in the most advanced present-day barnacles.  Thus, various of today's barnacles that are in these intermediate states are on their evolutionary way to the final one--they're just eons behind their most advanced compatriot species.

In a sense, I think this is part of a more general issue, the equating of induction with deduction--using what we can observe from the present, to make predictions about the future--and a deterministic force-like view of evolution.  Sometimes this means assuming that the future will be like the past, so that, for example, (as in the case of Darwin's barnacles) the present can be viewed tacitly or explicitly, as a cross section of a stream of events, each contemporary state leading to some future state that is already achieved by the most 'advanced' instance observed.

I think a way to understand the point, at least as I see it, is to use an example closer to home.  After all, it's possible that most people don't really know anything about barnacles because they've not seen them or, if they have on some beach trip, they didn't pay any attention, certainly not a Darwinian kind of attention.  So Darwin's barnacle work, even for those who would care to read it or about it, might seem too abstract.  Perhaps there are other ways we might explore his same point, about animals evolving toward a more mature or final stage.

For example, we could just as easily posit that other primates--lemurs, monkeys, and apes (orang-utans, gorillas, chimps), are each on their separate way to--are in separate stages of--becoming human-like.  Perhaps an equivalent suggestion to Darwin's about barnacles would be that non-human-primates are in various states on the road to 'humanity', which we can assume they will eventually reach.  What would this mean, and how could we tell--or could we?  If not, can we say anything about them, or, rather, about their future in this regard?

The fossil record and comparative genomics clearly show that all contemporary apes share relatively recent monkey-like common ancestry. We can take the living descendants and look, comparatively, at their various structures and behaviors.  Nothing stops us from making what would seem to be an obvious statement that we, humans, are more 'advanced' than the other primates.  Yes, of course, they do well today in their own ways, but we are obviously superior.  In some senses, it can be argued that other apes are more human-like than monkeys as if, though they are all contemporary (as with Darwin's barnacles), they can all be viewed as on the 'road to humanity', one of progress that is often to be taken.  Isn't that implicit in the famous figure from TH Huxley's Man's Place in Nature (1863)?

From Frontispiece in Huxley

He showed contemporary primates aligned as if, in a sense, they reflect our evolutionary history, from most primitive to most advanced (us).  Similar figures have been shown based on fossils representing stages, or prior times, during human evolution, such as this famous one grabbed from Google images.

After Time-Life books' image

These images have something in common: they use reconstructions of a linear evolutionary past, based on fossils, or they do the same by equating current species to past stages in our ancestry.  Yet, obviously, the other apes living today are no more our ancestors than living barnacles are the ancestors of the most 'advanced' species today.  Nor are they "less evolved".

Evolutionary inevitabilities?
What we have is a subtle kind of theory of inevitability, one that implicitly at least and in a sense explicitly asserts that there is in evolution a particular, in a sense pre-determined path.  Some species living today have reached a more advanced stage along that path (their future of course not yet knowable).  But others, though alive today, are as yet only part-way along the path.

Is there any reason to think, much less assume, that chimps are (if we don't wantonly kill them all off first) 'on the way' to becoming humanlike?  This kind of thinking was, I believe, implicit in Darwin, as if evolution were a kind of 'Newtonian' force, like gravity.  But does such thinking still lurk around?  If we share that kind of worldview we can ask, for example, why there are still chimps, if one branch of our shared ancestors evolved to become us humans.  If today's chimps are not on the way to becoming 'human', why did (some of) our chimp-like ancestors do so? If from a common ancestor we could evolve as we did, why not them and why haven't they yet--or will they?

We may have outgrown Darwin's more literal linear thinking, but perhaps not entirely.  As noted in our earlier post, the 'rewind the tape' argument is still raised from time to time.  Is evolution a 'force' that cannot be denied, a kind of inevitability, or is it so local, ad hoc, and related to existing genotypes at a time and place that has no real parallelism?

These are perhaps literally unanswerable questions, but perhaps they are in fact glibly answered by evolutionary biology. Darwin seemed, to me, to assert a sense of replicable inevitabilism, while we seem to have ad hoc accounts that don't suggest that.  If today's apes are on their evolutionary way to something else, it is legitimate at least to ask, at least rhetorically, whether that will be something human-like.  Today we generally say that despite similarities, our common human-chimp ancestors branched into our two present-day species and because of the random nature of mutation and local circumstance, we each evolved our separate ways.  But we'd also say that because of the randomness of mutations, and of environmental changes and so on, they are not destined to become like us.  But then, in the future, if the root abilities are there, and we share so much of our genomes......?

A plausible, indeed basically irrefutable view--so, rather, an axiom of life--is that today's species will either become extinct without issue, or will evolve into forms different from today's.  But can we make any sort of rigorous inference of the sort that today's apes will become human-like?  We know they will not become beetles, or trees--or mermaids--because they are constrained by what they already are, their future forms contingent upon the present.  But is there any rigorous or knowable sense in which a cross-section at any given time is a montage of the evolutionary future?  How precisely does their present form determine their evolutionary future?  Must they evolve into human-like creatures because our primate ancestors did?  If the rewind-the-tape argument has relevance for understanding the past, is the tape being played before our eyes today?  If not, is the biological future predictable with any kind of precision?

These are long-term questions, and modern biology would either just say we can't know or that the idea of parallelism is nonsensical since each species today is unique, as are its circumstances.  But it a non-trivial sense, when we take a sample of human populations for, say, disease-genetic mapping, and extend that to other people or populations, are we not making similar assumptions of parallelism, or the even stronger one that different populations are at the same stage?

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.    

References:

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.