The Australian comedian Tim Minchin has a routine that
consists of an 8 minute beat poem called Storm about a girl he met at a dinner
party. Minchin, who has made a name for himself as a vocal Skeptic (I
capitalize because there’s something of a cult of skepticism that has arisen in
recent years, and because calling them a cult will piss them off) was deeply
offended by Storm’s new age-y beliefs, which – to paraphrase – stemmed from a
belief that life was a mystery that we cannot fathom. Yes, we can, says
Minchin, and goes off on a diatribe about how what we discover about life
through the application of the scientific method is just as beautiful and
elegant and lyrical is anything that can be imagined through superstition. Or
something like that – you can watch the routine here,
I was thinking about Storm the other day, because I began to
wonder if I was inadvertently playing the role of hippie in a discussion about
the limits of scientific understanding. I tried to discuss this with my wife -
who had dragged me into the whole thing by enlisting me to review course
materials on phylogenetic systematics to be supplied to students and teachers -
and after several minutes of my trying to explain myself, she snapped. “If I
actually though you cared about this, I would never have asked you to do it.”
So that was the end of that. Or so she thinks. Ha! Because when I get annoyed
about something, I go and blog about it - so now the whole world knows what I
think. That’ll show her!
Anyway, the job in hand, as I said, involved course
materials for high school students and teachers about phylogenetic trees. Or,
as the writers put it, The Tree of Life. Note how I capitalized again, because
this is The Tree of Life we’re
talking about, like the One Ring to Rule Them All, or the Word of God. Hey
kids, when you do phylogenetic systematics, you’re not just figuring out how
things are related to each other. You’re helping to build the One True Tree of
Life, the Tree that links you to wombats, priapulid worms, and the fungus that
grows on your skin when you have poor hygiene. How Sick is that!
As you can tell, I’m a bit jaundiced about the Tree of Life.
Let’s be clear: the idea is a very important one. All living
things on this planet are linked to each other by varying degrees of common
ancestry in a pattern that is dictated by the process of evolution. By mapping
the pattern of shared characters between different organisms, you can
reconstruct that shared ancestry and hence the processes that generated the
pattern. And you can visualize the pattern as a series of nodes and branches
that looks kinda like a tree, which is a powerful metaphor to use when talking
to people about evolution, because trees are a living, naturally generated
structure, and because we also use them – in genealogy – to talk about our own
immediate ancestry.
Metaphors are dangerous things, because when stretched they can
take on a life of their own. “Genes” are a good example of this. When I say
“gene” in this context, I don’t mean what a molecular biologist would mean – a
stretch of nucleic acid, corresponding to a unit of inheritance, whose
molecular sequence codes for a functional polypeptide chain or RNA. I mean it
in the selfish, “Dawkins” sense, which is a phenotypic difference between two
organisms that has a heritable basis, and which affects the fitness of the
organism. I have a bigger tail than you,
females dig guys with big tails, I make more babies than you, and my babies
have bigger tails too. Voila, natural selection in action. Or, to put it
simply, I have the “big tail gene.”
Of course, there’s probably no such thing as “a” big tail
gene. More likely there are lots of different genes, whose combined affect is to
give me a bigger tail. But when you’re trying to explain something complicated,
like kin selection and the evolution of altruism, to a lay audience, simplicity
is vital. Hence “The Selfish Gene,” Richard Dawkins’ 1976 masterpiece, one of
the all-time greatest pieces of scientific writing and the book that made me
want to be a zoologist. I try to remember that when I see the author today, a
wild-eyed fundamentalist ranting away about God on the TV and calling otherscientists “Quislings.” What on earth happened to the guy that wrote that book?
It’s hard to believe they’re one and the same.
Like most people, my education in science followed a
well-trodden path. Each year you were taught something new. Then, at the
beginning of the next year, you were taught that what you learnt in the
previous year was not exactly true, because at some level you weren’t ready for
the truth. A vicar friend of mine (yes,
I am a scientist who has friends who are vicars. I am a Quisling!) referred to
this, in the context of bible study, as “milk and meat” – you start out on
milk, but as you get older and wiser you’re ready for some scriptural “meat.”
Why is it that everything evangelical Christians say comes out sounding
inadvertently sexual? But the fact that science and bible study have such
commonalities suggests that that maybe the Templeton Foundation is on to
something (“Quisling!”)
Unfortunately we live in a time when it’s all too easy to
call someone a liar for telling you less than the complete truth. This is the narrow
path we have to tread when we develop pedagogic materials for science. Hence my
concern over phylogenetic trees versus The Tree of Life. A phylogenetic tree is
just a hypothesis about evolutionary relationships, based on available
evidence. It cannot “be” the evolutionary history of the group of organisms in
question; most of them are missing due to the incompleteness of the fossil
record. It’s also essentially subjective, because you pick the characters that
you analyze, and – if you’re looking at anything more than a handful of them –
you run them through phylogenetic analysis software that makes a bunch of
assumptions about how evolution works.
This is not to say that phylogenetics is in any way bad
science. But neither is it infallible. Any tree can change with the emergence
of new evidence. Sometimes it’s a newly discovered organism, recent or fossil.
Sometimes it’s the inclusion of a known organism that had previously been
excluded from the analysis. Sometimes its because of a new character set, a new
analytical technique, a change in the assumptions underlying the software
algorithms that are being used to reconstruct the phylogeny. Some relationships
are more stable than others, being robustly supported by multiple lines of
evidence, but nothing is completely certain.
When I put this to one of my colleagues, she expressed
concern that if we didn’t give people the sense that there is real information
in the trees, they might not care about this at all. I understand this
perspective, but it’s a dangerous one. As scientists, we are comfortable with
uncertainty. In fact, for many of us, uncertainty is what drew us into science
in the first place. By our nature, we question everything, repeatedly, and this
questioning drives our understanding forward. Unfortunately, most people
outside of science want certainty. They expect us to be able to provide them
with answers – if I eat this, will I shorten my life expectancy: yes or no? And
they don’t like it when the answer is “maybe.”
But this does not mean we should give them certainty where none
exists. To use an example provided by the same colleague, it is true that we
are probably not going to change our minds next year and decide that gorillas
are more closely related to us than chimps. But neither can we say with
certainty that we won’t, if evidence emerges to support the alternate
relationship. Twenty years ago, you wouldn't have found many scientists who thought whales and hippos were closely related, but now it's a widely accepted hypothesis. This may be frustrating to the lay reader, but it’s also science. We go where the
evidence takes us, which is why we differ from creationists.
From uncertainty it’s a relatively short step to the
unknowable, and this is where a lot of my colleagues get really uncomfortable,
because we’re talking about the limits of science. Tim Minchin may think
“Storm” is a dumbass for believing that there are aspects of the human body
that are a mystery, but where paleontology is concerned there are all sorts of
mysteries. We are stuck in what my graduate advisor used to call “evolution's epistemological gap” – we can see the end result of evolution, in terms of
the diversity of species, and we can analyze and manipulate the processes that may have generated that diversity
at a micro-evolutionary level, but our lifespans aren’t long enough for us to directly observe one leading to the other. We have a lot of powerful supporting evidence, but the
most critical component – the fossil record – is hopelessly patchy.
In some cases, we can fill in the gaps with new fossil
discoveries, and these often change the evolutionary narrative significantly.
When I was a kid, I was told that we “knew” that our distant ancestors were
fish with lungs that developed 5-toed limbs from their fins. They were able to
survive out of water because of their lungs and the fins helped drag them back
to the water, and so evolved into limbs. Very neat and also (probably) not true. We
now know that things were more complicated than that; between 400 and 360
million years ago there were a bunch of different animals that were not quite
fish and not quite land animal, that developed feet with far more than five
toes, and that had gills, not lungs. The first feet may have had more to do
with moving in an aquatic environment than the terrestrial one. And we will continue
to find fossils that fill out the gaps in this story, in sometimes surprising
ways.
Or maybe we won’t. As I said, the fossil record is patchy.
The circumstances under which an animal is preserved are very specific and very
rare. The likelihood that erosion exposes that animal at a time and place where
someone can discover it is equally rare. And fossilization does not capture all
of the evidence about the organism. I may have an exquisitely preserved
oreodont skeleton, but I can’t tell you what color fur it had, what sounds it
made, what it smelt like, or how it digested its food. And given the circumstances under which
these animals are usually found, which do not allow for the sort of exceptional
preservation seen in other fossil deposits, I may never be able to tell you. I
don’t know about you, but I find that really exciting.
The “Tree of Life” is a bit like the Rudolph Zallinger
murals that we have on the walls of the Peabody Museum. At first sight, the Age
of Reptiles and the Age of Mammals look like panoramic views of the
evolutionary history of these groups. But they are not. We now know – because
of new evidence - that many of the animals pictured looked nothing like their
reconstructions and would not have done the things that they are shown doing.
Many of the species pictured together are actually separated by millions of
years, and sometimes tens of millions of years. Even when they are
contemporaries, the animals shown represent only a tiny fraction of the ones
that would have been present in the environments pictured, let alone all those
that have ever lived.
Does this mean the murals are useless? Not at all. Even now,
they remain a powerful tool for teaching about evolution. As I’ve discussedelsewhere in this blog, The Age of Mammals is one of the best summaries of the
effects of global climate change that one could ask for, while the Age of
Reptiles shows us how our ideas about dinosaurs have changed as a result of the
work of paleontologists, while still being an awe-inspiring work of art in its
own right.
The Tree of Life is the same thing. It is a powerful
metaphor for the interconnectedness of life, a good way to visualize our current
understanding of how different organisms are related to each other, and a
starting point for developing hypotheses of how evolution might have generated
this pattern. But the idea that its neat pattern of nodes and bifurcations
completely encapsulates the infinitely complex, intricate, and painstakingly
slow process by which populations of millions of individuals change over time
to give rise to new species is no more correct than saying that Zallinger’s
mammal mural is a photographic record of what life actually looked like in
the Oligocene.
When I first mentioned my concerns to my wife, she said (in
a tone of barely suppressed outrage) “Are you telling me that you don’t believe
in the Tree of Life?” (No, I’m not kidding – she really did say that). And yes,
at some abstract level, I do. It’s even possible to believe that one day we
might be able to build a complete Tree of Life – that we will describe every
species of life on Earth (in a way that accounts for individual and
population-level variability); that we will find all the fossils that we need
to reconstruct the complete evolutionary history of every group that has ever
lived; that we will develop unambiguous character sets that are devoid of
homoplasy and immune to the effects of convergence; that we will build
algorithms for phylogenetic reconstructionthat accurately reflect the true
process of evolution. And that, having done so, we will stand back and admire
our handiwork; a complete, entirely inclusive, 100% stable, and totally true
Tree of Life. But to believe this will happen requires, dare I say it, quite a
measure of faith.
I think I’ll stop there.
